2020 081 19 May

Dielectron production in pp and p–Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ =
5.02 TeV

ALICE Collaboration††thanks: See Appendix A for the list of collaboration
members ALICE Collaboration

The first measurements of dielectron production at midrapidity
($\left|\eta_{\rm e}\right|~{}<~{}0.8$) in proton–proton and proton–lead
collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV at the LHC are
presented. The dielectron cross section is measured with the ALICE detector as
a function of the invariant mass $m_{\rm ee}$ and the pair transverse momentum
$p_{\rm T,ee}$ in the ranges $m_{\rm ee}<{3.5}$ GeV/$c^{2}$ and $p_{\rm
T,ee}<{8}$ GeV/$c$, in both collision systems. In proton–proton collisions,
the charm and beauty cross sections are determined at midrapidity from a fit
to the data with two different event generators. This complements the existing
dielectron measurements performed at $\sqrt{s}$ = 7 and 13 TeV. The slope of
the $\sqrt{s}$ dependence of the three measurements is described by FONLL
calculations. The dielectron cross section measured in proton–lead collisions
is in agreement, within the current precision, with the expected dielectron
production without any nuclear matter effects for ${\rm e}^{+}{\rm e}^{-}$
pairs from open heavy-flavor hadron decays. For the first time at LHC
energies, the dielectron production in proton–lead and proton–proton
collisions are directly compared at the same $\sqrt{s_{\mathrm{NN}}}$ via the
dielectron nuclear modification factor $R_{\rm pPb}$. The measurements are
compared to model calculations including cold nuclear matter effects, or
additional sources of dielectrons from thermal radiation.

##  1 Introduction

ALICE [1], located at the Large Hadron Collider (LHC) at CERN, was designed to
study the quark–gluon plasma (QGP), a state of matter which consists of
deconfined quarks and gluons. The QGP is created at the high-energy densities
and temperatures reached in ultra-relativistic heavy-ion collisions. Under
these conditions, the chiral symmetry is expected to be restored in the QGP
phase [2, 3]. Dileptons (${\rm l}^{+}{\rm l}^{-}$, i.e. ${\rm e}^{+}{\rm
e}^{-}$or $\mu^{+}\mu^{-}$) are emitted during all stages of the heavy-ion
collision and carry information about the medium properties at the time of
their emission, as they do not interact strongly. This makes them a very
promising tool to understand the chiral symmetry restoration and the
thermodynamical properties of the QGP. In particular, the measurement of the
dilepton invariant mass ($m_{\rm ll}$) allows for the separation of the
different stages of the medium evolution. For $m_{\rm ee}<{1.1}$ GeV/$c^{2}$,
the main dilepton sources are Dalitz decays of pseudoscalar mesons ($\pi^{0}$,
$\eta$, $\eta$’) as well as Dalitz and two-body decays of vector mesons
($\rho$, $\omega$, $\phi$). In this mass range, the dilepton spectrum is
sensitive to the in-medium modification of the $\rho$ meson spectral function,
which is connected to the partial restoration of chiral symmetry in the hot
hadronic phase [3, 4]. At the same time, thermal radiation from the medium,
contributing over a broad mass range, provides insight into the temperature of
the medium and its space–time evolution.

Measurements of dilepton production in nucleus–nucleus collisions were
performed at the Super Proton Synchroton at CERN, among others, by CERES [5,
6] and NA60 [7] at a center-of-mass energy per nucleon–nucleon pair,
$\sqrt{s_{\mathrm{NN}}}$ $\leq~{}17.3$ GeV. An excess of dileptons compared to
the expectation from known hadron decays was observed. It can be ascribed to
thermal sources, primarily from thermal production of $\rho$ mesons for
$m_{\rm ll}<$ 1 GeV/$c^{2}$ with a strongly broadened $\rho$ spectral function
[7], as well as partonic thermal radiation for $m_{\rm ll}>$ 1 GeV/$c^{2}$
[8]. At higher energies $\sqrt{s_{\mathrm{NN}}}~{}=~{}200$ GeV, results from
PHENIX [9] and STAR [10] at the Relativistic Heavy Ion Collider (RHIC) are
also compatible with models involving a broadening of the $\rho$ spectral
function. The study of thermal radiation from the QGP in the intermediate-mass
region (IMR), ${1.1}<m_{\rm ll}<{2.7}$ GeV/$c^{2}$, is however challenging at
these center-of-mass energies due to the large background from correlated
${\rm l}^{+}{\rm l}^{-}$ pairs originating from open heavy-flavor hadron
decays. The first measurement of low-$m_{\rm ll}$ dileptons at the LHC,
performed by ALICE in Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}2.76$
TeV [11], does not provide data sensitive to a thermal signal due to the
limited statistical precision and the limited knowledge of the charm
contribution. Consequently, it is crucial to understand the dilepton
production in proton–proton (pp) collisions, in particular the contribution
from heavy-flavor hadron decays, in order to single out the characteristic
signals of the QGP.

In pp collisions, the production of charm and beauty quarks can be estimated
with perturbative quantum chromodynamics (pQCD) calculations in vacuum without
any initial- and final-state effects. Owing to flavor conservation, the heavy
quarks can only be produced in pairs. The resulting lepton pairs originating
from charm hadron decays reflect the initial kinematic correlations between
the charm and the anti-charm quarks, whereas in the case of beauty hadron
decays the correlation is weakened because of their large masses. In pp
collisions, where no thermal dilepton sources are expected, the ${\rm
l}^{+}{\rm l}^{-}$-pairs arising from heavy-flavor hadron decays are the main
contribution to the dilepton yield in the IMR. Hence, dileptons can be used to
study the heavy-quark production mechanisms. Together with the measurements of
single heavy-flavor hadrons and their decay products, accurate results on
dilepton production can provide constraints on the Monte Carlo (MC) event
generators aiming to describe heavy-flavor production. In studies with
dielectrons by PHENIX in pp collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}200$
GeV [12, 13], and more recently by ALICE in pp collisions at $\sqrt{s}$ = 7
and 13 TeV [14, 15] the charm and beauty cross sections at midrapidity and in
the full phase space were extracted by means of the analysis of the dielectron
invariant mass ($m_{\rm ee}$) and pair transverse momentum ($p_{\rm T,ee}$)
spectra. The measured cross sections at the LHC and RHIC were found to be
consistent with fixed order plus next-to-leading logarithms (FONLL)
calculations [16].

The production of dileptons in heavy-ion collisions can be modified with
respect to pp collisions not only by the presence of hot nuclear matter but
also by the presence of cold nuclear matter (CNM). The CNM effects include the
modification of the quark and gluon content in the initial state, that is
described by means of parton distribution functions (PDFs) of the incoming
nucleons in the collinear factorization framework. In nucleons that are bound
in the nucleus, the PDFs are altered by the presence of additional nuclear
matter with respect to free nucleons. This modification depends on the parton
momentum fraction $x$, the atomic mass number of the nucleus $A$, and the
momentum transfer $Q^{\rm 2}$ in the hard scattering process. Nuclear PDFs are
obtained from a global fit to data from different experiments [17, 18, 19].
When the phase space density of gluons within the hadron is high due to gluon
self-interactions, reaching a saturation regime, an appropriate theoretical
description is the Color Glass Condensate (CGC) theory [20, 21, 22, 23]. At
LHC energies at midrapidity, where small values of $x$ are probed by the charm
and beauty production ($x\leq 10^{-3}$), the most relevant effect on the PDFs
is shadowing [24]. The modification of the initial state in hadronic
collisions can significantly reduce the heavy-flavor production cross sections
at low transverse momentum ($p_{\rm T}$). In addition, multiple scattering of
partons in the nucleus, before and/or after the hard scattering, can change
the kinematic distribution of the produced hadrons and affect their azimuthal
correlation, such that the $m_{\rm ee}$ and $p_{\rm T,ee}$ distributions from
correlated heavy-flavor hadron decays could be modified [25, 26].

Initially, hot matter effects were not expected in proton–nucleus (pA)
collisions, so they were used as a baseline for measurements in heavy-ion
collisions to study possible CNM effects. At LHC energies in minimum bias (MB)
p–Pb collisions at midrapidity, the measured $p_{\rm T}$ differential
production cross sections of single open-charm hadrons [27, 28] and their
decay electrons [29, 30], as well as results on azimuthal correlations of D
mesons and charged particles [31], are compatible over the whole $p_{\rm T}$
range probed with the results in pp collisions scaled with the atomic mass
number $A$ of the Pb nucleus. Moreover, the yields of ${\rm J}/\psi$ from B
hadron decays as well as prompt ${\rm J}/\psi$ are found to be suppressed at
low $p_{\rm T}$ at midrapidity in MB p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV [32], but the measurements of B hadron
production cross sections at high $p_{\rm T}$ show no significant modification
of the spectra compared to perturbative QCD calculations of pp collisions
scaled with $A$. All of these results indicate that possible CNM effects are
small compared to the current uncertainties of the measurements for open
heavy-flavor production at midrapidity at the LHC. However, at forward and
backward rapidities, the measured $p_{\rm T}$ differential cross sections of D
[33] and B mesons [34], and of muons originating from heavy-flavor hadron
decays [35] in minimum bias p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV demonstrate the presence of CNM
effects and support shadowing as possible explanation. The forward and
backward results set constraints on models that also aim at reproducing the
midrapidity measurements. Accurate measurements in pA collisions provide
important inputs for the parametrizations of the nuclear PDFs, which are
currently suffering from large uncertainties [18, 19].

On the other hand, final-state effects may also play an important role in pA
collisions. In particular, in those with large multiplicities of produced
particles, as suggested by results from azimuthal anisotropy measurements
through two-particle [36, 37, 38, 39, 40, 41, 42] and multi-particle
correlations [43, 44], modifications of the $p_{\rm T}$ distributions of
identified hadrons with respect to the charged-particle multiplicity in the
event [45, 46], multiplicity dependence of strangeness production [47], and
$\psi$(2S) production [48, 49, 50]. Should such observations be linked to the
creation of a small volume of hot medium in high-multiplicity pA collisions,
the corresponding thermal radiation could lead to an enhanced dilepton
production [51, 52, 53]. At RHIC energies, results on dilepton production at
midrapidity in minimum bias d–Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200
GeV [12, 13] show no evidence of neither an additional source of lepton pairs,
nor of nuclear modification of the charm and beauty production. At the LHC,
where the density of final-state particles is larger, dilepton measurements in
p–Pb collisions can give more insight into the possible formation of a hot
medium in small systems and CNM effects.

In this article, the first measurements of ${\rm e}^{+}{\rm e}^{-}$production
in pp and p–Pb collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV at the
LHC are presented. The results are obtained with the ALICE detector. The data
are compared, in terms of the $m_{\rm ee}$ and $p_{\rm T,ee}$ distributions,
to the sum of the expected sources of ${\rm e}^{+}{\rm e}^{-}$ pairs from
known hadron decays, the so-called hadronic cocktail. The spectra are shown
after the application of fiducial requirements on single electrons
($\left|\eta_{\rm e}\right|~{}<~{}0.8$ and ${0.2}<p_{\rm T,e}<{10}$ GeV/$c$)
without an extrapolation to the full phase space. In addition, for the first
time at LHC energies, a direct comparison between the dielectron cross section
obtained in pp and p–Pb collisions is possible since both data sets were
recorded at the same $\sqrt{s_{\mathrm{NN}}}$. In particular, the analysis of
the pp data resolves the model dependence on the expected $m_{\rm ee}$ and
$p_{\rm T,ee}$ distributions of correlated ${\rm e}^{+}{\rm e}^{-}$ pairs from
open heavy-flavor hadron decays in pp collisions, used as reference for the
p–Pb study. This allows for the research of possible modifications to the
dielectron production in p–Pb collisions due to CNM or additional final-state
effects.

The article is organized as follows. The experimental setup and the used data
samples are described in Sec. 2. The analysis steps, including track selection
criteria, electron identification, signal extraction and efficiency
corrections, are described in Sec. 3, together with the corresponding
systematic uncertainties. The method to calculate the expected dielectron
cross section from known hadron decays is explained in Sec. 4. In Sec. 5, the
results are presented, covering the charm and beauty cross section extracted
in pp collisions, comparisons of the dielectron production in pp and p–Pb
collisions to the expectations from known hadron decays, and the resulting
dielectron nuclear modification factors.

##  2 The ALICE detector and data samples

The ALICE detector and its performance are described in [1, 54]. Electrons are
measured in the ALICE central barrel covering the midrapidity range
$|\eta|<0.9$. (Note that the term ‘electron’ is used for both electrons and
positrons throughout this paper.) The relevant subsystems used in the
dielectron analysis are the Inner Tracking System (ITS) [55], the Time
Projection Chamber (TPC) [56], and the Time-Of-Flight (TOF) [57] detector.

The innermost detector of the ALICE apparatus, closest to the nominal
interaction point, is the ITS. It consists of six silicon tracking layers
based on three different technologies. The two inner layers are Silicon Pixel
Detectors (SPD), the two middle layers are Silicon Drift Detectors, and the
two outer layers are Silicon Strip Detectors. About half of the pp and p–Pb
data samples were recorded without the Silicon Drift Detector information in
order to reach maximal data acquisition rates. For this reason, even when
available, the information from this detector is not used so as to have
uniform detector conditions over the entire data sets. The main detector for
particle identification (PID) and tracking is the TPC. This 500 cm long
cylindrical detector, with an outer radius of 247 cm, is located around the
ITS. The TPC readout is based on multi-wire proportional chambers and provides
up to 159 three-dimensional space points as well as the specific energy loss
of the particle. The outermost detector used in this analysis is the TOF. It
provides a time-of-flight measurement for particles from the interaction point
to its active volume, at a radius of 370 cm. The combined information from the
ITS, TPC, and TOF is used to reconstruct the track of a charged particle using
a Kalman-filter based algorithm [54].

The data used in this paper were recorded in collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV, with the p–Pb data taken in 2016, and
the pp data taken in 2017. Due to the asymmetric beam energies in the p–Pb
configuration, 4 TeV for the proton beam and 1.59 TeV per nucleon for the Pb
beam, the rapidity ($y$) of the center-of-mass system is shifted by $\Delta
y=0.465$ in the laboratory frame in the direction of the proton beam. For both
collision systems, events were recorded when a coincident signal in the V0
detector system [58] was registered. The V0 detector consists of two segmented
scintillators located at $+340$ cm and $-70$ cm along the beam axis from the
nominal interaction point. Additional selections are applied to the recorded
events. The background from beam–gas interactions and pileup events are
rejected by using the correlations between the V0 detector and ITS signals.
Only events with at least one track segment reconstructed in the ITS
contributing to the vertex reconstruction with the SPD are used. To assure a
uniform detector coverage at midrapidity, the vertex position along the beam
direction is restricted to $\pm 10$ cm with respect to the nominal interaction
point. A summary of the number of events $N_{\rm ev}$ passing the event
selection criteria and the corresponding integrated luminosity
$\mathcal{L}_{\rm int}$ is given in Table 1. These requirements are fulfilled
by 77% (75%) of the recorded events for the pp (p–Pb) data samples. The
$\mathcal{L}_{\rm int}$ is calculated as $\mathcal{L}_{\rm int}$ = $N_{\rm
MB}$ $/$ $\sigma_{\rm MB}$, with the number of analyzed events after the
vertex reconstruction efficiency correction $N_{\rm MB}$, and the minimum bias
trigger cross section $\sigma_{\rm MB}$ measured via a van der Meer scan in
the corresponding collision system [59, 60].

Table 1: The integrated luminosity ($\mathcal{L}_{\rm int}$) and the number of events ($N_{\rm ev}$) after event selection criteria are applied for the pp and p–Pb data samples. Data set | $\mathcal{L}_{\rm int}$ | $N_{\rm ev}$
---|---|---
pp | 19.93$\pm$0.4 $\rm{nb^{-1}}$ | 888$\times$106
p–Pb | 299$\pm$11 $\rm{\mu b^{-1}}$ | 535$\times$106

##  3 Data analysis

###  3.1 Track selection

The same track selection criteria are applied in the analysis of the pp and
p–Pb data samples. Electron candidates are selected from charged tracks
reconstructed in the ITS and TPC in the transverse-momentum range
${0.2}<p_{\rm T,e}<{10}$ GeV/$c$ and pseudorapidity range $\left|\eta_{\rm
e}\right|~{}<~{}0.8$. The tracks are required to have at least 80 space points
reconstructed in the TPC and at least three hits in the ITS assigned to them.
The maximum $\chi^{2}$ per space point measured in the TPC (ITS) is required
to be smaller than 4 (4.5). To reduce the contribution from secondary tracks,
the distance-of-closest approach of the track to the reconstructed primary
vertex is required to be smaller than 1 cm in the transverse plane to the
colliding beams and smaller than 3 cm in the longitudinal direction. In order
to further suppress the contribution of electrons from photon conversions in
the detector material, only tracks with a hit in the first layer of the SPD
and no ITS cluster shared with any other reconstructed track are used in the
analysis.

###  3.2 Electron identification

Electrons are identified by measuring their specific energy loss
$\textrm{d}E/\textrm{d}x$ in the TPC and their velocity with the TOF as a
function of their momentum. The momentum is estimated from the curvature of
the track measured in the ITS and TPC. The PID is based on the detector PID
response $n(\sigma_{i}^{\rm Det})$. This is expressed as the deviation between
the measured PID signal of the track in the detector (Det) and its expected
most probable value for a given particle hypothesis $i$ at the measured track
momentum. This deviation is normalized to the detector resolution $\sigma$.
Electrons are selected over the whole investigated momentum range in the
interval $|n(\sigma_{e}^{\rm TPC})|<3$, while the charged pion ($\pi^{\pm}$)
contribution is suppressed by requiring $n(\sigma_{\pi}^{\rm TPC})>3.5$.
Furthermore, the track must also fulfill at least one of the two following
conditions:

  1. 1.

The track is outside the hadron bands in the TPC, defined by $|n(\sigma_{\rm
K}^{\rm TPC})|<3$ and
${|n(\sigma_{\rm p}^{\rm TPC})|<3}$.

  2. 2.

The track has a valid hit in the TOF detector and falls within the range
$|n(\sigma_{e}^{\rm TOF})|<3$.

With this approach, the hadron contamination in the single-electron candidate
sample is less than 4% averaged over $p_{\rm T,e}$. The largest hadron
contamination, up to 9%, is observed where kaons ($p_{\rm T}\approx$ 0.5
GeV/$c$), protons ($p_{\rm T}\approx$ 1 GeV/$c$) or charged pions ($p_{\rm
T}>{7}$ GeV/$c$) have a similar $\textrm{d}E/\textrm{d}x$ as electrons in the
TPC. The final hadron contamination in the dielectron signal is negligible, as
pairs containing a misidentified hadron are further removed during the signal
extraction.

###  3.3 Signal extraction

A statistical approach is used to extract the true signal pairs ($S$) as a
function of $m_{\rm ee}$ and $p_{\rm T,ee}$, in which all electrons and
positrons in an event are combined to create an opposite-sign spectrum ($OS$).
The $OS$ contains not only signal, but also background ($B$) from
combinatorial pairs, as well as residual correlations from jets and
conversions of correlated decay photons originating from the same particle.
The background is estimated from the distribution of same-sign pairs ($SS$)
from the same event, as explained in [14]. The advantages of the same-sign
technique, with respect to an event-mixing approach, are the intrinsic correct
normalization of the $SS$ spectrum, and the inclusion of charge-symmetric
background sources, e.g. electrons from fragmentation in jets. The signal is
then extracted as $S=OS-R_{\rm acc}\times SS$, where $R_{\rm acc}$ is a
correction factor needed to account for the different acceptance of opposite-
sign and same-sign pairs. It is estimated using an event-mixing technique
detailed in [14].

For pairs with $m_{\rm ee}<{0.14}$ GeV/$c^{2}$, the angle $\varphi_{\kern
0.70004pt\rm V}$, which quantifies the orientation of the opening angle of the
pairs relative to the magnetic field [14] and allows for the rejection of
${\rm e}^{+}{\rm e}^{-}$ pairs from photon conversions, is required to be
smaller than 2 rad. After applying this criterion, the remaining contribution
from ${\rm e}^{+}{\rm e}^{-}$ pairs from photon conversions in the detector
material is less than 1.4%.

The signal-to-background ratio ($S/B$) and statistical significance
($S/\sqrt{S+2B}$) are depicted in the left and right panels of Fig. 1,
respectively, for the pp and p–Pb samples. Despite a worse $S$/$B$ in p–Pb
collisions, mostly due to the larger particle multiplicity, the statistical
significance of the measurement is similar in both collision systems.

Figure 1: (Color online) Signal-to-background ratio (left) and statistical
significance (right) of the dielectron measurements as a function of $m_{\rm
ee}$ in pp and in p–Pb collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV.

###  3.4 Efficiency correction

The efficiency of the single-electron and pair selection is calculated with
dedicated MC simulations. The simulated events are propagated through the
ALICE detector using the GEANT 3 [61, 62] transport code. The same strategy is
used for the pp and p–Pb analyses. Since the full kinematic range cannot be
fully populated by pairs originating only from the same-mother particle (SM)
or only from open heavy-flavor hadron decays (HF), the final efficiency
correction is estimated separately for each source. For SM pairs, pp and p–Pb
collisions are generated with the Monash2013 [63] tune of PYTHIA 8.1 [64]
(denoted as PYTHIA 8 from now on) and with DPMJET [65], respectively. In the
case of HF pairs, MC simulations of open heavy-flavor hadrons using PYTHIA 6
[66] are performed. In the p–Pb case, heavy-flavor events are embedded into
realistic p–Pb collisions simulated with EPOS-LHC [67]. The efficiency as a
function of $m_{\rm ee}$ and $p_{\rm T,ee}$ is calculated as

$\epsilon_{\rm ee}(m_{\rm ee},p_{\rm T,ee})=w_{\rm SM}\times\epsilon_{\rm
SM\rightarrow ee}(m_{\rm ee},p_{\rm T,ee})+w_{\rm HF}\times\epsilon_{\rm
HF\rightarrow ee}(m_{\rm ee},p_{\rm T,ee}).$ (1)

The weights $w_{\rm SM}$ and $w_{\rm HF}$ represent the relative cross
sections of the SM and HF sources, respectively. They are estimated with the
expected dielectron cross section from known hadron decays, explained in Sec.
4. The average reconstruction efficiency of a signal ${\rm e}^{+}{\rm e}^{-}$
pair is very similar in pp and p–Pb collisions and ranges from 25% to 30%.

The corrected differential dielectron cross section is calculated as

$\frac{{\rm d}^{2}\sigma_{\rm ee}}{{\rm d}m_{\rm ee}{\rm d}p_{\rm
T,ee}}=\frac{1}{\mathcal{L}_{int}}\frac{1}{\Delta m_{\rm ee}}\frac{1}{\Delta
p_{\rm T,ee}}\frac{S(m_{\rm ee},p_{\rm T,ee})}{\epsilon_{\rm ee}(m_{\rm
ee},p_{\rm T,ee})},$ (2)

with $\Delta m_{\rm ee}$ and $\Delta p_{\rm T,ee}$ being the width of the
$m_{\rm ee}$ and $p_{\rm T,ee}$ intervals, respectively, and
$\mathcal{L}_{int}$ the integrated luminosity. In pp collisions, the spectra
are corrected for the vertex reconstruction efficiency and for the efficiency
of the minimum bias trigger to select inelastic events with an ${\rm
e}^{+}{\rm e}^{-}$pair, which are found to be 96% and 98%, respectively. In
p–Pb collisions both efficiencies are unity.

###  3.5 Systematic uncertainties

Different sources of systematic uncertainties are taken into account. On the
single track level the effects of the required hit in the first ITS layer, the
ITS-TPC matching efficiency, and the selection of tracks without shared
clusters are studied. These uncertainties are calculated as a function of
$m_{\rm ee}$ and $p_{\rm T,ee}$. Effects from the track and PID selection as
well as the requirement on $\varphi_{\kern 0.70004pt\rm V}$ are estimated on
the pair level. For these uncertainties negligible $p_{\rm T,ee}$ dependence
is found and they are applied only as a function of $m_{\rm ee}$. In order to
suppress statistical fluctuations, that could influence the estimated
systematic uncertainties, they are evaluated in both analyses in wide mass
intervals. The resulting systematic uncertainties from the different sources
are summarized in Table 2 for the p–Pb and pp analyses.

The systematic uncertainties that arise from the limited knowledge of the
matching efficiency of the track segments reconstructed in the ITS and the
TPC, and from the requirement of a hit in the innermost ITS layer, are
determined with a two-step procedure. First, on the single-track level, the
efficiencies of these two track selection criteria are estimated for charged
pions in data and in MC as a function of $p_{\rm T}$. Second, the observed
difference is taken as input to a toy MC simulation, which generates particles
in the full $m_{\rm ee}$ and $p_{\rm T,ee}$ phase space decaying them into
${\rm e}^{+}{\rm e}^{-}$ pairs and applying the fiducial selection. The final
systematic uncertainty at the pair level is then calculated as the sum of the
uncertainties of the decay products, corresponding to the input.

The systematic uncertainty from the requirement of no shared clusters in the
ITS is evaluated by varying the maximum number of allowed shared ITS clusters
for the selected electron candidates. This provides a test of the
understanding of the background since it not only probes different single-
electron efficiencies but also different $S/B$ ratios. When no requirement is
applied the $S/B$ decreases by a factor two, which is due to the increased
contribution of electrons from photon conversions in the detector material in
the selected electron sample. The resulting dielectron spectra are compared
after the efficiency correction. The maximum deviation of the variations that
are considered statistically significant according to the Barlow criterion
[68] is used to assign the systematic uncertainty.

Similarly, the uncertainty from the remaining single-electron selection
criteria is determined by varying them simultaneously within reasonable
values. By changing the selection criteria for the tracks in the ITS and the
hadron rejection criteria, the evaluated systematic uncertainties are
sensitive to estimations of the background as well as a possible bias due to
the hadron contamination in the electron sample. The systematic uncertainty is
calculated as the root mean square of the variation of the final data points.
Finally, a possible bias due to the efficiency correction of the
$\varphi_{\kern 0.70004pt\rm V}$ selection is estimated. For this purpose, the
maximum $\varphi_{\kern 0.70004pt\rm V}$ requirement for ${\rm e}^{+}{\rm
e}^{-}$ pairs with $m_{\rm ee}<{0.14}$ GeV/$c^{2}$ is varied around its
default value from 1.5 to 2.7 rad.

Two additional sources of uncertainty are taken into account for the pp
analysis, namely the correction for the primary vertex reconstruction
efficiency and the trigger efficiency. Both are evaluated to be 2% based on MC
simulations. A priori, the reconstruction efficiency of an ${\rm e}^{+}{\rm
e}^{-}$ pair at a given $m_{\rm ee}$ and $p_{\rm T,ee}$ should not depend on
its source. However, in the p–Pb analysis, a difference in the efficiencies of
${\rm e}^{+}{\rm e}^{-}$ pairs originating from either light-flavor decays or
heavy-flavor decays is observed. Therefore, an additional uncertainty of 3% is
assigned to cover a possible bias in the spectra. The total systematic
uncertainty is calculated as the quadratic sum of the individual contributions
assuming they are all uncorrelated. The total uncertainty varies between 11%
and 4%, being equal to 5% in most of the $m_{\rm ee}$ range. The uncertainties
are partially correlated between different $m_{\rm ee}$ intervals.

Table 2: Systematic uncertainties on the requirement of a hit in the first ITS layer, the ITS-TPC matching efficiency (ME), the allowed number of shared clusters in the ITS, the variation of the $\varphi_{\kern 0.70004pt\rm V}$ selection, and the tracking and PID variations in coarse $m_{\rm ee}$ intervals for the p–Pb (pp) analysis. The uncertainties on the vertex reconstruction (2%) and trigger (2%) efficiencies in the pp analysis, as well as the uncertainty of the light- and heavy-flavor efficiency differences (3%) in the p–Pb analysis, are not listed. They are applied over the whole range of the measurement and included in the total uncertainty. The total systematic uncertainty is the quadratic sum of the single contributions assuming they are all uncorrelated. $m_{\rm ee}$ (GeV/$c^{2}$) | 1st ITS layer | ITS-TPC ME | Shared ITS cls. | $\varphi_{\rm V}$ | Tracking & PID | Total
---|---|---|---|---|---|---
$<0.14$ | 2 (1)% | 2 (2)% | 2 (1)% | 2 (1)% | 10 (6)% | 11 (7)%
$0.14-1.1$ | 2 (1)% | 2 (2)% | 2 (0)% | – | 2 (2)% | 5 (4)%
$1.1-2.7$ | 2 (2)% | 2 (3)% | 0 (0)% | – | 2 (2)% | 5 (5)%
$2.7-3.5$ | 2 (2)% | 2 (4)% | 0 (0)% | – | 2 (1)% | 5 (5)%

##  4 Cocktail of known hadron decays

The measured dielectron spectra in pp and p–Pb collisions are compared to a
hadronic cocktail, which represents the sum of the expected contributions of
dielectrons from known hadron decays, after the fiducial selection criteria on
single electrons are employed. A fast MC simulation of the ALICE central
barrel is performed, including realistic momentum and angular resolutions as
well as Bremsstrahlung effects, which are applied to the decay electrons as a
function of $p_{\rm T,e}$, azimuthal angle ($\varphi_{\rm e}$) and $\eta_{\rm
e}$ [69].

The Dalitz and dielectron decays of light neutral mesons are simulated with
the phenomenological event generator EXODUS [70], following the approach
described in [14]. The $p_{\rm T}$ spectra of light neutral mesons measured at
midrapidity in pp collisions at different center-of-mass energies and in p–Pb
collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV are parametrized and
taken as input to the calculations. Since the measured $p_{\rm T}$
distributions of $\pi^{\pm}$ mesons extend to lower $p_{\rm T}$, they are used
to determine the $\pi^{0}$ input parametrizations. The $p_{\rm T}$ spectra of
$\pi^{\pm}$ mesons measured by ALICE in pp and p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV [71, 72] are first parametrized with a
modified Hagedorn function [73]. A $p_{\rm T}$-dependent scaling factor is
then applied to the $\pi^{\pm}$ parametrization in order to account for the
difference between $\pi^{0}$ and $\pi^{\pm}$ due to isospin-violating decays,
mainly of the $\eta$ mesons. This factor is estimated using an effective model
that describes measured hadron spectra at low $p_{\rm T}$ and includes strong
and electromagnetic decays. The measured $p_{\rm T}$ spectra of $\phi$ mesons
in pp and p–Pb collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV [74, 75]
are fitted to obtain the $\phi$ input parametrizations. The $p_{\rm T}$
spectra of the other light mesons, $\eta$, $\eta^{\prime}$, $\rho$, and
$\omega$ are derived from the $\pi^{\pm}$ spectrum. The $p_{\rm T}$ spectrum
of the $\eta$ meson is estimated from a common fit to the ratios of the $\eta$
to $\pi^{0}$ $p_{\rm T}$ spectra in pp collisions at $\sqrt{s}$ = 7 TeV [76],
8 TeV [77], and in p–Pb collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV
[78] measured by ALICE as well measurements by CERES/TAPS in p–Au and p–Be
collisions at $\sqrt{s_{\rm NN}}$ = 29.1 GeV which extend to lower $p_{\rm T}$
($p_{\rm T}<{2}$ GeV/$c$) [79]. The $p_{\rm T}$ distributions of $\omega$ and
$\rho$ are obtained from the respective ratios to the $\pi^{\pm}$ $p_{\rm T}$
distributions in simulated pp collisions at $\sqrt{s}~{}=~{}5.02$ TeV with
PYTHIA 8. The $\eta$/$\pi^{0}$, $\rho$/$\pi^{\pm}$ and $\omega$/$\pi^{\pm}$
ratios as a function of $p_{\rm T}$ are assumed to be independent of the pA or
pp collision system and of the energy, as suggested by the measurements [76,
77, 78, 79]. Therefore, common parametrizations of these ratios are used for
the pp and p–Pb cocktails. Finally, the $\eta^{\prime}$ meson is generated
assuming $m_{\rm T}$ scaling [80, 81, 82], implying that the spectra of all
light mesons as a function of $m_{\rm T}=\sqrt{m_{\rm 0}^{2}+p_{\rm T}^{2}}$,
where $m_{\rm 0}$ is the pole mass of the considered mesons, follow the same
shape and only differ by a normalization factor. All contributions from the
decays of light-flavor hadrons as a function of $m_{\rm ee}$ are shown in Fig.
2. In order to estimate the ${\rm J}/\psi$ contribution, the measured ${\rm
J}/\psi$ $p_{\rm T}$ spectra in pp and p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV [83, 84] are parametrized and used as
inputs for the simulations. The ${\rm J}/\psi$ mesons are decayed using PHOTOS
[85] via the dielectron channel, which also includes the full QED radiative
channels.

Figure 2: (Color online) Expected cross section for dielectron production from
light-flavor hadron decays in pp collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV as a function of $m_{\rm ee}$. The sum
of the single light-flavor (LF) contributions is shown (solid black line) with
its uncertainties (gray band).

The contributions of correlated semileptonic decays of open charm and beauty
hadrons are calculated with two different MC event generators. They are
identical to the ones used in the dielectron analyses performed by ALICE in pp
collisions at $\sqrt{s}$ = 7 TeV [14] and $\sqrt{s}$ = 13 TeV [15]: PYTHIA 6.4
[66] with the Perugia2011 tune [86] and the next-to-leading order event
generator POWHEG [87, 88, 89, 90] with PYTHIA 6 to evolve the parton shower.
Only the shapes of the expected $m_{\rm ee}$ and $p_{\rm T,ee}$ dielectron
spectra are estimated with the MC event generators. The absolute normalization
is obtained from a fit of the measured dielectron cross sections in pp
collisions at $\sqrt{s}~{}=~{}5.02$ TeV, as shown in Sec. 5.1. For p–Pb
collisions, the $\mathrm{c\overline{c}}$ and $\mathrm{b\overline{b}}$ cross
sections, extracted in pp collisions, are scaled with the atomic mass number
$A$ of the Pb nucleus (208). This approach neglects any cold nuclear matter
effects, which will be discussed in Sec. 5.2.

The following sources of systematic uncertainties are taken into account: the
input parametrizations of the measured $\pi^{\pm}$, $\phi$ and ${\rm J}/\psi$
$p_{\rm T}$ spectra and $\eta/\pi^{0}$ ratios, the scaling factor applied to
the $\pi^{\pm}$ parametrizations, the $m_{\rm T}$ scaling parameters, and the
different decay branching ratios. The uncertainty of the $\pi^{\pm}$ scaling
factor is estimated from variations of the model parameters. For the $\rho$
and $\omega$ mesons, the uncertainty of the $\omega/\pi^{0}$ and
$\rho/\pi^{0}$ ratios are estimated by comparing the measured and simulated
ratios in pp collisions at $\sqrt{s}$ = 7 TeV [91] and $\sqrt{s}$ = 2.76 TeV
[92], respectively. The total uncertainties of the pp and p–Pb cocktails vary
from 5% to 20% depending on the $m_{\rm ee}$ and $p_{\rm T,ee}$ interval.

##  5 Results

The dielectron cross sections in pp and p–Pb collisions as well as the nuclear
modification factor are presented differentially as a function of $m_{\rm ee}$
for $p_{\rm T,ee}<{8}$ GeV/$c$ and as a function of $p_{\rm T,ee}$ in two
different mass regions, the low-mass region (LMR), ${0.5}<m_{\rm ee}<{1.1}$
GeV/$c^{2}$, and the intermediate-mass region (IMR), ${1.1}<m_{\rm ee}<{2.7}$
GeV/$c^{2}$.

###  5.1 Heavy-flavor cross sections in pp collisions

The differential ${\rm e}^{+}{\rm e}^{-}$ production cross sections
d$\sigma_{\rm ee}$/d$m_{\rm ee}$ and d$\sigma_{\rm ee}$/d$p_{\rm T,ee}$ in pp
collisions, measured in the IMR and in the range $p_{\rm T,ee}<{8}$ GeV/$c$ at
$\sqrt{s}~{}=~{}5.02$ TeV are presented in Fig. 3.

Figure 3: (Color online) Projections of the heavy-flavor dielectron fits as a
function of $m_{\rm ee}$ (left) and $p_{\rm T,ee}$ (right) using POWHEG (solid
black line) and PYTHIA 6 (dashed gray line) as event generators. The colored
lines show the charm (red) and beauty (magenta) contributions for both event
generators after the fit.

The data in the IMR are fitted in $m_{\rm ee}$ and $p_{\rm T,ee}$ with PYTHIA
6 and POWHEG templates of open-charm (red) and open-beauty (magenta)
production, keeping the light-flavor and ${\rm J}/\psi$ contributions fixed.
In this mass range, most of the ${\rm e}^{+}{\rm e}^{-}$ pairs originate from
open heavy-flavor hadron decays. The $\chi^{2}$/ndf between the data and the
cocktail sum is 110.9/123 for the POWHEG cocktail and 113.4/123 for the PYTHIA
6 cocktail. Both calculations are able to reproduce the measured spectra well
over the full kinematic range probed, however the full cocktail obtained with
POWHEG leads to a slightly better description of the data at low $m_{\rm ee}$
around $m_{\rm ee}$ = 0.5 GeV/$c^{2}$. The resulting cross sections are listed
in Table 3. The systematic uncertainties originating from the data were
determined by repeating the fit after moving the data points coherently up-
and downward by their systematic uncertainties. Additional uncertainties on
the effective beauty- and charm-to-electron branching ratios, arising from the
semi-leptonic decay branching ratios of open heavy-flavor hadrons and the
fragmentation functions of charm (beauty) quarks, amounting to 22% and 6% for
the charm and beauty cross sections, respectively, are also listed in the
table. All uncertainties are fully correlated between the two generators,
which differ only in the implementation of the heavy-quark production
mechanisms. In both calculations, the hadronization of the c- and b-quarks,
and the decays of the open heavy-flavor hadrons, are performed using PYTHIA 6.
For the following results, only calculations where the heavy-flavor
contribution is evaluated with POWHEG are presented, since the cocktail using
POWHEG and fitted to the data in the IMR can slightly better describe the
measured dielectron cross sections over the full $m_{\rm ee}$ and $p_{\rm
T,ee}$ range in pp collisions at $\sqrt{s}~{}=~{}5.02$ TeV.

Table 3: Heavy-flavor cross sections extracted via double differential fits in $m_{\rm ee}$ and $p_{\rm T,ee}$ to the measured dielectron spectra in pp collisions at $\sqrt{s}~{}=~{}5.02$ TeV using PYTHIA 6 and POWHEG. The statistical (stat.) and systematic (syst.) uncertainties on the data are quoted together with the 22% (6%) uncertainty on the branching ratio (BR) of the semi-leptonic decays of the open heavy-flavor hadrons and the fragmentation functions of charm (beauty) quarks is not listed. | PYTHIA | POWHEG
---|---|---
d$\sigma_{\rm c\overline{c}}$/d$y|_{y=0}$ | 524 $\pm$ 61 (stat.) $\pm$ 26 (syst.) $\pm$ 115 (BR) $\mu$b | 756 $\pm$ 80 (stat.) $\pm$ 38 (syst.) $\pm$ 166 (BR) $\mu$b
d$\sigma_{\rm b\overline{b}}$/d$y|_{y=0}$ | 34 $\pm$ 4 (stat.) $\pm$ 2 (syst.) $\pm$ 2 (BR) $\mu$b | 28 $\pm$ 5 (stat.) $\pm$ 1 (syst.) $\pm$ 2 (BR) $\mu$b

Figure 4: (Color online) Cross sections at midrapidity for
c$\overline{\textrm{c}}$ (left) and b$\overline{\textrm{b}}$ (right) as a
function of $\sqrt{s}$ in pp collisions. The colored markers represent the
measured midrapidity cross sections at $\sqrt{s}$ = 5.02, 7, and 13 TeV which
are derived using either PYTHIA 6 (blue circles) or POWHEG (red squares)
simulations. The systematic and statistical uncertainty of the data points are
summed in quadrature and represented by vertical bars. The measurements are
compared with FONLL calculations (black solid line), with model uncertainties
(dashed lines), and to single heavy-flavor hadron measurements (open markers).
The referenced $\rm c\overline{c}$ cross section at $\sqrt{s}$ = 7 TeV was
obtained from a measurement of prompt $\rm D^{0}$ meson production with
$p_{\rm T}>{0}$ GeV/$c$ and $|y|<0.5$ using the fragmentation fraction $\rm
f(c\rightarrow D^{0})=0.542\pm 0.024$ from ${\rm e}^{+}{\rm e}^{-}$ LEP data
[93].

A compilation of the measured d$\sigma_{\rm c\overline{c}}$/d$y|_{y=0}$ (left)
and d$\sigma_{\rm b\overline{b}}$/d$y|_{y=0}$ (right) in pp collisions at LHC
energies is shown in Fig. 4 as a function of $\sqrt{s}$. The difference in the
cross sections obtained with the two MC event generators in the present
analysis at $\sqrt{s}$ = 5.02 TeV is comparable with the results of previous
observations at $\sqrt{s}$ = 7 [14] and 13 TeV [15] performed with the same
models. This reflects the sensitivity of the dielectron measurement to the
implementation of the heavy-quark production mechanisms, in particular to the
initial correlation of charm quarks, which is not accessible with conventional
measurements of single open-charm hadrons and their decay products.
Nevertheless, the cross sections measured using POWHEG or PYTHIA are all in
agreement, within the current precision, with results from single heavy-flavor
hadron measurements [94, 95]. The measured total $\rm c\overline{c}$
production cross section in pp collisions at $\sqrt{s}~{}=~{}7$ TeV was
obtained from a measurement of prompt $\rm D^{0}$ meson production with
$p_{\rm T}>{0}$ GeV/$c$ and $|y|<0.5$ using the fragmentation fraction $\rm
f(c\rightarrow D^{0})=0.542\pm 0.024$ from ${\rm e}^{+}{\rm e}^{-}$ LEP data
[93]. Recent measurements of $\rm f(c\rightarrow D^{0})$ suggest that this
value is smaller in pp collisions at the LHC [96], which would result in a
larger cross section of charm production than assumed in [95]. FONLL
calculations [16] are able to reproduce the measurements within the model
uncertainties that are dominated by scale uncertainties, but also include PDF
and mass uncertainties. The slope of the center-of-mass energy dependence of
the cross sections is described by the calculations. The measured charm
production cross sections are however on the upper edge of the large
systematic uncertainties of the theory calculations for all three
measurements.

###  5.2 Dielectron production in pp and p–Pb collisions

The $m_{\rm ee}$-differential production cross sections of ${\rm e}^{+}{\rm
e}^{-}$ pairs measured in pp and p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV are compared to the expected
dielectrons from known hadron decays in Fig. 5. The light-flavor
contributions, summarized as "Light flavor" for readability, are based on
measurements in pp and p–Pb collisions as explained in detail in Sec. 4.

Figure 5: (Color online) Differential ${\rm e}^{+}{\rm e}^{-}$ cross section
as a function of $m_{\rm ee}$ measured in pp (left) and p–Pb (right)
collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV. The data are compared
to the hadronic cocktail, where the heavy-flavor contributions are fitted to
the pp spectrum in the intermediate-mass region, and for p–Pb collisions
scaled with the atomic mass number of the Pb nucleus $A$ = 208. The gray band
represents the total uncertainty on the hadronic cocktail.

The correlated pairs from heavy-flavor hadron decays are calculated with
POWHEG. Their contributions are normalized to the d$\sigma_{\rm
c\overline{c}}$/d$y|_{y=0}$ and the d$\sigma_{\rm b\overline{b}}$/d$y|_{y=0}$
in pp collisions obtained from the fit to the pp data, as discussed in the
previous section. For p–Pb collisions, the heavy-flavor contributions are
further scaled with the atomic mass number of the Pb nucleus. This assumes
that the production of heavy-flavor quarks in p–Pb collisions scales with the
number of binary nucleon–nucleon collisions. The total systematic uncertainty
of the cocktails is indicated by the gray band. The pp cocktail uncertainty in
the IMR is zero by construction since the heavy-flavor contribution is
directly fitted to the measured spectrum in pp collisions. The systematic
uncertainties of the heavy-flavor contribution in the p–Pb cocktail originate
from the statistical and systematic uncertainties of the extracted production
cross sections in the pp analysis listed in Table 3. Since the cross section
is based on the measurement of final state ${\rm e}^{+}{\rm e}^{-}$ pairs, the
uncertainties related to branching ratios of the semi-leptonic decays of open
heavy-flavor hadrons and the fragmentation functions of charm and beauty
quarks can be omitted, under the assumption that these do not change from pp
to p–Pb collisions. This is confirmed by the latest measurements of open
heavy-flavor hadrons in pp and p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV by ALICE [96]. The bottom panels in
Fig. 5 show the ratios of the data to the cocktail. The data are described by
the hadronic cocktails over the whole mass range ($m_{\rm ee}<{3.5}$
GeV/$c^{2}$) in both pp and p–Pb collisions, within the systematic and
statistical uncertainties. As seen in previous measurements in pp collisions
[14, 15], the heavy-flavor contribution dominates the spectrum for $m_{\rm
ee}>{0.8}$ GeV/$c^{2}$. In p–Pb collisions, the heavy-flavor contribution to
the hadronic cocktail does not include any modification beyond scaling with
binary nucleon–nucleon collisions with respect to the pp cocktail. No
significant deviation of the data from the vacuum expectation of the heavy-
flavor contributions can be observed in the mass spectrum. This suggests that
the CNM effects are small compared to the current uncertainties of the
measurements, as observed by other open heavy-flavor measurements at the LHC
at midrapidity [28], or compensated by an additional source of dielectrons in
p–Pb collisions compared with pp collisions, possibly related to the formation
of a hot medium in such collisions.

The $p_{\rm T,ee}$ spectra for pp and p–Pb collisions in the LMR and IMR are
compared to the hadronic cocktail in Figs. 6 and 7, respectively.

Figure 6: (Color online) Differential ${\rm e}^{+}{\rm e}^{-}$ cross section
as a function of $p_{\rm T,ee}$ in the low-mass region measured in pp (left)
and p–Pb (right) collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV. The
data are compared to the hadronic cocktail, where the heavy-flavor
contributions are fitted to the pp spectrum in the intermediate-mass region,
and for p–Pb collisions scaled with the atomic mass number of the Pb nucleus
$A$ = 208. The gray band represents the total uncertainty on the hadronic
cocktail.

Figure 7: (Color online) Differential ${\rm e}^{+}{\rm e}^{-}$ cross section
as a function of $p_{\rm T,ee}$ in the intermediate-mass region measured in pp
(left) and p–Pb (right) collisions at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV.
The data are compared to the hadronic cocktail, where the heavy-flavor
contributions are fitted to the pp spectrum in the intermediate-mass region,
and for p–Pb collisions scaled with the atomic mass number of the Pb nucleus
$A$ = 208. The gray band represents the total uncertainty on the hadronic
cocktail.

In the LMR, the hadronic cocktails in pp and p–Pb collisions are both composed
of ${\rm e}^{+}{\rm e}^{-}$ pairs from light-flavor, open-charm, and open-
beauty hadron decays. Most of the pairs in this mass interval are produced
from the decays of light-flavor hadrons, whose production at low $p_{\rm T}$
does not scale with $A$ in p–Pb collisions. Therefore, the relative expected
contribution of dielectrons from light-flavor hadron decays is smaller in p–Pb
collisions compared with pp collisions at the same $\sqrt{s_{\rm NN}}$. In
p–Pb collisions, the open-charm hadron decays are expected to contribute
significantly to the ${\rm e}^{+}{\rm e}^{-}$ cross section for $p_{\rm
T,ee}<{1}$ GeV/$c$. The open-beauty contribution only plays a significant role
for $p_{\rm T,ee}>{4}$ GeV/$c$ in both collision systems. In the IMR,
correlated ${\rm e}^{+}{\rm e}^{-}$ pairs from open-charm hadron decays are
the dominant dielectron source for $p_{\rm T,ee}<{2.5}$ GeV/$c$ in pp as well
as in p–Pb collisions, whereas most of the ${\rm e}^{+}{\rm e}^{-}$ pairs
originate from open-beauty hadron decays for $p_{\rm T,ee}>{3.5}$ GeV/$c$. The
contribution from ${\rm J}/\psi$ decays is small over the whole $p_{\rm T,ee}$
range. The dielectron production in pp and p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV is well described by the hadronic
cocktail, utilizing heavy-flavor cross sections fitted to the pp data and
assuming a scaling of the heavy-flavor cross sections with the $A$ of the Pb
nucleus. In particular, no significant modification of the heavy-flavor
production in the measured kinematic regions is justified by the analysis of
the p–Pb collisions data.

###  5.3 Nuclear modification factor

The nuclear modification factor, $R_{\rm pPb}$, is calculated as

$R_{\rm pPb}(\textit{m}_{\rm{ee}})=\frac{1}{A}\frac{\rm{d}\sigma^{\kern
0.70004ptpPb}_{ee}/\rm{d}\textit{m}_{\rm{ee}}}{\rm{d}\sigma^{\kern
0.70004ptpp}_{\rm ee}/\rm{d}\textit{m}_{\rm{ee}}},$ (3)

with $\sigma^{\kern 0.70004pt\rm{pPb}}_{\rm ee}$ and $\sigma^{\kern
0.70004pt\rm{pp}}_{\rm ee}$ representing the cross sections of dielectron
production in p–Pb and pp collisions, respectively, and $A$ denoting the mass
number of the Pb nucleus (208). The $R_{\rm pPb}$ allows for a direct
comparison of the measurements in the pp and p–Pb collision systems. The
systematic uncertainties of the p–Pb and pp measurements are treated as
independent and, thus, added in quadrature. The dielectron $R_{\rm pPb}$ as a
function of $m_{\rm ee}$ for $p_{\rm T,ee}<{8}$ GeV/$c$ is shown in Fig. 8.

Figure 8: (Color online) Measured dielectron nuclear modification factor as a
function of $m_{\rm ee}$ at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV. The data
are shown in blue, with their statistical and systematic uncertainties
depicted as vertical bars and boxes. The baseline expectation, calculated from
the pp and p–Pb cocktails outlined in Sec. 4, is shown as a black line with a
gray band indicating its uncertainties. Two additional cocktails, one
incorporating a modified charm production due to CNM effects and another one
including thermal radiation from the hadronic and partonic phases, are shown
as red and orange dashed lines, respectively.

The data are compared to the $R_{\rm pPb}$ of the hadronic cocktails as
described in Sec. 4 (solid black line). In the cocktail $R_{\rm pPb}$, the
uncertainties from the open heavy-flavor contributions as well as those from
the scaling factor applied to the $\pi^{\pm}$ parametrizations, the
$\rho/\pi^{\pm}$, $\omega/\pi^{\pm}$, and $\eta/\pi^{0}$ $p_{\rm T}$ ratios
are fully correlated, and therefore cancel out. The uncertainties on the
parametrized $\pi$, $\phi$, and ${\rm J}/\psi$ spectra are propagated to the
$R_{\rm pPb}$. Since they are based on independent measurements they are added
quadratically. The measured $R_{\rm pPb}$ is below the expectation of binary
collision scaling for $m_{\rm ee}<{1.1}$ GeV/$c^{2}$, where the fraction of
dielectrons from light-flavor hadron decays to the total expected ${\rm
e}^{+}{\rm e}^{-}$ cross section in p–Pb collisions, denoted by the green
area, is not negligible. The $R_{\rm pPb}$ is consistent with unity in the IMR
within uncertainties, displaying a step between the two mass regions. The
behavior is reproduced, within uncertainties, by the hadronic cocktail
assuming no further modification of the open heavy-flavor cross sections
beyond binary collision scaling. This suggests a different scaling behavior of
the light-flavor production from binary collision scaling, as already
indicated in previous measurements [97].

An additional cocktail calculation incorporating a modification of the open-
charm contribution via CNM effects is shown by a dashed red line in Fig. 8.
The CNM effects on the production of dielectrons from open-charm hadron decays
are incorporated by using the EPS09 nPDF [18] in the POWHEG calculations. In
the mass region below 1 GeV/$c^{2}$, where the admixture of charm is
significant, the modification of the charm contribution improves the
description of the measured $R_{\rm pPb}$. In the IMR, the data are just
beyond the upper edge of the systematic uncertainties of the calculations
including CNM for the charm production. On one hand, it suggests negligible
CNM effects compared to the current precision of the measurement in this mass
range, where the $p_{\rm T}$ of D mesons, from which the dielectrons
originate, is larger than 2 GeV/$c$ according to calculations performed with
PYTHIA 6. This is in agreement with previous results on the D meson $R_{\rm
pPb}$ at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV by ALICE, which show no
significant modification of the $p_{\rm T}$ spectra above 2 GeV/$c$ [27, 28]
compared to pp collisions. The dielectron cross section from charm at lower
$m_{\rm ee}$ however is sensitive to the production of low $p_{\rm T}$ D
mesons ($p_{\rm T}$ $<$ 2 GeV/$c$). On the other hand, a possible additional
source of electron pairs in p–Pb collisions compared to pp collisions could
compensate CNM effects on the heavy-flavor production.

The measured $R_{\rm pPb}$ is further compared to calculations including
thermal radiation from the hadronic and partonic phases, based on a model
which describes the dilepton enhancement measured in heavy-ion collisions at
the SPS and RHIC [51, 98, 7, 52]. The contribution of thermal dielectrons is
obtained from an expanding thermal fireball model for p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV, corresponding to a mean charged-
particle multiplicity at midrapidity of $\langle{\rm d}N_{\rm ch}/{\rm
d}y\rangle$ = 20, corrected for weak decay feeddown. The equation of state was
extracted from lattice QCD computations with a crossover transition around the
critical temperature $T_{\rm c}$ = 170 MeV. A broadening of the $\rho$
electromagnetic spectral function is expected as an effect of interactions in
the hot hadronic phase. The thermal emission rate of dielectrons from the
hadronic phase is calculated based on the hadronic many-body theory. The
effects of the detector resolution are not included in the calculations and no
modification of the heavy-flavor contribution is considered. A hadronic
cocktail including these calculations is shown as the orange dotted line
(HG+QGP). In the range ${0.2}<m_{\rm ee}<{0.6}$ GeV/$c^{2}$, the model tends
to slightly overestimate the measured $R_{\rm pPb}$, whereas in the IMR it
agrees with the data within their uncertainties. An additional thermal source
of dielectrons in p–Pb collisions compared to pp collisions can not be
excluded by the data.

To further investigate the modifications of the open-charm contribution to the
${\rm e}^{+}{\rm e}^{-}$ spectrum, the dielectron $R_{\rm pPb}$ as a function
of $p_{\rm T,ee}$ is shown in the LMR and IMR in Fig. 9.

Figure 9: (Color online) Measured dielectron nuclear modification factor as a
function of $p_{\rm T,ee}$ in the low-mass region (left) and intermediate-mass
region (right) at $\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV. The data are shown
in blue, with their statistical and systematic uncertainties depicted as
vertical bars and boxes. The baseline expectation, calculated from the pp and
p–Pb cocktails outlined in Sec. 4, is shown as a black line with a gray band
indicating its uncertainties. Two additional cocktails, one incorporating a
modified charm production due to CNM effects and another one including thermal
radiation from the hadronic and partonic phases, are shown as red and orange
dashed lines, respectively.

In the LMR, the fraction of ${\rm e}^{+}{\rm e}^{-}$ pairs from light-flavor
hadron decays ranges from about 40% to 60% depending on $p_{\rm T,ee}$. For
$p_{\rm T,ee}$ larger than about 1 GeV/$c$ the data are compatible with binary
collision scaling, indicating that the production of light-flavor hadrons is
driven by the initial hard scatterings of the incoming partons and is not
affected by CNM effects. This no longer holds true for $p_{\rm T,ee}<{1}$
GeV/$c$, pointing to a change in the production mechanism of the light-flavor
hadrons. These features can be reproduced by the hadronic cocktail. Inclusion
of CNM effects for the charm contribution in the hadronic cocktail only have a
small effect. The uncertainties on the data as well as the CNM calculations
themselves are too large to draw any conclusion. The addition of the thermal
contributions in the LMR is disfavored by the data at low-$p_{\rm T,ee}$
($p_{\rm T,ee}<{1}$ GeV/$c$), whereas at higher $p_{\rm T,ee}$ the
uncertainties on the data do not allow for any discrimination between the
three models.

In the IMR, the contribution from light-flavor hadron decays is negligible.
The $R_{\rm pPb}$ is consistent with unity, indicating that the heavy-flavor
cross sections approximately scale with the number of binary collisions in
this range. According to the calculations using EPS09 nPDFs, a suppression of
the total ${\rm e}^{+}{\rm e}^{-}$ cross section is expected for $p_{\rm
T,ee}<{3.5}$ GeV/$c$ due to CNM effects on dielectrons from open-charm hadron
decays. Nevertheless, it is disfavored by these data. On the contrary, the
cocktail calculation including thermal contributions would be preferred by the
data. In particular, for $p_{\rm T,ee}<{1}$ GeV/$c$ a thermal contribution
significantly helps to improve the description of the data.

Finally, a potential interplay between CNM effects and the thermal
contribution cannot be ruled out. Therefore, it is mandatory to separate the
dielectrons from heavy-flavor hadron decays and those from thermal radiation.
This could be achieved by an analysis as a function of the distance-of-closest
approach of the ${\rm e}^{+}{\rm e}^{-}$ pairs to the collision vertex [14].

##  6 Conclusions

The dielectron production at midrapidity ($\left|\eta_{\rm
e}\right|~{}<~{}0.8$) was measured with the ALICE detector as a function of
invariant mass and pair transverse momentum in pp and p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV.

In pp collisions, the dielectron continuum can be well described by the
expected contributions from light-flavor hadron decays and calculations of
${\rm e}^{+}{\rm e}^{-}$ pairs from heavy-flavor hadron decays fitted to the
data. The cross sections of $\mathrm{c\overline{c}}$ and
$\mathrm{b\overline{b}}$ production at midrapidity are extracted from the
measurement by a double-differential fit to the $m_{\rm ee}$ and $p_{\rm
T,ee}$ spectrum in the intermediate-mass region. Templates from two different
event generators, PYTHIA 6 [66] and POWHEG [87, 88, 89, 90], are used. Both
calculations can describe the data well, yet they yield significantly
different results for the cross sections of the single
$\mathrm{c\overline{c}}$ and $\mathrm{b\overline{b}}$ contributions. The
hadronization of c- and b-quarks as well as the decay of the heavy-flavor
hadrons is done in both PYTHIA 6 and POWHEG simulations with the Perugia 2011
tune of PYTHIA 6.4 [66, 86]. Therefore, the model dependence of the extracted
cross sections directly reflects the sensitivity of the dielectron measurement
to the different implementation of the heavy-quark production mechanisms in
the Monte Carlo event generators. The measured ${\rm d}\sigma_{\rm
c\overline{c}}/{\rm d}y|_{y=0}$ and ${\rm d}\sigma_{\rm b\overline{b}}/{\rm
d}y|_{y=0}$ are compared to existing results from dielectron measurements, as
well as measurements of identified charm hadrons and semi-leptonic decays of
beauty hadrons, in pp collisions at different $\sqrt{s}$. The difference
between the cross sections extracted in this analysis with the two event
generators is comparable to those reported in previous observations at
$\sqrt{s}$ = 7 and 13 TeV. The slope of the center-of-mass energy dependence
of the cross sections can be described by FONLL calculations.

The dielectron $m_{\rm ee}$ and $p_{\rm T,ee}$ spectra in p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}~{}=~{}5.02$ TeV, reported here for the first time, are
compared to a hadronic cocktail composed of the expected dielectron cross
sections from the known hadron decays. Whereas ${\rm e}^{+}{\rm e}^{-}$ pairs
from light-flavor and ${\rm J}/\psi$ hadron decays are estimated using
independent measurements of hadrons, the contributions of dielectrons from
open heavy-flavor hadron decays are determined from the dielectron measurement
in pp collisions at the same center-of-mass energy using POWHEG as the event
generator. The heavy-flavor cross sections are assumed to scale with the
atomic mass number of the Pb nucleus in p–Pb collisions, with respect to the
measured pp reference. Good agreement is observed between the measured and
expected total ${\rm e}^{+}{\rm e}^{-}$ cross section.

The dielectron $R_{\rm pPb}$ as a function of $m_{\rm ee}$ highlights the
different scaling behavior of the light- and heavy-flavor dielectron sources.
While the measured $R_{\rm pPb}$ is below one for $m_{\rm ee}<{1}$
GeV/$c^{2}$, it is consistent with unity within uncertainties in the IMR where
most of the ${\rm e}^{+}{\rm e}^{-}$ pairs originate from correlated open
heavy-flavor hadron decays. On the one hand, calculations including a
suppression of the charm production using the nPDF EPS09 do not describe the
data as well as the hadronic cocktail using the atomic mass number scaling
hypothesis in the intermediate-mass region. The central value of the
computations including CNM effects is nevertheless closer to the measured
$R_{\rm pPb}$ at masses around 0.5 GeV/$c^{2}$. On the other hand, including a
thermal contribution from a hot hadronic and partonic phase to the dielectron
cocktail helps in the description of the data in the IMR. The thermal
radiation calculations seem however to overestimate the production of
dielectrons in the LMR. The hadronic cocktail calculations including CNM
effects and thermal radiation show that both play a role at low $p_{\rm T,ee}$
with opposite trends, although the current uncertainties on the measured
$p_{\rm T,ee}$ dependence of $R_{\rm pPb}$ are still too large to reject any
of the calculations presented. Moreover, CNM effects on the charm production
and thermal radiation from a hot medium possibly formed in p–Pb collisions
could cancel each other, if both are present, which makes it necessary to
disentangle them in a more sophisticated approach.

A more detailed study of the dielectron production in p–Pb collisions requires
the separation of ${\rm e}^{+}{\rm e}^{-}$ pairs from prompt sources and those
from the displaced open heavy-flavor hadron decays. The distance-of-closest
approach of the ${\rm e}^{+}{\rm e}^{-}$ pair to the collision vertex,
pioneered at the LHC by ALICE in the dielectron analysis of the pp data at
$\sqrt{s}=7$ TeV [14], could enable the search for the presence of a possible
additional contribution from thermal radiation in p–Pb collisions, in
particular in high-multiplicity events. In the near future, the dielectron
analysis will greatly benefit from the upgrades of the ALICE TPC [99, 100],
the ITS [101] and a completely new readout system [102] and computing
framework [103]. The data acquisition rate will increase by a factor of 100,
while the pointing resolution of primary tracks will improve by a factor of 3
to 6, depending on their orientation with respect to the magnetic field. This
will open up the possibility to study the dielectron production with
unprecedented precision and detail.

## Acknowledgements

The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467-N36] and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC) , Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation, Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research | Natural Sciences, the VILLUM FONDEN and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Energie Atomique (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; General Secretariat for Research and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE), Department of Science and Technology, Government of India (DST), University Grants Commission, Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology , Nagasaki Institute of Applied Science (IIST), Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Science and Higher Education, National Science Centre and WUT ID-UB, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Ministry of Research and Innovation and Institute of Atomic Physics, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation, National Research Centre Kurchatov Institute, Russian Science Foundation and Russian Foundation for Basic Research, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSDTA) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America.

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##  Appendix A The ALICE Collaboration

S. Acharyaorg141&D. Adamováorg95&A. Adlerorg74&J. Adolfssonorg81&M.M.
Aggarwalorg100&G. Aglieri Rinellaorg34&M. Agnelloorg30&N.
Agrawalorg10,org54&Z. Ahammedorg141&S. Ahmadorg16&S.U. Ahnorg76&Z.
Akbarorg51&A. Akindinovorg92&M. Al-Turanyorg107&S.N. Alamorg40,org141&D.S.D.
Albuquerqueorg122&D. Aleksandrovorg88&B. Alessandroorg59&H.M. Alfandaorg6&R.
Alfaro Molinaorg71&B. Aliorg16&Y. Aliorg14&A. Aliciorg10,org26,org54&N.
Alizadehvandchaliorg125&A. Alkinorg2,org34&J. Almeorg21&T. Altorg68&L.
Altenkamperorg21&I. Altsybeevorg113&M.N. Anaamorg6&C. Andreiorg48&D.
Andreouorg34&A. Andronicorg144&M. Angelettiorg34&V. Anguelovorg104&C.
Ansonorg15&T. Antičićorg108&F. Antinoriorg57&P. Antonioliorg54&N.
Apadulaorg80&L. Aphecetcheorg115&H. Appelshäuserorg68&S. Arcelliorg26&R.
Arnaldiorg59&M. Arratiaorg80&I.C. Arseneorg20&M. Arslandokorg104&A.
Augustinusorg34&R. Averbeckorg107&S. Azizorg78&M.D. Azmiorg16&A.
Badalàorg56&Y.W. Baekorg41&S. Bagnascoorg59&X. Baiorg107&R. Bailhacheorg68&R.
Balaorg101&A. Balbinoorg30&A. Baldisseriorg137&M. Ballorg43&S.
Balouzaorg105&D. Banerjeeorg3&R. Barberaorg27&L. Barioglioorg25&G.G.
Barnaföldiorg145&L.S. Barnbyorg94&V. Barretorg134&P. Bartaliniorg6&C.
Bartelsorg127&K. Barthorg34&E. Bartschorg68&F. Baruffaldiorg28&N.
Bastidorg134&S. Basuorg143&G. Batigneorg115&B. Batyunyaorg75&D.
Bauriorg49&J.L. Bazo Albaorg112&I.G. Beardenorg89&C. Beattieorg146&C.
Beddaorg63&N.K. Beheraorg61&I. Belikovorg136&A.D.C. Bell Hechavarriaorg144&F.
Belliniorg34&R. Bellwiedorg125&V. Belyaevorg93&G. Bencediorg145&S.
Beoleorg25&A. Bercuciorg48&Y. Berdnikovorg98&D. Berenyiorg145&R.A.
Bertensorg130&D. Berzanoorg59&M.G. Besoiuorg67&L. Betevorg34&A.
Bhasinorg101&I.R. Bhatorg101&M.A. Bhatorg3&H. Bhattorg49&B.
Bhattacharjeeorg42&A. Bianchiorg25&L. Bianchiorg25&N. Bianchiorg52&J.
Bielčíkorg37&J. Bielčíkováorg95&A. Bilandzicorg105&G. Biroorg145&R.
Biswasorg3&S. Biswasorg3&J.T. Blairorg119&D. Blauorg88&C. Blumeorg68&G.
Bocaorg139&F. Bockorg96&A. Bogdanovorg93&S. Boiorg23&J. Bokorg61&L.
Boldizsárorg145&A. Bolozdynyaorg93&M. Bombaraorg38&G. Bonomiorg140&H.
Borelorg137&A. Borissovorg93&H. Bossiorg146&E. Bottaorg25&L. Bratrudorg68&P.
Braun-Munzingerorg107&M. Bregantorg121&M. Brozorg37&E. Brunaorg59&G.E.
Brunoorg33,org106&M.D. Bucklandorg127&D. Budnikovorg109&H. Bueschingorg68&S.
Bufalinoorg30&O. Bugnonorg115&P. Buhlerorg114&P. Buncicorg34&Z.
Butheleziorg72,org131&J.B. Buttorg14&S.A. Bysiakorg118&D. Caffarriorg90&A.
Calivaorg107&E. Calvo Villarorg112&J.M.M. Camachoorg120&R.S. Camachoorg45&P.
Cameriniorg24&F.D.M. Canedoorg121&A.A. Caponorg114&F. Carnesecchiorg26&R.
Caronorg137&J. Castillo Castellanosorg137&A.J. Castroorg130&E.A.R.
Casulaorg55&F. Catalanoorg30&C. Ceballos Sanchezorg75&P. Chakrabortyorg49&S.
Chandraorg141&W. Changorg6&S. Chapelandorg34&M. Chartierorg127&S.
Chattopadhyayorg141&S. Chattopadhyayorg110&A. Chauvinorg23&C.
Cheshkovorg135&B. Cheynisorg135&V. Chibante Barrosoorg34&D.D.
Chinellatoorg122&S. Choorg61&P. Chochulaorg34&T. Chowdhuryorg134&P.
Christakoglouorg90&C.H. Christensenorg89&P. Christiansenorg81&T.
Chujoorg133&C. Cicaloorg55&L. Cifarelliorg10,org26&L.D. Cilladiorg25&F.
Cindoloorg54&M.R. Ciupekorg107&G. Claiorg54orgI&J. Cleymansorg124&F.
Colamariaorg53&D. Colellaorg53&A. Colluorg80&M. Colocciorg26&M.
Concasorg59orgII&G. Conesa Balbastreorg79&Z. Conesa del Valleorg78&G.
Continorg24,org60&J.G. Contrerasorg37&T.M. Cormierorg96&Y. Corrales
Moralesorg25&P. Corteseorg31&M.R. Cosentinoorg123&F. Costaorg34&S.
Costanzaorg139&P. Crochetorg134&E. Cuautleorg69&P. Cuiorg6&L.
Cunqueiroorg96&D. Dabrowskiorg142&T. Dahmsorg105&A. Daineseorg57&F.P.A.
Damasorg115,org137&M.C. Danischorg104&A. Danuorg67&D. Dasorg110&I.
Dasorg110&P. Dasorg86&P. Dasorg3&S. Dasorg3&A. Dashorg86&S. Dashorg49&S.
Deorg86&A. De Caroorg29&G. de Cataldoorg53&J. de Cuvelandorg39&A. De
Falcoorg23&D. De Gruttolaorg10&N. De Marcoorg59&S. De Pasqualeorg29&S.
Deborg50&H.F. Degenhardtorg121&K.R. Dejaorg142&A. Delofforg85&S.
Delsantoorg25,org131&W. Dengorg6&P. Dhankherorg49&D. Di Bariorg33&A. Di
Mauroorg34&R.A. Diazorg8&T. Dietelorg124&P. Dillensegerorg68&Y. Dingorg6&R.
Diviàorg34&D.U. Dixitorg19&Ø. Djuvslandorg21&U. Dmitrievaorg62&A.
Dobrinorg67&B. Dönigusorg68&O. Dordicorg20&A.K. Dubeyorg141&A.
Dublaorg90,org107&S. Dudiorg100&M. Dukhishyamorg86&P. Dupieuxorg134&R.J.
Ehlersorg96&V.N. Eikelandorg21&D. Eliaorg53&B. Erazmusorg115&F.
Erhardtorg99&A. Erokhinorg113&M.R. Ersdalorg21&B. Espagnonorg78&G.
Eulisseorg34&D. Evansorg111&S. Evdokimovorg91&L. Fabbiettiorg105&M.
Fagginorg28&J. Faivreorg79&F. Fanorg6&A. Fantoniorg52&M. Faselorg96&P.
Fecchioorg30&A. Felicielloorg59&G. Feofilovorg113&A. Fernández Téllezorg45&A.
Ferreroorg137&A. Ferrettiorg25&A. Festantiorg34&V.J.G. Feuillardorg104&J.
Figielorg118&S. Filchaginorg109&D. Finogeevorg62&F.M. Fiondaorg21&G.
Fiorenzaorg53&F. Flororg125&A.N. Floresorg119&S. Foertschorg72&P.
Fokaorg107&S. Fokinorg88&E. Fragiacomoorg60&U. Frankenfeldorg107&U.
Fuchsorg34&C. Furgetorg79&A. Fursorg62&M. Fusco Girardorg29&J.J.
Gaardhøjeorg89&M. Gagliardiorg25&A.M. Gagoorg112&A. Galorg136&C.D.
Galvanorg120&P. Ganotiorg84&C. Garabatosorg107&J.R.A. Garciaorg45&E. Garcia-
Solisorg11&K. Gargorg115&C. Gargiuloorg34&A. Garibliorg87&K. Garnerorg144&P.
Gasikorg105,org107&E.F. Gaugerorg119&M.B. Gay Ducatiorg70&M. Germainorg115&J.
Ghoshorg110&P. Ghoshorg141&S.K. Ghoshorg3&M. Giacaloneorg26&P.
Gianottiorg52&P. Giubellinoorg59,org107&P. Giubilatoorg28&A.M.C.
Glaenzerorg137&P. Glässelorg104&A. Gomez Ramirezorg74&V.
Gonzalezorg107,org143&L.H. González-Truebaorg71&S. Gorbunovorg39&L.
Görlichorg118&A. Goswamiorg49&S. Gotovacorg35&V. Grabskiorg71&L.K.
Graczykowskiorg142&K.L. Grahamorg111&L. Greinerorg80&A. Grelliorg63&C.
Grigorasorg34&V. Grigorievorg93&A. Grigoryanorg1&S. Grigoryanorg75&O.S.
Groettvikorg21&F. Grosaorg30,org59&J.F. Grosse-Oetringhausorg34&R.
Grossoorg107&R. Guernaneorg79&M. Guittiereorg115&K. Gulbrandsenorg89&T.
Gunjiorg132&A. Guptaorg101&R. Guptaorg101&I.B. Guzmanorg45&R. Haakeorg146&M.K.
Habiborg107&C. Hadjidakisorg78&H. Hamagakiorg82&G. Hamarorg145&M. Hamidorg6&R.
Hanniganorg119&M.R. Haqueorg63,org86&A. Harlenderovaorg107&J.W.
Harrisorg146&A. Hartonorg11&J.A. Hasenbichlerorg34&H. Hassanorg96&Q.U.
Hassanorg14&D. Hatzifotiadouorg10,org54&P. Hauerorg43&L.B. Havenerorg146&S.
Hayashiorg132&S.T. Heckelorg105&E. Hellbärorg68&H. Helstruporg36&A.
Herghelegiuorg48&T. Hermanorg37&E.G. Hernandezorg45&G. Herrera Corralorg9&F.
Herrmannorg144&K.F. Hetlandorg36&H. Hillemannsorg34&C. Hillsorg127&B.
Hippolyteorg136&B. Hohlwegerorg105&J. Honermannorg144&D. Horakorg37&A.
Hornungorg68&S. Hornungorg107&R. Hosokawaorg15,org133&P. Hristovorg34&C.
Huangorg78&C. Hughesorg130&P. Huhnorg68&T.J. Humanicorg97&H.
Hushnudorg110&L.A. Husovaorg144&N. Hussainorg42&S.A. Hussainorg14&D.
Hutterorg39&J.P. Iddonorg34,org127&R. Ilkaevorg109&H. Ilyasorg14&M.
Inabaorg133&G.M. Innocentiorg34&M. Ippolitovorg88&A. Isakovorg95&M.S.
Islamorg110&M. Ivanovorg107&V. Ivanovorg98&V. Izucheevorg91&B. Jacakorg80&N.
Jacazioorg34,org54&P.M. Jacobsorg80&S. Jadlovskaorg117&J. Jadlovskyorg117&S.
Jaelaniorg63&C. Jahnkeorg121&M.J. Jakubowskaorg142&M.A. Janikorg142&T.
Jansonorg74&M. Jercicorg99&O. Jevonsorg111&M. Jinorg125&F.
Jonasorg96,org144&P.G. Jonesorg111&J. Jungorg68&M. Jungorg68&A. Juskoorg111&P.
Kalinakorg64&A. Kalweitorg34&V. Kaplinorg93&S. Karorg6&A. Karasu Uysalorg77&D.
Karatovicorg99&O. Karavichevorg62&T. Karavichevaorg62&P. Karczmarczykorg142&E.
Karpechevorg62&A. Kazantsevorg88&U. Kebschullorg74&R. Keidelorg47&M.
Keilorg34&B. Ketzerorg43&Z. Khabanovaorg90&A.M. Khanorg6&S. Khanorg16&A.
Khanzadeevorg98&Y. Kharlovorg91&A. Khatunorg16&A. Khuntiaorg118&B.
Kilengorg36&B. Kimorg61&B. Kimorg133&D. Kimorg147&D.J. Kimorg126&E.J.
Kimorg73&H. Kimorg17&J. Kimorg147&J.S. Kimorg41&J. Kimorg104&J. Kimorg147&J.
Kimorg73&M. Kimorg104&S. Kimorg18&T. Kimorg147&T. Kimorg147&S. Kirschorg68&I.
Kiselorg39&S. Kiselevorg92&A. Kisielorg142&J.L. Klayorg5&C. Kleinorg68&J.
Kleinorg34,org59&S. Kleinorg80&C. Klein-Bösingorg144&M. Kleinerorg68&A.
Klugeorg34&M.L. Knichelorg34&A.G. Knospeorg125&C. Kobdajorg116&M.K.
Köhlerorg104&T. Kolleggerorg107&A. Kondratyevorg75&N. Kondratyevaorg93&E.
Kondratyukorg91&J. Konigorg68&S.A. Konigstorferorg105&P.J. Konopkaorg34&G.
Kornakovorg142&L. Koskaorg117&O. Kovalenkoorg85&V. Kovalenkoorg113&M.
Kowalskiorg118&I. Králikorg64&A. Kravčákováorg38&L. Kreisorg107&M.
Krivdaorg64,org111&F. Krizekorg95&K. Krizkova Gajdosovaorg37&M. Krügerorg68&E.
Kryshenorg98&M. Krzewickiorg39&A.M. Kuberaorg97&V. Kučeraorg34,org61&C.
Kuhnorg136&P.G. Kuijerorg90&L. Kumarorg100&S. Kunduorg86&P. Kurashviliorg85&A.
Kurepinorg62&A.B. Kurepinorg62&A. Kuryakinorg109&S. Kushpilorg95&J.
Kvapilorg111&M.J. Kweonorg61&J.Y. Kwonorg61&Y. Kwonorg147&S.L. La
Pointeorg39&P. La Roccaorg27&Y.S. Laiorg80&M. Lamannaorg34&R. Langoyorg129&K.
Lapidusorg34&A. Lardeuxorg20&P. Larionovorg52&E. Laudiorg34&R. Lavickaorg37&T.
Lazarevaorg113&R. Leaorg24&L. Leardiniorg104&J. Leeorg133&S. Leeorg147&S.
Lehnerorg114&J. Lehrbachorg39&R.C. Lemmonorg94&I. León Monzónorg120&E.D.
Lesserorg19&M. Lettrichorg34&P. Lévaiorg145&X. Liorg12&X.L. Liorg6&J.
Lienorg129&R. Lietavaorg111&B. Limorg17&V. Lindenstruthorg39&A.
Lindnerorg48&C. Lippmannorg107&M.A. Lisaorg97&A. Liuorg19&J. Liuorg127&S.
Liuorg97&W.J. Llopeorg143&I.M. Lofnesorg21&V. Loginovorg93&C. Loizidesorg96&P.
Loncarorg35&J.A. Lopezorg104&X. Lopezorg134&E. López Torresorg8&J.R.
Luhderorg144&M. Lunardonorg28&G. Luparelloorg60&Y.G. Maorg40&A.
Maevskayaorg62&M. Magerorg34&S.M. Mahmoodorg20&T. Mahmoudorg43&A.
Maireorg136&R.D. Majkaorg146org*&M. Malaevorg98&Q.W. Malikorg20&L.
Malininaorg75orgIII&D. Mal’Kevichorg92&P. Malzacherorg107&G.
Mandaglioorg32,org56&V. Mankoorg88&F. Mansoorg134&V. Manzariorg53&Y.
Maoorg6&M. Marchisoneorg135&J. Marešorg66&G.V. Margagliottiorg24&A.
Margottiorg54&A. Marínorg107&C. Markertorg119&M. Marquardorg68&C.D.
Martinorg24&N.A. Martinorg104&P. Martinengoorg34&J.L. Martinezorg125&M.I.
Martínezorg45&G. Martínez Garcíaorg115&S. Masciocchiorg107&M. Maseraorg25&A.
Masoniorg55&L. Massacrierorg78&E. Massonorg115&A. Mastroserioorg53,org138&A.M.
Mathisorg105&O. Matonohaorg81&P.F.T. Matuokaorg121&A. Matyjaorg118&C.
Mayerorg118&F. Mazzaschiorg25&M. Mazzilliorg53&M.A. Mazzoniorg58&A.F.
Mechlerorg68&F. Meddiorg22&Y. Melikyanorg62,org93&A. Menchaca-Rochaorg71&C.
Mengkeorg6&E. Meninnoorg29,org114&A.S. Menonorg125&M. Meresorg13&S.
Mhlangaorg124&Y. Miakeorg133&L. Michelettiorg25&L.C. Migliorinorg135&D.L.
Mihaylovorg105&K. Mikhaylovorg75,org92&A.N. Mishraorg69&D. Miśkowiecorg107&A.
Modakorg3&N. Mohammadiorg34&A.P. Mohantyorg63&B. Mohantyorg86&M. Mohisin
Khanorg16orgIV&Z. Moravcovaorg89&C. Mordasiniorg105&D.A. Moreira De
Godoyorg144&L.A.P. Morenoorg45&I. Morozovorg62&A. Morschorg34&T.
Mrnjavacorg34&V. Mucciforaorg52&E. Mudnicorg35&D. Mühlheimorg144&S.
Muhuriorg141&J.D. Mulliganorg80&A. Mulliriorg23,org55&M.G. Munhozorg121&R.H.
Munzerorg68&H. Murakamiorg132&S. Murrayorg124&L. Musaorg34&J.
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Nairorg85&B.K. Nandiorg49&R. Naniaorg10,org54&E. Nappiorg53&M.U.
Naruorg14&A.F. Nassirpourorg81&C. Nattrassorg130&R. Nayakorg49&T.K.
Nayakorg86&S. Nazarenkoorg109&A. Neaguorg20&R.A. Negrao De Oliveiraorg68&L.
Nellenorg69&S.V. Nesboorg36&G. Neskovicorg39&D. Nesterovorg113&L.T.
Neumannorg142&B.S. Nielsenorg89&S. Nikolaevorg88&S. Nikulinorg88&V.
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Oskarssonorg81&J. Otwinowskiorg118&K. Oyamaorg82&Y. Pachmayerorg104&V.
Pacikorg89&S. Padhanorg49&D. Paganoorg140&G. Paićorg69&J. Panorg143&S.
Panebiancoorg137&P. Pareekorg50,org141&J. Parkorg61&J.E. Parkkilaorg126&S.
Parmarorg100&S.P. Pathakorg125&B. Paulorg23&J. Pazziniorg140&H. Peiorg6&T.
Peitzmannorg63&X. Pengorg6&L.G. Pereiraorg70&H. Pereira Da Costaorg137&D.
Peresunkoorg88&G.M. Perezorg8&S. Perrinorg137&Y. Pestovorg4&V.
Petráčekorg37&M. Petroviciorg48&R.P. Pezziorg70&S. Pianoorg60&M. Piknaorg13&P.
Pillotorg115&O. Pinazzaorg34,org54&L. Pinskyorg125&C. Pintoorg27&S.
Pisanoorg10,org52&D. Pistoneorg56&M. Płoskońorg80&M. Planinicorg99&F.
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Cahuantziorg45&K. Røedorg20&R. Rogalevorg91&E. Rogochayaorg75&D. Rohrorg34&D.
Röhrichorg21&P.F. Rojasorg45&P.S. Rokitaorg142&F. Ronchettiorg52&A.
Rosanoorg56&E.D. Rosasorg69&K. Roslonorg142&A. Rossiorg28,org57&A.
Rotondiorg139&A. Royorg50&P. Royorg110&O.V. Ruedaorg81&R. Ruiorg24&B.
Rumyantsevorg75&A. Rustamovorg87&E. Ryabinkinorg88&Y. Ryabovorg98&A.
Rybickiorg118&H. Rytkonenorg126&O.A.M. Saarimakiorg44&R. Sadekorg115&S.
Sadhuorg141&S. Sadovskyorg91&K. Šafaříkorg37&S.K. Sahaorg141&B. Sahooorg49&P.
Sahooorg49&R. Sahooorg50&S. Sahooorg65&P.K. Sahuorg65&J. Sainiorg141&S.
Sakaiorg133&S. Sambyalorg101&V. Samsonovorg93,org98&D. Sarkarorg143&N.
Sarkarorg141&P. Sarmaorg42&V.M. Sartiorg105&M.H.P. Sasorg63&E.
Scapparoneorg54&J. Schambachorg119&H.S. Scheidorg68&C. Schiauaorg48&R.
Schickerorg104&A. Schmahorg104&C. Schmidtorg107&H.R. Schmidtorg103&M.O.
Schmidtorg104&M. Schmidtorg103&N.V. Schmidtorg68,org96&A.R. Schmierorg130&J.
Schukraftorg89&Y. Schutzorg136&K. Schwarzorg107&K. Schwedaorg107&G.
Scioliorg26&E. Scomparinorg59&J.E. Segerorg15&Y. Sekiguchiorg132&D.
Sekihataorg132&I. Selyuzhenkovorg93,org107&S. Senyukovorg136&D.
Serebryakovorg62&A. Sevcencoorg67&A. Shabanovorg62&A. Shabetaiorg115&R.
Shahoyanorg34&W. Shaikhorg110&A. Shangaraevorg91&A. Sharmaorg100&A.
Sharmaorg101&H. Sharmaorg118&M. Sharmaorg101&N. Sharmaorg100&S.
Sharmaorg101&O. Sheibaniorg125&K. Shigakiorg46&M. Shimomuraorg83&S.
Shirinkinorg92&Q. Shouorg40&Y. Sibiriakorg88&S. Siddhantaorg55&T.
Siemiarczukorg85&D. Silvermyrorg81&G. Simatovicorg90&G. Simonettiorg34&B.
Singhorg105&R. Singhorg86&R. Singhorg101&R. Singhorg50&V.K. Singhorg141&V.
Singhalorg141&T. Sinhaorg110&B. Sitarorg13&M. Sittaorg31&T.B. Skaaliorg20&M.
Slupeckiorg44&N. Smirnovorg146&R.J.M. Snellingsorg63&C. Sonccoorg112&J.
Songorg125&A. Songmoolnakorg116&F. Soramelorg28&S. Sorensenorg130&I.
Sputowskaorg118&J. Stachelorg104&I. Stanorg67&P.J. Steffanicorg130&E.
Stenlundorg81&S.F. Stiefelmaierorg104&D. Stoccoorg115&M.M.
Storetvedtorg36&L.D. Strittoorg29&A.A.P. Suaideorg121&T. Sugitateorg46&C.
Suireorg78&M. Suleymanovorg14&M. Suljicorg34&R. Sultanovorg92&M.
Šumberaorg95&V. Sumberiaorg101&S. Sumowidagdoorg51&S. Swainorg65&A.
Szaboorg13&I. Szarkaorg13&U. Tabassamorg14&S.F. Taghaviorg105&G.
Taillepiedorg134&J. Takahashiorg122&G.J. Tambaveorg21&S. Tangorg6,org134&M.
Tarhiniorg115&M.G. Tarzilaorg48&A. Tauroorg34&G. Tejeda Muñozorg45&A.
Telescaorg34&L. Terlizziorg25&C. Terrevoliorg125&D. Thakurorg50&S.
Thakurorg141&D. Thomasorg119&F. Thoresenorg89&R. Tieulentorg135&A.
Tikhonovorg62&A.R. Timminsorg125&A. Toiaorg68&N. Topilskayaorg62&M.
Toppiorg52&F. Torales-Acostaorg19&S.R. Torresorg37&A. Trifiróorg32,org56&S.
Tripathyorg50,org69&T. Tripathyorg49&S. Trogoloorg28&G. Trombettaorg33&L.
Tropporg38&V. Trubnikovorg2&W.H. Trzaskaorg126&T.P. Trzcinskiorg142&B.A.
Trzeciakorg37,org63&A. Tumkinorg109&R. Turrisiorg57&T.S. Tveterorg20&K.
Ullalandorg21&E.N. Umakaorg125&A. Urasorg135&G.L. Usaiorg23&M. Valaorg38&N.
Valleorg139&S. Valleroorg59&N. van der Kolkorg63&L.V.R. van Doremalenorg63&M.
van Leeuwenorg63&P. Vande Vyvreorg34&D. Vargaorg145&Z. Vargaorg145&M. Varga-
Kofaragoorg145&A. Vargasorg45&M. Vasileiouorg84&A. Vasilievorg88&O. Vázquez
Doceorg105&V. Vecherninorg113&E. Vercellinorg25&S. Vergara Limónorg45&L.
Vermuntorg63&R. Vernetorg7&R. Vértesiorg145&L. Vickovicorg35&Z.
Vilakaziorg131&O. Villalobos Baillieorg111&G. Vinoorg53&A. Vinogradovorg88&T.
Virgiliorg29&V. Vislaviciusorg89&A. Vodopyanovorg75&B. Volkelorg34&M.A.
Völklorg103&K. Voloshinorg92&S.A. Voloshinorg143&G. Volpeorg33&B. von
Hallerorg34&I. Vorobyevorg105&D. Voscekorg117&J. Vrlákováorg38&B.
Wagnerorg21&M. Weberorg114&S.G. Weberorg144&A. Wegrzynekorg34&S.C.
Wenzelorg34&J.P. Wesselsorg144&J. Wiechulaorg68&J. Wikneorg20&G. Wilkorg85&J.
Wilkinsonorg10,org54&G.A. Willemsorg144&E. Willsherorg111&B.
Windelbandorg104&M. Winnorg137&W.E. Wittorg130&J.R. Wrightorg119&Y.
Wuorg128&R. Xuorg6&S. Yalcinorg77&Y. Yamaguchiorg46&K. Yamakawaorg46&S.
Yangorg21&S. Yanoorg137&Z. Yinorg6&H. Yokoyamaorg63&I.-K. Yooorg17&J.H.
Yoonorg61&S. Yuanorg21&A. Yuncuorg104&V. Yurchenkoorg2&V. Zaccoloorg24&A.
Zamanorg14&C. Zampolliorg34&H.J.C. Zanoliorg63&N. Zardoshtiorg34&A.
Zarochentsevorg113&P. Závadaorg66&N. Zaviyalovorg109&H. Zbroszczykorg142&M.
Zhalovorg98&S. Zhangorg40&X. Zhangorg6&Z. Zhangorg6&V. Zherebchevskiiorg113&Y.
Zhiorg12&D. Zhouorg6&Y. Zhouorg89&Z. Zhouorg21&J. Zhuorg6,org107&Y. Zhuorg6&A.
Zichichiorg10,org26&G. Zinovjevorg2&N. Zurloorg140&

## Affiliation notes

org*Deceased

orgIItalian National Agency for New Technologies, Energy and Sustainable
Economic Development (ENEA), Bologna, Italy

orgIIDipartimento DET del Politecnico di Torino, Turin, Italy

orgIIIM.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of
Nuclear, Physics, Moscow, Russia

orgIVDepartment of Applied Physics, Aligarh Muslim University, Aligarh, India

orgVInstitute of Theoretical Physics, University of Wroclaw, Poland

## Collaboration Institutes

org1A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute)
Foundation, Yerevan, Armenia

org2Bogolyubov Institute for Theoretical Physics, National Academy of Sciences
of Ukraine, Kiev, Ukraine

org3Bose Institute, Department of Physics and Centre for Astroparticle Physics
and Space Science (CAPSS), Kolkata, India

org4Budker Institute for Nuclear Physics, Novosibirsk, Russia

org5California Polytechnic State University, San Luis Obispo, California,
United States

org6Central China Normal University, Wuhan, China

org7Centre de Calcul de l’IN2P3, Villeurbanne, Lyon, France

org8Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana,
Cuba

org9Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City
and Mérida, Mexico

org10Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche
“Enrico Fermi’, Rome, Italy

org11Chicago State University, Chicago, Illinois, United States

org12China Institute of Atomic Energy, Beijing, China

org13Comenius University Bratislava, Faculty of Mathematics, Physics and
Informatics, Bratislava, Slovakia

org14COMSATS University Islamabad, Islamabad, Pakistan

org15Creighton University, Omaha, Nebraska, United States

org16Department of Physics, Aligarh Muslim University, Aligarh, India

org17Department of Physics, Pusan National University, Pusan, Republic of
Korea

org18Department of Physics, Sejong University, Seoul, Republic of Korea

org19Department of Physics, University of California, Berkeley, California,
United States

org20Department of Physics, University of Oslo, Oslo, Norway

org21Department of Physics and Technology, University of Bergen, Bergen,
Norway

org22Dipartimento di Fisica dell’Università ’La Sapienza’ and Sezione INFN,
Rome, Italy

org23Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy

org24Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy

org25Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy

org26Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Bologna, Italy

org27Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Catania, Italy

org28Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Padova, Italy

org29Dipartimento di Fisica ‘E.R. Caianiello’ dell’Università and Gruppo
Collegato INFN, Salerno, Italy

org30Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy

org31Dipartimento di Scienze e Innovazione Tecnologica dell’Università del
Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy

org32Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy

org33Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari,
Italy

org34European Organization for Nuclear Research (CERN), Geneva, Switzerland

org35Faculty of Electrical Engineering, Mechanical Engineering and Naval
Architecture, University of Split, Split, Croatia

org36Faculty of Engineering and Science, Western Norway University of Applied
Sciences, Bergen, Norway

org37Faculty of Nuclear Sciences and Physical Engineering, Czech Technical
University in Prague, Prague, Czech Republic

org38Faculty of Science, P.J. Šafárik University, Košice, Slovakia

org39Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-
Universität Frankfurt, Frankfurt, Germany

org40Fudan University, Shanghai, China

org41Gangneung-Wonju National University, Gangneung, Republic of Korea

org42Gauhati University, Department of Physics, Guwahati, India

org43Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-
Wilhelms-Universität Bonn, Bonn, Germany

org44Helsinki Institute of Physics (HIP), Helsinki, Finland

org45High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico

org46Hiroshima University, Hiroshima, Japan

org47Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation
(ZTT), Worms, Germany

org48Horia Hulubei National Institute of Physics and Nuclear Engineering,
Bucharest, Romania

org49Indian Institute of Technology Bombay (IIT), Mumbai, India

org50Indian Institute of Technology Indore, Indore, India

org51Indonesian Institute of Sciences, Jakarta, Indonesia

org52INFN, Laboratori Nazionali di Frascati, Frascati, Italy

org53INFN, Sezione di Bari, Bari, Italy

org54INFN, Sezione di Bologna, Bologna, Italy

org55INFN, Sezione di Cagliari, Cagliari, Italy

org56INFN, Sezione di Catania, Catania, Italy

org57INFN, Sezione di Padova, Padova, Italy

org58INFN, Sezione di Roma, Rome, Italy

org59INFN, Sezione di Torino, Turin, Italy

org60INFN, Sezione di Trieste, Trieste, Italy

org61Inha University, Incheon, Republic of Korea

org62Institute for Nuclear Research, Academy of Sciences, Moscow, Russia

org63Institute for Subatomic Physics, Utrecht University/Nikhef, Utrecht,
Netherlands

org64Institute of Experimental Physics, Slovak Academy of Sciences, Košice,
Slovakia

org65Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India

org66Institute of Physics of the Czech Academy of Sciences, Prague, Czech
Republic

org67Institute of Space Science (ISS), Bucharest, Romania

org68Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt,
Frankfurt, Germany

org69Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México,
Mexico City, Mexico

org70Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS),
Porto Alegre, Brazil

org71Instituto de Física, Universidad Nacional Autónoma de México, Mexico
City, Mexico

org72iThemba LABS, National Research Foundation, Somerset West, South Africa

org73Jeonbuk National University, Jeonju, Republic of Korea

org74Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik,
Fachbereich Informatik und Mathematik, Frankfurt, Germany

org75Joint Institute for Nuclear Research (JINR), Dubna, Russia

org76Korea Institute of Science and Technology Information, Daejeon, Republic
of Korea

org77KTO Karatay University, Konya, Turkey

org78Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France

org79Laboratoire de Physique Subatomique et de Cosmologie, Université
Grenoble-Alpes, CNRS-IN2P3, Grenoble, France

org80Lawrence Berkeley National Laboratory, Berkeley, California, United
States

org81Lund University Department of Physics, Division of Particle Physics,
Lund, Sweden

org82Nagasaki Institute of Applied Science, Nagasaki, Japan

org83Nara Women’s University (NWU), Nara, Japan

org84National and Kapodistrian University of Athens, School of Science,
Department of Physics , Athens, Greece

org85National Centre for Nuclear Research, Warsaw, Poland

org86National Institute of Science Education and Research, Homi Bhabha
National Institute, Jatni, India

org87National Nuclear Research Center, Baku, Azerbaijan

org88National Research Centre Kurchatov Institute, Moscow, Russia

org89Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

org90Nikhef, National institute for subatomic physics, Amsterdam, Netherlands

org91NRC Kurchatov Institute IHEP, Protvino, Russia

org92NRC «Kurchatov» Institute - ITEP, Moscow, Russia

org93NRNU Moscow Engineering Physics Institute, Moscow, Russia

org94Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United
Kingdom

org95Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy,
Czech Republic

org96Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States

org97Ohio State University, Columbus, Ohio, United States

org98Petersburg Nuclear Physics Institute, Gatchina, Russia

org99Physics department, Faculty of science, University of Zagreb, Zagreb,
Croatia

org100Physics Department, Panjab University, Chandigarh, India

org101Physics Department, University of Jammu, Jammu, India

org102Physics Department, University of Rajasthan, Jaipur, India

org103Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen,
Germany

org104Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg,
Heidelberg, Germany

org105Physik Department, Technische Universität München, Munich, Germany

org106Politecnico di Bari, Bari, Italy

org107Research Division and ExtreMe Matter Institute EMMI, GSI
Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany

org108Rudjer Bošković Institute, Zagreb, Croatia

org109Russian Federal Nuclear Center (VNIIEF), Sarov, Russia

org110Saha Institute of Nuclear Physics, Homi Bhabha National Institute,
Kolkata, India

org111School of Physics and Astronomy, University of Birmingham, Birmingham,
United Kingdom

org112Sección Física, Departamento de Ciencias, Pontificia Universidad
Católica del Perú, Lima, Peru

org113St. Petersburg State University, St. Petersburg, Russia

org114Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria

org115SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes,
France

org116Suranaree University of Technology, Nakhon Ratchasima, Thailand

org117Technical University of Košice, Košice, Slovakia

org118The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy
of Sciences, Cracow, Poland

org119The University of Texas at Austin, Austin, Texas, United States

org120Universidad Autónoma de Sinaloa, Culiacán, Mexico

org121Universidade de São Paulo (USP), São Paulo, Brazil

org122Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil

org123Universidade Federal do ABC, Santo Andre, Brazil

org124University of Cape Town, Cape Town, South Africa

org125University of Houston, Houston, Texas, United States

org126University of Jyväskylä, Jyväskylä, Finland

org127University of Liverpool, Liverpool, United Kingdom

org128University of Science and Technology of China, Hefei, China

org129University of South-Eastern Norway, Tonsberg, Norway

org130University of Tennessee, Knoxville, Tennessee, United States

org131University of the Witwatersrand, Johannesburg, South Africa

org132University of Tokyo, Tokyo, Japan

org133University of Tsukuba, Tsukuba, Japan

org134Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France

org135Université de Lyon, Université Lyon 1, CNRS/IN2P3, IPN-Lyon,
Villeurbanne, Lyon, France

org136Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg,
France, Strasbourg, France

org137Université Paris-Saclay Centre d’Etudes de Saclay (CEA), IRFU,
Départment de Physique Nucléaire (DPhN), Saclay, France

org138Università degli Studi di Foggia, Foggia, Italy

org139Università degli Studi di Pavia, Pavia, Italy

org140Università di Brescia, Brescia, Italy

org141Variable Energy Cyclotron Centre, Homi Bhabha National Institute,
Kolkata, India

org142Warsaw University of Technology, Warsaw, Poland

org143Wayne State University, Detroit, Michigan, United States

org144Westfälische Wilhelms-Universität Münster, Institut für Kernphysik,
Münster, Germany

org145Wigner Research Centre for Physics, Budapest, Hungary

org146Yale University, New Haven, Connecticut, United States

org147Yonsei University, Seoul, Republic of Korea