# Charmonium production at mid-rapidity in Pb-Pb and p-Pb collisions with
ALICE

Steffen Georg Weber on behalf of the ALICE Collaboration  Research Division,
GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany
<EMAIL_ADDRESS>
###### Abstract

We present an overview of the ALICE measurements on the production of
$\mathrm{J/}\psi$ in Pb-Pb collisions at a centre-of-mass energy per nucleon
pair of $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{TeV}$ and p-Pb collisions at
$\sqrt{s_{\mathrm{NN}}}=5.02\,\mathrm{TeV}$ at mid-rapidity
($|y_{\mathrm{lab}}|<0.8$) down to zero transverse momentum. The cold nuclear
matter effects estimated from the p-Pb measurements and their impact on the
interpretation of Pb-Pb results are discussed, based on comparison of data to
model calculations.

##  1 Introduction

Charmonium production has been proposed as a sensitive probe of the hot and
dense medium created in ultra-relativistic heavy-ion collisions. The original
idea [1] was that in nucleus-nucleus collisions the binding potential between
the charm and anticharm quarks is screened due to the high density of colour
charges via a mechanism similar to the Debye screening. Results at SPS [2] and
RHIC [3] energies indeed showed a suppression of the $\mathrm{J/}\psi$ yield
in most central collisions, beyond what was expected from cold nuclear matter
(CNM) effects.

At LHC energies, charm-anticharm quark pairs are abundantly produced in
initial hard partonic processes. For this reason, it has been suggested that a
new production mechanism based on the (re)combination of independently
produced charm and anticharm quarks to charmonium pairs becomes relevant.
These models assume either a creation of charmonium states at the
hadronization stage, like the Statistical Hadronization Model (SHM) [4] or a
continuous dissociation and regeneration throughout the lifetime of the hot
medium, addressed in transport models [5].

The experimental observable used to quantify modifications of charmonium
production in nucleus-nucleus collisions compared to proton-proton collisions
is the nuclear modification factor, $R_{\mathrm{AA}}$, defined as

$R_{\mathrm{AA}}=\frac{1}{\langle
T_{\mathrm{AA}}\rangle}\frac{\mathrm{d}^{2}N_{\mathrm{AA}}^{\mathrm{J/}\psi}/\mathrm{d}p_{\mathrm{T}}\mathrm{d}y}{\mathrm{d}^{2}\sigma_{\mathrm{pp}}^{\mathrm{J/}\psi}/\mathrm{d}p_{\mathrm{T}}\mathrm{d}y},$

where $T_{\mathrm{AA}}$ is the nuclear overlap function determined by Glauber
model calculations.

In addition to hot medium effects, also cold nuclear matter effects can
influence the production of charmonia, the most relevant ones at LHC energies
are listed below. The parton distribution functions (PDF) in a nucleus are
modified compared to the one in a free nucleon, this effect is known as
shadowing [6]. There is a limited knowledge on gluon PDFs in heavy ion
collisions at the moment, new data on charmonium production can provide
further constraints.

Figure 1: Left: The nuclear modification factor $R_{\mathrm{AA}}$ measured by
ALICE in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{TeV}$ as a
function of the mean number of participants $\langle
N_{\mathrm{part}}\rangle$[7], compared with PHENIX [8] results in Au-Au
collisions at $\sqrt{s_{\mathrm{NN}}}=200\,\mathrm{GeV}$. Right:
$R_{\mathrm{AA}}$ as function of transverse momentum [9], compared with
measurements from CMS [10] and PHENIX [3] and with theoretical models [11,
12].

Another way to describe the depletion of the nuclear gluon PDFs in the
kinematical domain relevant for the charmonium production is via gluon
saturation, which is addressed in the Color Glass Condensate (CGC) model [13].
Other theoretical models take into account energy loss mechanisms on the
partonic level, before and after the hard collision [14].

The cold nuclear matter effects are experimentally accessible via proton-
nucleus collisions, where the creation of a hot medium is not expected. These
measurements are thus crucial for the interpretation of data from nucleus-
nucleus collisions.

##  2 Experimental Results

ALICE has measured inclusive $\mathrm{J/}\psi$ production at mid-rapidity in
the dielectron decay channel
$\mathrm{J/}\psi\rightarrow\mathrm{e}^{+}\mathrm{e}^{-}$ and in forward
rapidity in the dimuon decay channel
$\mathrm{J/}\psi\rightarrow\mu^{+}\mu^{-}$. In both cases, $\mathrm{J/}\psi$
are reconstructed down to $p_{\mathrm{T}}=0$. Here, measurements at mid-
rapidity will be discussed. The pseudorapidity coverage for measurements in
Pb-Pb collisions is $|\eta|<0.8$, for p-Pb collisions, the coverage is shifted
to $-1.37<\eta_{\mathrm{cms}}<0.43$ in the centre-of-mass-frame due to the
different per nucleon energy of the two collision partners. The main detectors
which have been used for the presented results are the Inner Tracking System
(ITS) for collision vertex determination and tracking, the Time Projection
Chamber (TPC), for tracking and particle identification, and the V0 and Zero
Degree Calorimeter (ZDC) detectors for centrality determination and event
selection.

The raw $\mathrm{J/}\psi$ yield has been obtained using the bin counting
method from the invariant mass distribution of dielectron pairs in the window
$2.92<m_{\mathrm{ee}}<3.16\,\mathrm{GeV}/c^{2}$ after combinatorial background
subtraction using the mixed-event technique. For the calculation of the
nuclear modification factor, the pp reference was obtained with an
interpolation procedure including data at mid-rapidity from ALICE [15], PHENIX
[16] and CDF [17].

The presented results from Pb-Pb collisions are based on data taken in 2010
and 2011 at a collision energy of $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{TeV}$
with an integrated luminosity
$\mathcal{L}_{\mathrm{int}}=27.7\,\mu\mathrm{b}^{-1}$, the results from p-Pb
collisions are based on data from 2013 at a collision energy of
$\sqrt{s_{\mathrm{NN}}}=5.02\,\mathrm{TeV}$ with
$\mathcal{L}_{\mathrm{int}}=51.4\,\mu\mathrm{b}^{-1}$. More details of the
analysis can be found in [7] for the Pb-Pb analysis and in [18] for the p-Pb
analysis.

Figure 2: Left: The nuclear modification factor $R_{\mathrm{pPb}}$ measured by
ALICE in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\,\mathrm{TeV}$ as a
function of $p_{\mathrm{T}}$ [18], together with model calculations [19, 20,
21]. Right: Squared nuclear modification factor $R_{\mathrm{pPb}}^{2}$ as an
estimate for CNM effects in Pb-Pb collisions and $R_{\mathrm{AA}}$from Pb-Pb
collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{TeV}$.

The left panel of Fig. 1 shows the $p_{\mathrm{T}}$-integrated nuclear
modification factor $R_{\mathrm{AA}}$ from Pb-Pb collisions at mid-rapidity as
a function of the average number of participating nucleons $\langle
N_{\mathrm{part}}\rangle$. The ALICE results [7] are compared to measurements
by PHENIX in Au-Au collisions at $\sqrt{s_{\mathrm{NN}}}=200\,\mathrm{GeV}$
[8]. The results for central collisions are significantly different, with the
ALICE data showing a factor of three less suppression than the PHENIX results.
Also the PHENIX results indicate a strong centrality dependence, with a
suppression increasing with centrality, whereas the ALICE results are
compatible with no centrality dependence. The same behaviour is seen at
forward rapidity [22]. The pt dependence of the $\mathrm{J/}\psi$
$R_{\mathrm{AA}}$ for the 40% most central collisions is shown in the right
panel of Fig. 1. Our data is compared to PHENIX results in Au-Au collisions at
$\sqrt{s_{\mathrm{NN}}}=200\,\mathrm{GeV}$ [3], CMS results at higher
$p_{\mathrm{T}}$ [10] and model calculations [11, 12]. The ALICE data suggests
a much lower degree of suppression at low $p_{\mathrm{T}}$ compared to lower
energy measurements. Our data is well described by transport model predictions
by Zhao et al. [12]. Newer predictions by Zhou et al. [11] are systematically
below the measurement and exhibit a $p_{\mathrm{T}}$ dependence similar to the
one in the data. The model calculations show substantial theoretical
uncertainties due to the limited knowledge of charm cross section and cold
nuclear matter effects at LHC energies.

Another experimental observable adopted to gain further insight on charmonium
production mechanisms is the mean transverse momentum $\langle
p_{\mathrm{T}}\rangle$ of $\mathrm{J/}\psi$ produced in different collisions
systems. ALICE measurements [9] have shown that the $\langle
p_{\mathrm{T}}\rangle$ for Pb-Pb collisions is significantly smaller than that
for pp collisions. Such a behaviour is not observed at smaller centre-of-mass
energies from meausrements by PHENIX and NA50, for which no significant system
size dependence of $\langle p_{\mathrm{T}}\rangle$ is seen. This might
indicate the onset of processes which either deplete the high $p_{\mathrm{T}}$
region or enhance the $\mathrm{J/}\psi$ production at low $p_{\mathrm{T}}$ in
heavy-ion collisions at the LHC. The latter effect would be expected as a
consequence of a significant contribution from charmonium recombination.

On the left panel of Fig. 2 the nuclear modification factor in p-Pb collisions
$R_{\mathrm{pPb}}$ is shown as a function of $p_{\mathrm{T}}$, together with
model calculations. These consider either pure gluon shadowing effects [19],
gluon saturation [20] or partonic energy loss, with or without inclusion of
nuclear shadowing contributions [21]. All considered models describe the data
reasonably well.

In order to compare the results obtained in Pb-Pb and in p-Pb collisions, it
is important to notice that for the investigated energies and rapidity
regions, the probed Bjorken-$x$ ranges are comparable. Using 2 $\rightarrow$ 1
($gg\rightarrow\mathrm{J/}\psi{}$) kinematics, in Pb-Pb collisions the probed
range is $7.0\times 10^{-4}<x<3.5\times 10^{-3}$ and in p-Pb collisions
$6.1\times 10^{-4}<x<3.0\times 10^{-3}$. Under the assumption that shadowing
is the main CNM-related mechanism that plays a role in the charmonium
production and that its effect on the two colliding nuclei in Pb-Pb collisions
can be factorized, the squared nuclear modification factor
$R_{\mathrm{pPb}}^{2}$ can be considered as an estimate of CNM effects in Pb-
Pb collisions [23, 24].

The right panel of Fig. 2 shows $R_{\mathrm{pPb}}^{2}$ together with
$R_{\mathrm{AA}}$ as a function of $p_{\mathrm{T}}$. The
$R_{\mathrm{pPb}}^{2}$ shows a clear $p_{\mathrm{T}}$ dependence with a
stronger effect at low $p_{\mathrm{T}}$, whereas the $R_{\mathrm{AA}}$ shows
the largest suppression at high $p_{\mathrm{T}}$. These differences suggest
that the $\mathrm{J/}\psi$ suppression in Pb-Pb collisions cannot be explained
by CNM effects alone. The data at low $p_{\mathrm{T}}$ hint at the presence of
charmonium production in Pb-Pb consistent with (re)combination scenarios,
whereas at high $p_{\mathrm{T}}$ suppression effects seem to be present in the
hot medium.

##  3 Conclusions

Measurements by ALICE of inclusive $\mathrm{J/}\psi$ production at mid-
rapidity as a function of centrality and transverse momentum in Pb-Pb
collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{TeV}$ and in p-Pb
collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\,\mathrm{TeV}$ have been presented.
Uniquely at the LHC, these measurements reach down to $p_{\mathrm{T}}=0$. In
Pb-Pb collisions, a suppression of $\mathrm{J/}\psi$ production is observed,
with no strong centrality dependence and a weak suppression at low transverse
momentum. The results are in contrast to measurements at RHIC energies, which
show an increase of the suppression with centrality, especially at low
$p_{\mathrm{T}}$. Our data can be well described by theoretical models which
take into account a (re)combination component in the $\mathrm{J/}\psi$
production.

In p-Pb collisions, a suppression of $\mathrm{J/}\psi$ production at low
transverse momenta was observed, which can be well described by models taking
into account either shadowing effects, gluon saturation, or partonic energy
loss. An estimate of CNM effects in Pb-Pb via the squared nuclear modification
factor $R_{\mathrm{pPb}}^{2}$ in p-Pb collisions underlines the importance of
hot nuclear matter effects for the interpretation of the results in Pb-Pb
collisions and supports the idea of charmonium production in the hot medium or
at hadronization.

## References

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