EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

 CERN-EP-2019-122 LHCb-PAPER-2019-008 02 August 2019

Precision measurement of the ${{\mathchar 28931\relax}^{+}_{c}}$, ${{\mathchar
28932\relax}^{+}_{c}}$ and ${{\mathchar 28932\relax}^{0}_{c}}$ baryon
lifetimes

LHCb collaboration†††Authors are listed at the end of this paper.

We report measurements of the lifetimes of the ${{\mathchar
28931\relax}^{+}_{c}}$, ${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{0}_{c}}$ charm baryons using proton-proton collision data at
center-of-mass energies of 7 and 8 TeV, corresponding to an integrated
luminosity of 3.0$\text{\,fb}^{-1}$, collected by the LHCb experiment. The
charm baryons are reconstructed through the decays ${{\mathchar
28931\relax}^{+}_{c}}\rightarrow p{{K}^{-}}{{\pi}^{+}}$, ${{\mathchar
28932\relax}^{+}_{c}}\rightarrow p{{K}^{-}}{{\pi}^{+}}$ and ${{\mathchar
28932\relax}^{0}_{c}}\rightarrow p{{K}^{-}}{{K}^{-}}{{\pi}^{+}}$, and
originate from semimuonic decays of beauty baryons. The lifetimes are measured
relative to that of the ${{D}^{+}}$ meson, and are determined to be

$\displaystyle\tau_{{{\mathchar 28931\relax}^{+}_{c}}}$
$\displaystyle=203.5\pm 1.0\pm 1.3\pm 1.4~{}{\rm fs},$
$\displaystyle\tau_{{{\mathchar 28932\relax}^{+}_{c}}}$
$\displaystyle=456.8\pm 3.5\pm 2.9\pm 3.1~{}{\rm fs},$
$\displaystyle\tau_{{{\mathchar 28932\relax}^{0}_{c}}}$
$\displaystyle=154.5\pm 1.7\pm 1.6\pm 1.0~{}{\rm fs},$

where the uncertainties are statistical, systematic, and due to the
uncertainty in the ${{D}^{+}}$ lifetime. The measurements are approximately
3–4 times more precise than the current world average values. The ${{\mathchar
28931\relax}^{+}_{c}}$ and ${{\mathchar 28932\relax}^{+}_{c}}$ lifetimes are
in agreement with previous measurements; however, the ${{\mathchar
28932\relax}^{0}_{c}}$ baryon lifetime is approximately 3.3 standard
deviations larger than the world average value.

Published in Phys. Rev. D100 (2019) 032001

© 2024 CERN for the benefit of the LHCb collaboration. CC-BY-4.0 licence.

Measurements of the lifetimes of hadrons containing heavy ($b$ or $c$) quarks
play an important role in testing theoretical approaches that are used to
perform Standard Model calculations. The validation of such tools is
important, as they can then be used to search for deviations from Standard
Model expectations in other processes. One of the most predictive tools in
quark flavor physics is the heavy quark expansion (HQE) [1, 2, 3, 4,
*Blok:1992he, 6, 7, 8], which can be used to calculate the decay widths of
hadrons containing heavy quarks, $Q$, through an expansion in inverse powers
of the heavy quark mass, $m_{Q}$. The lowest-order term in the expansion
depends only on $m_{Q}$, and therefore contributes equally to the decay width
of all hadrons with a single heavy quark $Q$. Higher-order terms in the HQE
are related to non-perturbative corrections, and to effects due to the
presence of the other light (spectator) quark(s) in the heavy hadron. These
corrections generally increase as the mass of the heavy quark decreases, and
therefore measurements of charm-hadron lifetimes are sensitive to these
higher-order contributions [9, 10, 11, 12, 13, 14].

Particle lifetimes are also required to compare measured $b$\- or $c$-hadron
decay branching fractions to corresponding predictions for partial decay
widths. Improved precision on the lifetimes thus allows for more stringent
tests of theoretical predictions. Lastly, improving the knowledge of the
properties of all Standard Model particles is important, as they serve as
input directly, or through simulation, into a wide variety of studies both
within and beyond the Standard Model.

Recently, the LHCb collaboration reported a measurement of the ${{\mathchar
28938\relax}^{0}_{c}}$ lifetime [15] that was nearly four times larger than,
and inconsistent with, the world average value. The lifetimes of the other
three ground state singly charmed baryons (${{\mathchar
28931\relax}^{+}_{c}}$, ${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{0}_{c}}$) were last measured almost twenty years ago, and are
only known with precisions of 3%, 6% and 10%, respectively [16]. The most
precise measurements contributing to the average lifetimes are those from the
FOCUS collaboration [17, 18, 19] based on signal sample sizes of approximately
8000 ${{\mathchar 28931\relax}^{+}_{c}}$, 500 ${{\mathchar
28932\relax}^{+}_{c}}$ and 100 ${{\mathchar 28932\relax}^{0}_{c}}$ decays. For
the ${{\mathchar 28931\relax}^{+}_{c}}$ baryon, there is mild tension between
the average lifetime obtained from fixed target experiments [17, 20, 21] and
that obtained by the CLEO collaboration [22].

The LHCb experiment has recorded samples of charm baryons that are larger than
any previous sample by several orders of magnitude, through both prompt
production and as secondary products of $b$-hadron decays. Given the large
deviation seen in the recent ${{\mathchar 28938\relax}^{0}_{c}}$ lifetime
measurement, the tension in the ${{\mathchar 28931\relax}^{+}_{c}}$ lifetime
measurements, and the overall relatively poor precision on the ${{\mathchar
28931\relax}^{+}_{c}}$, ${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{0}_{c}}$ lifetimes compared to those for the charm mesons, it is
important to have additional precise measurements of the lifetimes of these
baryons.

This paper reports new measurements of the lifetimes of the ${{\mathchar
28931\relax}^{+}_{c}}$, ${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{0}_{c}}$ baryons using samples of semileptonic ${{\mathchar
28931\relax}^{0}_{b}}\rightarrow{{\mathchar
28931\relax}^{+}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$, ${{\mathchar
28932\relax}^{0}_{b}}\rightarrow{{\mathchar
28932\relax}^{+}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$, and ${{\mathchar
28932\relax}^{-}_{b}}\rightarrow{{\mathchar
28932\relax}^{0}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$ decays,
respectively.111Throughout the text, charge-conjugate processes are implicitly
included. The symbol $X$ represents any additional undetected particles. The
${{\mathchar 28931\relax}^{+}_{c}}$ and ${{\mathchar 28932\relax}^{+}_{c}}$
baryons are both reconstructed in the $p{{K}^{-}}{{\pi}^{+}}$ final state and
the ${{\mathchar 28932\relax}^{0}_{c}}$ baryon is observed through its decay
to $p{{K}^{-}}{{K}^{-}}{{\pi}^{+}}$. The technique employed to measure the
charm-baryon lifetimes follows that used to measure the ${{\mathchar
28938\relax}^{0}_{c}}$ lifetime in Ref. [15].

To reduce the uncertainties associated with systematic effects, the lifetime
ratio

$\displaystyle r_{H_{c}}\equiv\frac{\tau_{H_{c}}}{\tau_{{{D}^{+}}}}$ (1)

is measured, where the ${{D}^{+}}$ meson is reconstructed using
$B\rightarrow{{D}^{+}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$ decays, with
${{D}^{+}}\rightarrow{{K}^{-}}{{\pi}^{+}}{{\pi}^{+}}$. The symbols $H_{b}$ and
$H_{c}$ are used here and throughout to refer to the $b$ or $c$ hadron in any
of the modes indicated above.

The measurements presented in this paper use proton-proton ($pp$) collision
data samples collected by the LHCb experiment, corresponding to an integrated
luminosity of 3.0$\text{\,fb}^{-1}$, of which 1.0$\text{\,fb}^{-1}$ was
recorded at a center-of-mass energy of 7 TeV and 2.0$\text{\,fb}^{-1}$ at 8
TeV. The LHCb detector [23, 24] is a single-arm forward spectrometer covering
the pseudorapidity range $2<\eta<5$, designed for the study of particles
containing $b$ or $c$ quarks. The tracking system provides a measurement of
the momentum, $p$, of charged particles with a relative uncertainty that
varies from 0.5% at low momentum to 1.0% at
200$\text{\,Ge\kern-1.00006ptV\\!/}c$. The minimum distance of a track to a
primary vertex (PV), the impact parameter (IP), is measured with a resolution
of $\sigma_{\rm IP}=(15+29/p_{\mathrm{T}})\,\upmu\text{m}$ [25], where
$p_{\mathrm{T}}$ is the component of the momentum transverse to the beam, in
$\text{\,Ge\kern-1.00006ptV\\!/}c$. Charged hadrons are identified using
information from two ring-imaging Cherenkov (RICH) detectors [26]. Muons are
identified by a system composed of alternating layers of iron and multiwire
proportional chambers [27]. The online event selection is performed by a
trigger [28], which consists of a hardware stage, based on information from
the calorimeter and muon systems, followed by a software stage, which applies
a full event reconstruction.

Simulation is required to model the effects of the detector acceptance and
resolution, as well as the imposed selection requirements. Proton-proton
collisions are simulated using Pythia [29, *Sjostrand:2007gs] with a specific
LHCb configuration [31]. Decays of hadronic particles are described by EvtGen
[32], in which final-state radiation is generated using Photos [33]. The
interaction of the generated particles with the detector and its response are
implemented using the Geant4 toolkit [34, *Agostinelli:2002hh] as described in
Ref. [36].

Samples of candidate semileptonic $H_{b}$ decays are formed by combining a
${\mu^{-}}$ candidate with a charm-hadron candidate, reconstructed through one
of the following modes: ${{\mathchar 28931\relax}^{+}_{c}}\rightarrow
p{{K}^{-}}{{\pi}^{+}}$, ${{\mathchar 28932\relax}^{+}_{c}}\rightarrow
p{{K}^{-}}{{\pi}^{+}}$, ${{\mathchar 28932\relax}^{0}_{c}}\rightarrow
p{{K}^{-}}{{K}^{-}}{{\pi}^{+}}$, or
${{D}^{+}}\rightarrow{{K}^{-}}{{\pi}^{+}}{{\pi}^{+}}$. All final-state charged
particles are required to be detached from all PVs in the event. This
selection is based upon a quantity $\chi^{2}_{\text{IP}}$, which is the
difference in the $\chi^{2}$ of the PV fit with and without the inclusion of
the particle under consideration. The requirement on $\chi^{2}_{\text{IP}}$
for the $p$, ${{K}^{-}}$ and ${{\pi}^{+}}$ (${\mu^{-}}$) candidates
corresponds to about 2$\sigma_{\rm IP}$ (3$\sigma_{\rm IP}$). The muon is
required to have $p_{\mathrm{T}}>1\text{\,Ge\kern-1.00006ptV\\!/}c$,
$p>6\text{\,Ge\kern-1.00006ptV\\!/}c$ and have particle identification (PID)
information consistent with that of a muon. The final-state hadrons must have
PID information consistent with their assumed particle hypotheses, and have
$p_{\mathrm{T}}>0.25\text{\,Ge\kern-1.00006ptV\\!/}c$ and
$p>2\text{\,Ge\kern-1.00006ptV\\!/}c$. To remove the contribution from
promptly produced charm baryons, the reconstructed trajectory of the $H_{c}$
candidate must not point back to any PV in the event. Only $H_{c}$ candidates
that have an invariant mass within 60$\text{\,Me\kern-1.00006ptV\\!/}c^{2}$ of
their known mass are retained.

The $H_{c}{\mu^{-}}$ combinations are required to form a good quality vertex
and satisfy the invariant mass requirement,
$m(H_{c}{\mu^{-}})<8.0$$\text{\,Ge\kern-1.00006ptV\\!/}c^{2}$. Random
combinations of $H_{c}$ and ${\mu^{-}}$ are suppressed by requiring the
$H_{c}$ decay vertex to be downstream of the reconstructed $H_{c}{\mu^{-}}$
decay vertex. In events with more than one PV, the $b$-hadron candidate and
its decay products are associated to the PV for which the
$\chi^{2}_{\text{IP}}$ of the $b$ hadron is smallest.

The dominant source of background in the $H_{b}\rightarrow H_{c}{\mu^{-}}$
samples is from other semileptonic $b$-hadron decays. To suppress the
background in the ${{\mathchar 28931\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{+}_{c}}$ samples from misidentified
${{D}^{+}_{s}}\rightarrow{{K}^{+}}{{K}^{-}}{{\pi}^{+}}$,
${{D}^{+}}\rightarrow{{K}^{-}}{{\pi}^{+}}{{\pi}^{+}}$,
${{D}^{*+}}\rightarrow{{D}^{0}}(\rightarrow{{K}^{-}}{{\pi}^{+}}){{\pi}^{+}}$,
and $D\rightarrow\phi(\rightarrow{{K}^{+}}{{K}^{-}})X$ decays, a set of vetoes
is employed. The vetoes are only applied to candidates that have an invariant
mass consistent (within $\sim 2.5$ times the mass resolution) with either the
known ${{D}^{+}_{s}}$ mass, ${{D}^{+}}$ mass, the ${{D}^{*+}}-{{D}^{0}}$ mass
difference, or the $\phi$ meson mass, after substituting either the kaon or
pion mass in place of the proton mass in the reconstructed decay chain. For
those candidates, tighter PID requirements are imposed such that any peaking
contribution is removed. The veto removes about 1–2% of the signal, and
reduces the total background by about 15% (25%) in the ${{\mathchar
28931\relax}^{+}_{c}}$ (${{\mathchar 28932\relax}^{+}_{c}}$) samples.
Potential contamination in the ${{\mathchar 28932\relax}^{0}_{c}}$ sample from
fully reconstructed, but misidentified, four-body ${{D}^{0}}$ meson decays has
been investigated, and is found to be negligible. After all selections, the
dominant source of background is from real muons combined with partially
reconstructed or misidentified charm-hadron decays.

After applying the above selections, the ${{\mathchar 28932\relax}^{+}_{c}}$
sample still has a lower signal-to-background ratio than the ${{\mathchar
28931\relax}^{+}_{c}}$ and ${{\mathchar 28932\relax}^{0}_{c}}$ samples. To
improve the signal-to-background ratio in the ${{\mathchar
28932\relax}^{+}_{c}}{\mu^{-}}$ sample, a boosted decision tree (BDT)
discriminant [37, 38] is built from 18 variables. The variables are the
$\chi^{2}$ for the ${{\mathchar 28932\relax}^{0}_{b}}$ and ${{\mathchar
28932\relax}^{+}_{c}}$ decay-vertex fits, and for each final-state hadron:
$p$, $p_{\mathrm{T}}$, $\chi^{2}_{\text{IP}}$ to the associated PV, and a PID
response variable. The BDT is trained using simulated ${{\mathchar
28932\relax}^{0}_{b}}\rightarrow{{\mathchar
28932\relax}^{+}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$ decays for the
signal, while background is taken from the ${{\mathchar 28932\relax}^{+}_{c}}$
mass sidebands, ${30<|m(p{{K}^{-}}{{\pi}^{+}})-m_{{{\mathchar
28932\relax}^{+}_{c}}}|<50\text{\,Me\kern-1.00006ptV\\!/}c^{2}}$, where
$m_{{{\mathchar 28932\relax}^{+}_{c}}}$ is the known ${{\mathchar
28932\relax}^{+}_{c}}$ mass [16]. Only a loose requirement on the BDT is
employed, which provides an efficiency of about 97% for signal decays while
suppressing 40% of the background.

Signal candidates must satisfy a well-defined set of hardware and software
trigger requirements. At the hardware level, signal candidates are required to
include a high $p_{\mathrm{T}}$ muon. At the software level, they must pass a
topological multivariate selection designed to provide an enriched sample of
beauty hadrons decaying to multibody final states containing a muon [39].

The invariant-mass distributions for the selected ${{D}^{+}}$, ${{\mathchar
28931\relax}^{+}_{c}}$, ${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{0}_{c}}$ candidates in the $H_{c}{\mu^{-}}$ final states are
shown in Fig. 1. For the ${{\mathchar 28931\relax}^{+}_{c}}$ and ${{D}^{+}}$
samples only a 10% randomly selected sub-sample of events is used in this
analysis, since the full yield is much larger than is needed in this analysis
given the anticipated size of the systematic uncertainties. A binned maximum-
likelihood fit is performed to each of the four samples to obtain the signal
yields. For each mass distribution, the signal shape is parametrized as the
sum of two Gaussian functions with a common mean, and the background shape is
described using an exponential function. All signal and background shape
parameters are freely varied in the fit. The resulting signal yields are given
in Table 1. The ${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{0}_{c}}$ yields are about 100 times larger than any previous
sample used to measure the lifetimes of these baryons, and the ${{\mathchar
28931\relax}^{+}_{c}}$ sample is about 40 times larger.

Figure 1: Invariant-mass distributions for candidate (top left) ${{D}^{+}}$ in $B\rightarrow{{D}^{+}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$, (top right) ${{\mathchar 28931\relax}^{+}_{c}}$ in ${{\mathchar 28931\relax}^{0}_{b}}\rightarrow{{\mathchar 28931\relax}^{+}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$, (bottom left) ${{\mathchar 28932\relax}^{+}_{c}}$ in ${{{\mathchar 28932\relax}^{0}_{b}}\rightarrow{{\mathchar 28932\relax}^{+}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X}$, and (bottom right) ${{\mathchar 28932\relax}^{0}_{c}}$ in ${{{\mathchar 28932\relax}^{-}_{b}}\rightarrow{{\mathchar 28932\relax}^{0}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X}$ candidate decays. The results of the fits, as described in the text, are overlaid. Table 1: Yields from the binned maximum-likelihood fits to the $H_{c}$ invariant mass spectra in $H_{c}{\mu^{-}}$ signal candidates. For the ${{\mathchar 28931\relax}^{+}_{c}}$ and ${{D}^{+}}$ modes, only 10% of the sample is used, since the yields in the full data set are much larger than needed in this analysis. $H_{c}$ | Yield ($10^{3}$)
---|---
${{D}^{+}}$ | $809.4\pm 1.3$
${{\mathchar 28931\relax}^{+}_{c}}$ | $303.5\pm 0.7$
${{\mathchar 28932\relax}^{+}_{c}}$ | $~{}55.8\pm 0.5$
${{\mathchar 28932\relax}^{0}_{c}}$ | $~{}21.6\pm 0.2$

The decay time of each $H_{c}$ candidate is determined from the positions of
the $H_{b}$ and $H_{c}$ decay vertices, and the measured $H_{c}$ momentum.
Because $b$ hadrons have a mean lifetime of about 1.5 ps, the decay vertices
are well separated from the PV. As a result, systematic effects due to
lifetime-biasing selections in the trigger or offline analysis are greatly
reduced compared to promptly produced charmed baryons.

The background-subtracted decay-time spectra are obtained using the sPlot
technique [40], where the measured $H_{c}$ mass is used as the discriminating
variable. To improve the accuracy of the sPlot background subtraction, a
correction to the $H_{c}$ mass is applied to remove a small dependence of the
mean reconstructed $H_{c}$ mass on its reconstructed decay time, $t_{\rm
rec}$. This correction is obtained by first fitting for the peak position of
the reconstructed mass, $M_{\rm peak}^{H_{c}}(t_{\rm rec})$, in bins of
reconstructed decay time, followed by a fit for the dependence of $M_{\rm
peak}^{H_{c}}(t_{\rm rec})$ on $t_{\rm rec}$, using the functional form

$\displaystyle M^{H_{c}}(0)+A[1-\exp(-t_{\rm rec}/C)].$ (2)

The second term represents the deviation from a constant value, and is used to
correct the measured $H_{c}$ mass of every candidate used in the sPlot. For
the four modes under study, the values of $A$ and $C$ range from
2.7–4.1$\text{\,Me\kern-1.00006ptV\\!/}c^{2}$ and 0.06–0.17 ps, respectively.
The uncertainties in the signal yields reflect both the finite signal yield
and the statistical uncertainty associated with the background subtraction.

Potential backgrounds from random $H_{c}{\mu^{-}}$ combinations, where the
muon is not produced directly at the $H_{b}$ decay vertex, could lead to a
bias on the lifetime. Such decays include $H_{b}\rightarrow
H_{c}{\tau^{-}}{{\overline{\nu}}_{\tau}}$,
${\tau^{-}}\rightarrow{\mu^{-}}{{\nu}_{\tau}}{{\overline{\nu}}_{\mu}}$ and
$H_{b}\rightarrow H_{c}{\kern 1.79993pt\overline{\kern-1.79993ptD}},~{}{\kern
1.79993pt\overline{\kern-1.79993ptD}}\rightarrow{\mu^{-}}X$, where ${\kern
1.79993pt\overline{\kern-1.79993ptD}}$ represents a ${{D}^{-}_{s}}$,
${{D}^{-}}$ or ${{\kern 1.79993pt\overline{\kern-1.79993ptD}}{}^{0}}$ meson.
These backgrounds are a small fraction of the observed signal, about 3% in
total, and have decay-time spectra that are similar to the genuine
$H_{c}{\mu^{-}}{{\overline{\nu}}_{\mu}}$ final state due to the $\chi^{2}$
requirements on the $H_{b}$ vertex fit. The effect of these backgrounds is
studied with simulation and pseudoexperiments, and is included as a source of
systematic uncertainty.

The decay-time spectra for the ${{D}^{+}}$, ${{\mathchar
28931\relax}^{+}_{c}}$, ${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{0}_{c}}$ signals are shown in Fig. 2, along with the results of
the fits described below. Only $H_{c}$ candidates with decay time larger than
zero are used in the fit. The decrease in signal yield as the decay time
approaches zero is mainly due to the $H_{c}$ decay-time resolution, typically
in the 85–100 fs range, which results in migration of the signal into the
negative decay-time region.

Figure 2: Decay-time spectra for (top left) ${{D}^{+}}$ signal in
$B\rightarrow{{D}^{+}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$, (top right)
${{\mathchar 28931\relax}^{+}_{c}}$ signal in ${{\mathchar
28931\relax}^{0}_{b}}\rightarrow{{\mathchar
28931\relax}^{+}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$, (bottom left)
${{\mathchar 28932\relax}^{+}_{c}}$ signal in ${{\mathchar
28932\relax}^{0}_{b}}\rightarrow{{\mathchar
28932\relax}^{+}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$, and (bottom right)
${{\mathchar 28932\relax}^{0}_{c}}$ signal in ${{\mathchar
28932\relax}^{-}_{b}}\rightarrow{{\mathchar
28932\relax}^{0}_{c}}{\mu^{-}}{{\overline{\nu}}_{\mu}}X$ candidate decays.
Overlaid are the fit results, as described in the text, along with the
uncertainties due to finite sizes of the simulated samples.

The charm-hadron lifetimes are determined by fitting the decay-time spectra
using a binned $\chi^{2}$ fit over the ranges shown in Fig. 2. The signal
decay-time model takes the form

$\displaystyle S(t_{\rm rec};\tau_{\rm sim}^{H_{c}})=f(t_{\rm rec};\tau_{\rm
sim}^{H_{c}})g(t_{\rm rec})\beta(t_{\rm rec}),$ (3)

where $f(t_{\rm rec};\tau_{\rm sim}^{H_{c}})$ is a signal template of
reconstructed decay times obtained from the full LHCb simulation with input
lifetime $\tau_{\rm sim}^{H_{c}}$. The selection requirements applied to the
simulation are identical to those applied to the data. The function

$\displaystyle g(t_{\rm rec})=\exp(-t_{\rm rec}/\tau_{\rm
fit}^{H_{c}})/\exp(-t_{\rm rec}/\tau_{\rm sim}^{H_{c}})$ (4)

weights the simulated template with lifetime $\tau_{\rm sim}^{H_{c}}$ to a
lifetime value $\tau_{\rm fit}^{H_{c}}$. Because the weighting function
$g(t_{\rm rec})$ depends on the reconstructed decay time, $t_{\rm rec}$,
rather than the true decay time, there is a dependence of $S(t_{\rm
rec};\tau_{\rm sim}^{H_{c}})$ on the $\tau_{\rm sim}^{H_{c}}$ value used to
generate the template. The simulation uses the known ${{D}^{+}}$ lifetime,
$\tau_{\rm sim}^{{{D}^{+}}}=1040$ fs, which is accurately measured by many
experiments [16]. Since the charm-baryon lifetimes in this analysis are
expected to have a better precision than the existing world average values, a
number of different $\tau_{\rm sim}^{H_{c}}$ templates are produced. An
optimization procedure, as described below, is used to determine the best
choice of $\tau_{\rm sim}^{H_{c}}$ to use for the charm-baryon templates. The
simulation includes contributions from
$H_{c}{\tau^{-}}{{\overline{\nu}}_{\tau}}X$ final states as well as excited
charm hadrons.

The function $\beta(t_{\rm rec})$ corrects for a small difference in the
efficiency between data and simulation for reconstructing tracks in the vertex
detector that originate far from the beamline [41]. As discussed in Ref. [15],
$\beta(t)$ is calibrated using the precisely known value of the ${{D}^{+}}$
lifetime, from which it is found that $\beta(t_{\rm rec})=1+\beta_{0}t_{\rm
rec}$, with $\beta_{0}=(-0.89\pm 0.32)\times 10^{-2}$ ps-1. The result of the
binned $\chi^{2}$ fit to the ${{D}^{+}}$ decay-time spectrum after this
correction is applied is shown in Fig. 2 (top left), where the fitted lifetime
is found to be ${\tau_{\rm fit}^{{{D}^{+}}}=1042.0\pm
1.7\text{\,(stat)}~{}\rm{fs}}$. The inclusion of the $\beta(t)$ term in
$S(t_{\rm rec})$ amounts to about a 1% positive correction to the measured
lifetime.

Since beauty baryonic decays are not perfectly described by the simulation,
the simulated events are weighted in bins of $(p_{\mathrm{T}},\eta)$ of the
beauty baryon and the mass $m(H_{c}{\mu^{-}})$ of the $H_{c}{\mu^{-}}$ system
to match that which is observed in background-subtracted data. The simulation
is also weighted to match all of the two-body invariant mass projections among
the $H_{c}$ decay products. After all of these weights are applied, excellent
agreement is seen for a wide range of observables in these decays, most
notably those that are used in the BDT. These weights are applied in the
formation of the $f(t_{\rm rec};\tau_{\rm sim}^{H_{c}})$ templates.

The lifetime of each charmed baryon is determined from a simultaneous fit to
its decay-time spectrum and that of the ${{D}^{+}}$ meson. In these fits,
$\tau_{\rm fit}^{H_{c}}$ in Eq. (4) is replaced by $r_{H_{c}}\tau_{\rm
fit}^{{{D}^{+}}}$ in order to reduce systematic uncertainties. Thus, the free
parameters in the fit are $r_{H_{c}}$, as shown in Eq. (1), and $\tau_{\rm
fit}^{{{D}^{+}}}$. In the ${{\mathchar 28932\relax}^{0}_{c}}$ decay-time fit,
$\beta_{0}$ is scaled by 4/3 since the effect scales with the number of
charged final-state particles in the $H_{c}$ decay [41].

The procedure for determining the optimal values of $\tau_{\rm sim}^{H_{c}}$
to use in forming the templates $f(t_{\rm rec};\tau_{\rm sim}^{H_{c}})$ is
first developed and validated using simulation. A series of templates,
$f_{i}(t_{\rm rec};\tau_{\rm sim}^{H_{c}})$, spanning a wide range of
$\tau_{\rm sim}^{H_{c}}$ values is produced for each charm baryon. From one
template with true lifetime $\tau_{\rm sim}^{H_{c},\rm true}$, a pseudo-
dataset set of decay times is formed that has comparable yield to that of the
data. The decay-time fit is then performed using each template $f_{i}(t_{\rm
rec};\tau_{\rm sim}^{H_{c}})$ to this pseudo-dataset, with each fit yielding a
value of $\tau_{\rm fit}$ and a $\chi^{2}$ of the fit. Examination of the
results show, as expected, that when the pseudo-dataset are fit using the
correct template, $\tau_{\rm fit}$ is consistent with $\tau_{\rm
sim}^{H_{c},\rm true}$. Conversely, when the same pseudo-dataset is fit with
an alternate template that is produced with a significantly different input
value of $\tau_{\rm sim}^{H_{c}}$, $\tau_{\rm fit}$ deviates from $\tau_{\rm
sim}^{H_{c},\rm true}$. Thus the criterion for choosing the optimal template
is to select that in which $\tau_{\rm fit}$ is closest to $\tau_{\rm
sim}^{H_{c}}$. The chosen template is also found to have the lowest fit
$\chi^{2}$, which provides additional support for the method of determining
the optimal template.

Applying this same criterion to the data, the optimal values of $\tau_{\rm
sim}^{H_{c}}$ are found to be $\tau_{\rm sim}^{{{\mathchar
28931\relax}^{+}_{c}}}=203$ fs, $\tau_{\rm sim}^{{{\mathchar
28932\relax}^{+}_{c}}}=455$ fs, and $\tau_{\rm sim}^{{{\mathchar
28932\relax}^{0}_{c}}}=155$ fs. As with the pseudo-data, these optimal values
also yield the lowest $\chi^{2}$ value for the decay-time fit. Slightly
different values of $\tau_{\rm sim}^{H_{c}}$ are not excluded by the
procedure, and are considered as a source of systematic uncertainty.

The results of the fits to the ${{\mathchar 28931\relax}^{+}_{c}}$,
${{\mathchar 28932\relax}^{+}_{c}}$ and ${{\mathchar 28932\relax}^{0}_{c}}$
decay-time distributions using the best-fit templates are shown in Fig. 2 and
corresponds to the ratios

$\displaystyle r_{{{\mathchar 28931\relax}^{+}_{c}}}$ $\displaystyle=0.1956\pm
0.0010,$ $\displaystyle r_{{{\mathchar 28932\relax}^{+}_{c}}}$
$\displaystyle=0.4392\pm 0.0034,$ $\displaystyle r_{{{\mathchar
28932\relax}^{0}_{c}}}$ $\displaystyle=0.1485\pm 0.0017,$

where the uncertainties are statistical only. Multiplying these ratios by the
${{D}^{+}}$ lifetime [16], leads to the lifetimes

$\displaystyle\tau_{{{\mathchar 28931\relax}^{+}_{c}}}$
$\displaystyle=203.5\pm 1.0~{}{\rm fs},$ $\displaystyle\tau_{{{\mathchar
28932\relax}^{+}_{c}}}$ $\displaystyle=456.8\pm 3.5~{}{\rm fs},$
$\displaystyle\tau_{{{\mathchar 28932\relax}^{0}_{c}}}$
$\displaystyle=154.5\pm 1.7~{}{\rm fs}.$

The statistical precision of these measurements is 5–8 times better than those
of the current world average values [16].

A number of sources of systematic uncertainty on the measured ratios
$r_{H_{c}}$ are summarized in Table 2. The decay-time acceptance correction,
$\beta(t_{\rm rec})$, leads to an uncertainty of 0.5% on $r_{H_{c}}$. This
uncertainty includes a contribution from the finite
$B\rightarrow{{D}^{+}}{\mu^{-}}X$ sample sizes and the choice of fit function.

The technique for finding the correct template is based on choosing that in
which the fitted lifetime is most consistent with the value used in the
simulation. The uncertainty due to this choice is estimated by repeating the
decay-time fit using alternative templates that have simulated lifetimes that
differ from the nominal one by two times the uncertainty on the fitted
lifetime. The difference between the fitted values of $r_{H_{c}}$ for these
alternative templates and the nominal one is assigned as a systematic
uncertainty.

The ${{\mathchar 28931\relax}^{0}_{b}}$, ${{\mathchar 28932\relax}^{0}_{b}}$
and ${{\mathchar 28932\relax}^{-}_{b}}$ lifetimes are not known precisely, and
this has a small effect on the decay-time acceptance. To study this effect,
simulated decays are weighted to produce either a shorter or longer $H_{b}$
lifetime, based on the known uncertainties on the $b$-baryon lifetimes [16].
New signal templates are formed, and the fits are repeated. The change in the
fitted value of $r_{H_{c}}$ is assigned as a systematic uncertainty.

Studies of the ${{D}^{+}}$ calibration mode show a small difference in the
reconstruction efficiency between data and simulation, which is described by
the $\beta_{0}$ parameter. This parameter has a small dependence on the
$p_{\mathrm{T}}$ and $\eta$ of the $H_{b}$ hadron. While the signal mode
simulations are weighted to match the ($p_{\mathrm{T}}$, $\eta$) spectrum
observed in data, the weighting is imperfect. A difference would lead to a
small bias in the average value of $\beta_{0}$. The uncertainty on $r_{H_{c}}$
is obtained by taking into account the variation of $\beta_{0}$ in different
$p_{\mathrm{T}}$ and $\eta$ ranges, and the extent to which the
($p_{\mathrm{T}}$, $\eta$) spectrum differs between data and simulation for
each of the decay modes.

The decay-time resolution is checked by comparing the ${{D}^{0}}$ decay-time
spectra in ${{{B}^{-}}}\rightarrow{{D}^{0}}{{\pi}^{-}}$ decays between data
and simulation, where no explicit requirement on the ${{D}^{0}}$ flight
distance is applied. The simulation reproduces the data well. A second check
is performed where the ${{\mathchar 28931\relax}^{+}_{c}}$ lifetime is fitted
using a template that is produced with an additional smearing which increases
the decay-time resolution by 2.5%. The change increases the fit $\chi^{2}$
substantially, with only a small change of 0.3 fs in the fitted lifetime. This
difference is considered negligible, and no systematic uncertainty due to
modeling the decay-time resolution is assigned.

The method for background subtraction uses the sPlot technique, which relies
on a specific choice for modeling the signal and background distributions in
the charm-hadron invariant-mass spectra. To quantify a possible systematic
effect on $r_{H_{c}}$, the decay-time spectra in data are obtained using a
different background-subtraction technique. Instead of the sPlot method,
signal and sideband regions are defined for each of the mass spectra, and for
each charm baryon the decay-time spectrum of candidates from the sideband
regions are subtracted from the spectrum obtained from the signal region. The
resulting background-subtracted decay-time spectra are then fitted using the
decay-time fit described previously. The difference between this result and
the nominal one is assigned as a systematic uncertainty.

The decay-time spectra in the $H_{c}{\mu^{-}}$ samples have small
contributions from random combinations of $H_{c}$ and ${\mu^{-}}$ candidates
[($0.8\pm 0.2)$% of the signal], as well as backgrounds where the muon comes
from either a ${\tau^{-}}$ [($1.8\pm 0.3$)%] or a semileptonic $D$ decay
[($0.5\pm 0.2)$%]. The impact of these backgrounds is assessed using
pseudoexperiments, as described in Ref. [15].

The systematic uncertainty due to the finite size of the simulated samples
used to produce the signal templates is assessed by repeating the fit to the
data many times, where in each fit the simulated-template bin contents are
fluctuated within their uncertainties. The standard deviation of the
distribution of the fitted $r_{H_{c}}$ values is assigned as a systematic
uncertainty. The total systematic uncertainty on $r_{H_{c}}$ is about 0.6% for
the ${{\mathchar 28931\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{+}_{c}}$ measurements, and about 1.2% for that of the
${{\mathchar 28932\relax}^{0}_{c}}$ baryon.

Table 2: Summary of systematic uncertainties on the ratio of the charm baryon to ${{D}^{+}}$ meson lifetimes (in units of $10^{-4}$). The statistical uncertainty on the measurements is also provided for reference. Source | $r_{{{\mathchar 28931\relax}^{+}_{c}}}$ | $r_{{{\mathchar 28932\relax}^{+}_{c}}}$ | $r_{{{\mathchar 28932\relax}^{0}_{c}}}$
---|---|---|---
Decay-time acceptance | 6 | 13 | 4
$H_{c}$ lifetime | 4 | 4 | 12
$H_{b}$ lifetime | 1 | 3 | 0
$H_{b}$ production spectra | 2 | 4 | 1
Background subtraction | 8 | 17 | 7
$H_{c}({\tau^{-}},~{}D$, random ${\mu^{-}}$) | 5 | 11 | 3
Simulated sample size | 4 | 13 | 5
Total systematic | 13 | 28 | 16
Statistical uncertainty | 10 | 34 | 17

In summary, $pp$ collision data samples at 7 TeV and 8 TeV center-of-mass
energies collected by the LHCb experiment, corresponding to
3.0$\text{\,fb}^{-1}$ of integrated luminosity, are used to measure the
lifetimes of the ${{\mathchar 28931\relax}^{+}_{c}}$, ${{\mathchar
28932\relax}^{+}_{c}}$ and ${{\mathchar 28932\relax}^{0}_{c}}$ baryons. For
the ${{\mathchar 28931\relax}^{+}_{c}}$ and ${{D}^{+}}$ samples, only 10% of
the integrated luminosity is used for this measurement. The lifetimes,
measured relative to that of the ${{D}^{+}}$ meson, are determined to be

$\displaystyle r_{{{\mathchar 28931\relax}^{+}_{c}}}$ $\displaystyle=0.1956\pm
0.0010\pm 0.0013,$ $\displaystyle r_{{{\mathchar 28932\relax}^{+}_{c}}}$
$\displaystyle=0.4392\pm 0.0034\pm 0.0028,$ $\displaystyle r_{{{\mathchar
28932\relax}^{0}_{c}}}$ $\displaystyle=0.1485\pm 0.0017\pm 0.0016,$

where the first uncertainty is statistical and the second is systematic. After
multiplying by the known ${{D}^{+}}$ lifetime of $1040\pm 7$ fs [16], the
charm-baryon lifetimes are measured to be

$\displaystyle\tau_{{{\mathchar 28931\relax}^{+}_{c}}}$
$\displaystyle=203.5\pm 1.0\pm 1.3\pm 1.4~{}{\rm fs},$
$\displaystyle\tau_{{{\mathchar 28932\relax}^{+}_{c}}}$
$\displaystyle=456.8\pm 3.5\pm 2.9\pm 3.1~{}{\rm fs},$
$\displaystyle\tau_{{{\mathchar 28932\relax}^{0}_{c}}}$
$\displaystyle=154.5\pm 1.7\pm 1.6\pm 1.0~{}{\rm fs},$

where the last uncertainty is due to the uncertainty in the ${{D}^{+}}$
lifetime. The ${{\mathchar 28931\relax}^{+}_{c}}$ and ${{\mathchar
28932\relax}^{+}_{c}}$ lifetimes are measured with about 1% precision and are
consistent with the existing world averages. The ${{\mathchar
28932\relax}^{0}_{c}}$ lifetime is measured with about 1.8% precision, and is
$3.3\sigma$ larger than the world average value of $112^{+13}_{-10}$ fs. These
measurements have uncertainties that are approximately 3–4 times smaller than
those of the existing world average values, and have precision comparable to
that achieved for charm mesons.

## Acknowledgements

We express our gratitude to our colleagues in the CERN accelerator departments
for the excellent performance of the LHC. We thank the technical and
administrative staff at the LHCb institutes. We acknowledge support from CERN
and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); MOST
and NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN
(Italy); NWO (Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MSHE
(Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC
(United Kingdom); DOE NP and NSF (USA). We acknowledge the computing resources
that are provided by CERN, IN2P3 (France), KIT and DESY (Germany), INFN
(Italy), SURF (Netherlands), PIC (Spain), GridPP (United Kingdom), RRCKI and
Yandex LLC (Russia), CSCS (Switzerland), IFIN-HH (Romania), CBPF (Brazil), PL-
GRID (Poland) and OSC (USA). We are indebted to the communities behind the
multiple open-source software packages on which we depend. Individual groups
or members have received support from AvH Foundation (Germany); EPLANET, Marie
Skłodowska-Curie Actions and ERC (European Union); ANR, Labex P2IO and OCEVU,
and Région Auvergne-Rhône-Alpes (France); Key Research Program of Frontier
Sciences of CAS, CAS PIFI, and the Thousand Talents Program (China); RFBR, RSF
and Yandex LLC (Russia); GVA, XuntaGal and GENCAT (Spain); the Royal Society
and the Leverhulme Trust (United Kingdom).

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Ajaltouni7, S. Akar61, P. Albicocco20, J. Albrecht12, F. Alessio44, M.
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Alves Jr43, S. Amato2, Y. Amhis9, L. An19, L. Anderlini19, G. Andreassi45, M.
Andreotti18, J.E. Andrews62, F. Archilli29, J. Arnau Romeu8, A. Artamonov41,
M. Artuso63, K. Arzymatov39, E. Aslanides8, M. Atzeni46, B. Audurier24, S.
Bachmann14, J.J. Back52, S. Baker57, V. Balagura9,b, W. Baldini18,44, A.
Baranov39, R.J. Barlow58, S. Barsuk9, W. Barter57, M. Bartolini21, F.
Baryshnikov73, V. Batozskaya33, B. Batsukh63, A. Battig12, V. Battista45, A.
Bay45, F. Bedeschi26, I. Bediaga1, A. Beiter63, L.J. Bel29, S. Belin24, N.
Beliy4, V. Bellee45, N. Belloli22,i, K. Belous41, I. Belyaev36, G.
Bencivenni20, E. Ben-Haim10, S. Benson29, S. Beranek11, A. Berezhnoy37, R.
Bernet46, D. Berninghoff14, E. Bertholet10, A. Bertolin25, C. Betancourt46, F.
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Bieker12, S. Bifani49, P. Billoir10, A. Birnkraut12, A. Bizzeti19,u, M.
Bjørn59, M.P. Blago44, T. Blake52, F. Blanc45, S. Blusk63, D. Bobulska55, V.
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Bozzi18,44, S. Braun14, M. Brodski44, J. Brodzicka31, A. Brossa Gonzalo52, D.
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Calabrese18,g, S. Cali20, R. Calladine49, M. Calvi22,i, M. Calvo Gomez42,m, A.
Camboni42,m, P. Campana20, D.H. Campora Perez44, L. Capriotti17,e, A.
Carbone17,e, G. Carboni27, R. Cardinale21, A. Cardini24, P. Carniti22,i, K.
Carvalho Akiba2, G. Casse56, M. Cattaneo44, G. Cavallero21, R. Cenci26,p, M.G.
Chapman50, M. Charles10,44, Ph. Charpentier44, G. Chatzikonstantinidis49, M.
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C. Marin Benito9, M. Marinangeli45, P. Marino45, J. Marks14, P.J. Marshall56,
G. Martellotti28, M. Martinelli44,22,i, D. Martinez Santos43, F. Martinez
Vidal76, A. Massafferri1, M. Materok11, R. Matev44, A. Mathad46, Z. Mathe44,
V. Matiunin36, C. Matteuzzi22, K.R. Mattioli77, A. Mauri46, E. Maurice9,b, B.
Maurin45, M. McCann57,44, A. McNab58, R. McNulty15, J.V. Mead56, B. Meadows61,
C. Meaux8, N. Meinert70, D. Melnychuk33, M. Merk29, A. Merli23,q, E.
Michielin25, D.A. Milanes69, E. Millard52, M.-N. Minard6, O. Mineev36, L.
Minzoni18,g, D.S. Mitzel14, A. Mödden12, A. Mogini10, R.D. Moise57, T.
Mombächer12, I.A. Monroy69, S. Monteil7, M. Morandin25, G. Morello20, M.J.
Morello26,t, J. Moron32, A.B. Morris8, R. Mountain63, F. Muheim54, M.
Mukherjee68, M. Mulder29, D. Müller44, J. Müller12, K. Müller46, V. Müller12,
C.H. Murphy59, D. Murray58, P. Naik50, T. Nakada45, R. Nandakumar53, A.
Nandi59, T. Nanut45, I. Nasteva2, M. Needham54, N. Neri23,q, S. Neubert14, N.
Neufeld44, R. Newcombe57, T.D. Nguyen45, C. Nguyen-Mau45,n, S. Nieswand11, R.
Niet12, N. Nikitin37, N.S. Nolte44, A. Oblakowska-Mucha32, V. Obraztsov41, S.
Ogilvy55, D.P. O’Hanlon17, R. Oldeman24,f, C.J.G. Onderwater71, J. D.
Osborn77, A. Ossowska31, J.M. Otalora Goicochea2, T. Ovsiannikova36, P.
Owen46, A. Oyanguren76, P.R. Pais45, T. Pajero26,t, A. Palano16, M. Palutan20,
G. Panshin75, A. Papanestis53, M. Pappagallo54, L.L. Pappalardo18,g, W.
Parker62, C. Parkes58,44, G. Passaleva19,44, A. Pastore16, M. Patel57, C.
Patrignani17,e, A. Pearce44, A. Pellegrino29, G. Penso28, M. Pepe Altarelli44,
S. Perazzini17, D. Pereima36, P. Perret7, L. Pescatore45, K. Petridis50, A.
Petrolini21,h, A. Petrov72, S. Petrucci54, M. Petruzzo23,q, B. Pietrzyk6, G.
Pietrzyk45, M. Pikies31, M. Pili59, D. Pinci28, J. Pinzino44, F. Pisani44, A.
Piucci14, V. Placinta34, S. Playfer54, J. Plews49, M. Plo Casasus43, F.
Polci10, M. Poli Lener20, M. Poliakova63, A. Poluektov8, N. Polukhina73,c, I.
Polyakov63, E. Polycarpo2, G.J. Pomery50, S. Ponce44, A. Popov41, D.
Popov49,13, S. Poslavskii41, E. Price50, C. Prouve43, V. Pugatch48, A. Puig
Navarro46, H. Pullen59, G. Punzi26,p, W. Qian4, J. Qin4, R. Quagliani10, B.
Quintana7, N.V. Raab15, B. Rachwal32, J.H. Rademacker50, M. Rama26, M. Ramos
Pernas43, M.S. Rangel2, F. Ratnikov39,74, G. Raven30, M. Ravonel Salzgeber44,
M. Reboud6, F. Redi45, S. Reichert12, F. Reiss10, C. Remon Alepuz76, Z. Ren3,
V. Renaudin59, S. Ricciardi53, S. Richards50, K. Rinnert56, P. Robbe9, A.
Robert10, A.B. Rodrigues45, E. Rodrigues61, J.A. Rodriguez Lopez69, M.
Roehrken44, S. Roiser44, A. Rollings59, V. Romanovskiy41, A. Romero Vidal43,
J.D. Roth77, M. Rotondo20, M.S. Rudolph63, T. Ruf44, J. Ruiz Vidal76, J.J.
Saborido Silva43, N. Sagidova35, B. Saitta24,f, V. Salustino Guimaraes65, C.
Sanchez Gras29, C. Sanchez Mayordomo76, B. Sanmartin Sedes43, R.
Santacesaria28, C. Santamarina Rios43, M. Santimaria20,44, E. Santovetti27,j,
G. Sarpis58, A. Sarti20,k, C. Satriano28,s, A. Satta27, M. Saur4, D.
Savrina36,37, S. Schael11, M. Schellenberg12, M. Schiller55, H. Schindler44,
M. Schmelling13, T. Schmelzer12, B. Schmidt44, O. Schneider45, A. Schopper44,
H.F. Schreiner61, M. Schubiger45, S. Schulte45, M.H. Schune9, R. Schwemmer44,
B. Sciascia20, A. Sciubba28,k, A. Semennikov36, E.S. Sepulveda10, A.
Sergi49,44, N. Serra46, J. Serrano8, L. Sestini25, A. Seuthe12, P. Seyfert44,
M. Shapkin41, T. Shears56, L. Shekhtman40,x, V. Shevchenko72, E. Shmanin73,
B.G. Siddi18, R. Silva Coutinho46, L. Silva de Oliveira2, G. Simi25,o, S.
Simone16,d, I. Skiba18, N. Skidmore14, T. Skwarnicki63, M.W. Slater49, J.G.
Smeaton51, E. Smith11, I.T. Smith54, M. Smith57, M. Soares17, l. Soares
Lavra1, M.D. Sokoloff61, F.J.P. Soler55, B. Souza De Paula2, B. Spaan12, E.
Spadaro Norella23,q, P. Spradlin55, F. Stagni44, M. Stahl14, S. Stahl44, P.
Stefko45, S. Stefkova57, O. Steinkamp46, S. Stemmle14, O. Stenyakin41, M.
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Zonneveld54, S. Zucchelli17,e.

1Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil
2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil
3Center for High Energy Physics, Tsinghua University, Beijing, China
4University of Chinese Academy of Sciences, Beijing, China
5Institute Of High Energy Physics (ihep), Beijing, China
6Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IN2P3-LAPP, Annecy,
France
7Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France
8Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
9LAL, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay, France
10LPNHE, Sorbonne Université, Paris Diderot Sorbonne Paris Cité, CNRS/IN2P3,
Paris, France
11I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany
12Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
13Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany
14Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg,
Germany
15School of Physics, University College Dublin, Dublin, Ireland
16INFN Sezione di Bari, Bari, Italy
17INFN Sezione di Bologna, Bologna, Italy
18INFN Sezione di Ferrara, Ferrara, Italy
19INFN Sezione di Firenze, Firenze, Italy
20INFN Laboratori Nazionali di Frascati, Frascati, Italy
21INFN Sezione di Genova, Genova, Italy
22INFN Sezione di Milano-Bicocca, Milano, Italy
23INFN Sezione di Milano, Milano, Italy
24INFN Sezione di Cagliari, Monserrato, Italy
25INFN Sezione di Padova, Padova, Italy
26INFN Sezione di Pisa, Pisa, Italy
27INFN Sezione di Roma Tor Vergata, Roma, Italy
28INFN Sezione di Roma La Sapienza, Roma, Italy
29Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands
30Nikhef National Institute for Subatomic Physics and VU University Amsterdam,
Amsterdam, Netherlands
31Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of
Sciences, Kraków, Poland
32AGH - University of Science and Technology, Faculty of Physics and Applied
Computer Science, Kraków, Poland
33National Center for Nuclear Research (NCBJ), Warsaw, Poland
34Horia Hulubei National Institute of Physics and Nuclear Engineering,
Bucharest-Magurele, Romania
35Petersburg Nuclear Physics Institute NRC Kurchatov Institute (PNPI NRC KI),
Gatchina, Russia
36Institute of Theoretical and Experimental Physics NRC Kurchatov Institute
(ITEP NRC KI), Moscow, Russia, Moscow, Russia
37Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow,
Russia
38Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS),
Moscow, Russia
39Yandex School of Data Analysis, Moscow, Russia
40Budker Institute of Nuclear Physics (SB RAS), Novosibirsk, Russia
41Institute for High Energy Physics NRC Kurchatov Institute (IHEP NRC KI),
Protvino, Russia, Protvino, Russia
42ICCUB, Universitat de Barcelona, Barcelona, Spain
43Instituto Galego de Física de Altas Enerxías (IGFAE), Universidade de
Santiago de Compostela, Santiago de Compostela, Spain
44European Organization for Nuclear Research (CERN), Geneva, Switzerland
45Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL),
Lausanne, Switzerland
46Physik-Institut, Universität Zürich, Zürich, Switzerland
47NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine
48Institute for Nuclear Research of the National Academy of Sciences (KINR),
Kyiv, Ukraine
49University of Birmingham, Birmingham, United Kingdom
50H.H. Wills Physics Laboratory, University of Bristol, Bristol, United
Kingdom
51Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
52Department of Physics, University of Warwick, Coventry, United Kingdom
53STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
54School of Physics and Astronomy, University of Edinburgh, Edinburgh, United
Kingdom
55School of Physics and Astronomy, University of Glasgow, Glasgow, United
Kingdom
56Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom
57Imperial College London, London, United Kingdom
58School of Physics and Astronomy, University of Manchester, Manchester,
United Kingdom
59Department of Physics, University of Oxford, Oxford, United Kingdom
60Massachusetts Institute of Technology, Cambridge, MA, United States
61University of Cincinnati, Cincinnati, OH, United States
62University of Maryland, College Park, MD, United States
63Syracuse University, Syracuse, NY, United States
64Laboratory of Mathematical and Subatomic Physics , Constantine, Algeria,
associated to 2
65Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de
Janeiro, Brazil, associated to 2
66South China Normal University, Guangzhou, China, associated to 3
67School of Physics and Technology, Wuhan University, Wuhan, China, associated
to 3
68Institute of Particle Physics, Central China Normal University, Wuhan,
Hubei, China, associated to 3
69Departamento de Fisica , Universidad Nacional de Colombia, Bogota, Colombia,
associated to 10
70Institut für Physik, Universität Rostock, Rostock, Germany, associated to 14
71Van Swinderen Institute, University of Groningen, Groningen, Netherlands,
associated to 29
72National Research Centre Kurchatov Institute, Moscow, Russia, associated to
36
73National University of Science and Technology “MISIS”, Moscow, Russia,
associated to 36
74National Research University Higher School of Economics, Moscow, Russia,
associated to 39
75National Research Tomsk Polytechnic University, Tomsk, Russia, associated to
36
76Instituto de Fisica Corpuscular, Centro Mixto Universidad de Valencia -
CSIC, Valencia, Spain, associated to 42
77University of Michigan, Ann Arbor, United States, associated to 63
78Los Alamos National Laboratory (LANL), Los Alamos, United States, associated
to 63

aUniversidade Federal do Triângulo Mineiro (UFTM), Uberaba-MG, Brazil
bLaboratoire Leprince-Ringuet, Palaiseau, France
cP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS),
Moscow, Russia
dUniversità di Bari, Bari, Italy
eUniversità di Bologna, Bologna, Italy
fUniversità di Cagliari, Cagliari, Italy
gUniversità di Ferrara, Ferrara, Italy
hUniversità di Genova, Genova, Italy
iUniversità di Milano Bicocca, Milano, Italy
jUniversità di Roma Tor Vergata, Roma, Italy
kUniversità di Roma La Sapienza, Roma, Italy
lAGH - University of Science and Technology, Faculty of Computer Science,
Electronics and Telecommunications, Kraków, Poland
mLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain
nHanoi University of Science, Hanoi, Vietnam
oUniversità di Padova, Padova, Italy
pUniversità di Pisa, Pisa, Italy
qUniversità degli Studi di Milano, Milano, Italy
rUniversità di Urbino, Urbino, Italy
sUniversità della Basilicata, Potenza, Italy
tScuola Normale Superiore, Pisa, Italy
uUniversità di Modena e Reggio Emilia, Modena, Italy
vH.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom
wMSU - Iligan Institute of Technology (MSU-IIT), Iligan, Philippines
xNovosibirsk State University, Novosibirsk, Russia
ySezione INFN di Trieste, Trieste, Italy
zSchool of Physics and Information Technology, Shaanxi Normal University
(SNNU), Xi’an, China
aaPhysics and Micro Electronic College, Hunan University, Changsha City, China
abLanzhou University, Lanzhou, China
†Deceased