Published for SISSA by
Springer
Received: April 28, 2015
Accepted: June 17, 2015
Published: July 10, 2015
NN
The ALICE collaboration
E-mail: ALICE-publications@cern.ch
Abstract: The transverse momentum (pT ) dependence of the nuclear modification factor
RAA and the centrality dependence of the average transverse momentum hpT i for inclusive
√
J/ψ have been measured with ALICE for Pb-Pb collisions at sNN = 2.76 TeV in the
e+ e− decay channel at mid-rapidity (|y| < 0.8). The hpT i is significantly smaller than the
one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase
of RAA is observed towards low pT . These observations might be indicative of a sizable
contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction
of non-prompt J/ψ from beauty hadron decays, fB , has been determined in the region
1.5 < pT < 10 GeV/c in three centrality intervals. No significant centrality dependence of
fB is observed. Finally, the RAA of non-prompt J/ψ is discussed and compared with model
predictions. The nuclear modification in the region 4.5 < pT < 10 GeV/c is found to be
stronger than predicted by most models.
Keywords: Hadron-Hadron Scattering
ArXiv ePrint: 1504.07151
Open Access, Copyright CERN,
for the benefit of the ALICE Collaboration.
Article funded by SCOAP3 .
doi:10.1007/JHEP07(2015)051
JHEP07(2015)051
Inclusive, prompt and non-prompt J/ψ production at
√
mid-rapidity in Pb-Pb collisions at s = 2.76 TeV
Contents
1 Introduction
1
2 Data analysis
2.1 Inclusive J/ψ
2.2 Non-prompt J/ψ
2
3
8
13
4 Conclusions
19
The ALICE collaboration
26
1
Introduction
Heavy-ion collisions at high energies allow the study of strongly interacting matter under
extreme conditions. Calculations based on Quantum-Chromo-Dynamics (QCD) on the
lattice indicate that the hot and dense medium created in these collisions behaves like
a strongly coupled Quark-Gluon Plasma (QGP) [1–4]. Heavy quarks are an important
probe for the properties of this state of matter, since they are produced via hard partonic
collisions at a very early stage and thus experience the complete evolution of the system.
Quarkonium states, i.e. bound states of a heavy quark and anti-quark such as the J/ψ meson
(cc̄ state) are of particular interest. It was predicted that the J/ψ formation is suppressed
in a QGP due to the screening of the cc̄ potential in the presence of free colour charges [5].
Experimentally, a suppression of the inclusive J/ψ yield in heavy-ion collisions relative to
the corresponding yield in pp, scaled by the number of binary nucleon-nucleon collisions, has
been observed at the Super Proton Synchrotron (SPS) [6–8] and the Relativistic Heavy Ion
Collider (RHIC) [9, 10]. The level of suppression was found to be similar at SPS and RHIC,
despite the significantly different collision energy. More recently, the nuclear modification
of J/ψ was also measured for Pb-Pb collisions at the LHC [11–13]. While at high transverse
momentum (pT > 4 GeV/c) the suppression factor is at the same level as the one observed
at RHIC in the low pT region, a significant reduction of the suppression is measured
towards lower pT . This has been interpreted as the effect of an additional contribution
to J/ψ production at low pT , due to the combination of correlated or uncorrelated c and
c̄ quarks [14, 15]. This contribution becomes sizable at LHC energies, since the number
of cc̄ pairs is much higher than at lower energies. Assuming that a deconfined phase is
produced and that all the J/ψ are dissociated, this process happens at the chemical freezeout stage of the fireball evolution. This is the approach followed within the statistical
hadronization models described in refs. [16, 17]. Alternatively, J/ψ could be generated via
coalescence throughout the full evolution of the QGP phase, if their survival probability in
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JHEP07(2015)051
3 Results
2
Data analysis
A detailed description of the ALICE detector can be found in [38]. For the analysis presented here the detectors of the central barrel have been used, in particular the Inner
Tracking System (ITS) and the Time Projection Chamber (TPC). These detectors are
located inside a large solenoidal magnet with a field strength of 0.5 T. They allow the
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JHEP07(2015)051
this environment is large enough. This scenario has been implemented in several partonic
transport models [18, 19]. It was found that both approaches can provide a description of
the measured nuclear modification factors [12] and of the elliptic flow of inclusive J/ψ [20].
The production of open beauty hadrons is expected to be sensitive to the density of the
medium created in heavy-ion collisions due to the energy loss experienced by the parent
parton (a beauty quark) which hadronizes into the beauty hadron. This energy loss is
expected to occur via medium-induced gluon radiation [21, 22] and elastic collisional energy
loss processes [23–25] and it depends on the QCD Casimir coupling factor of the parton
(larger for gluons than for quarks) and on the parton mass [26–29]. Other mechanisms,
such as in-medium hadron formation and dissociation, can be envisaged as particularly
relevant for heavy-flavour hadrons due to their small formation times [30–32].
Inclusive J/ψ production is the sum of several contributions. In addition to the directly produced J/ψ, the decays of heavier charmonium states, such as the χc and ψ(2S),
also contribute to the inclusive J/ψ yield. These two sources (direct and charmonium decays) are defined as prompt J/ψ, where the contribution from charmonium decays is about
35% as measured in pp collisions [33]. Since heavier charmonia are less strongly bound
than the J/ψ they should be more easily dissolved in a deconfined medium [34]. The J/ψ
suppression measured at the SPS is indeed compatible with the assumption that only the
excited states are dissolved and not the directly produced J/ψ [6, 7]. On top of the prompt
J/ψ production, there is an additional non-prompt contribution to the inclusive J/ψ at
high centre-of-mass energies, coming from the decay of beauty hadrons. Since these decays
proceed via weak interactions, the resulting J/ψ will originate from a decay vertex that is
displaced from the main interaction vertex. Their measurement provides a direct determination of the nuclear modification of beauty hadrons. By subtracting the non-prompt
contribution from the inclusive J/ψ yield one can also provide an unbiased information
on medium modification of prompt charmonia. The non-prompt J/ψ contribution at mid√
rapidity has already been measured in pp collisions at s = 7 TeV by ATLAS [35], CMS [36]
√
and ALICE [37]. For Pb-Pb collisions at sNN = 2.76 TeV CMS has also published prompt
and non-prompt J/ψ production results at mid-rapidity for pT > 6.5 GeV/c [13].
In this paper we present a differential measurement of the inclusive J/ψ production at
√
mid-rapidity in Pb-Pb collisions at sNN = 2.76 TeV. The pT dependence of the nuclear
modification factor and the centrality dependence of the average transverse momentum of
J/ψ have been obtained, extending the set of results presented in [12]. A measurement of
the prompt and non-prompt contributions to the inclusive J/ψ production is also presented.
The nuclear modification factor of non-prompt J/ψ is determined down to pT = 1.5 GeV/c
and compared to model predictions.
2.1
Inclusive J/ψ
J/ψ candidates are reconstructed by combining opposite-sign (OS) pairs of electron/positron candidates and calculating their invariant mass mee . These candidates are
selected from tracks reconstructed in the ITS and the TPC by employing the set of quality
criteria described in [12, 44]. In order to reject the background from photon conversions
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JHEP07(2015)051
measurement of J/ψ mesons via the dielectron decay channel in the central rapidity region
down to zero pT . The ITS [39] consists of six layers of silicon detectors surrounding the
beam pipe at radial positions between 3.9 cm and 43.0 cm. Its two innermost layers are
composed of Silicon Pixel Detectors (SPD), which provide the spatial resolution to separate
on a statistical basis the non-prompt J/ψ. The active volume of the TPC [40] covers the
range along the beam direction −250 < z < 250 cm relative to the Interaction Point (IP)
and extends in radial direction from 85 cm to 247 cm. It is the main tracking device in the
central barrel and is in addition used for particle identification via the measurement of the
specific ionization (dE/dx) in the detector gas.
Triggering and event characterization is performed via forward detectors, the V0 [41]
and two Zero Degree Calorimeters (ZDC) [42]. The V0 detectors consist of two scintillator
arrays positioned at z = −90 cm and z = +340 cm and cover the pseudo-rapidity ranges
−3.7 ≤ η ≤ −1.7 and 2.8 ≤ η ≤ 5.1. The ZDCs, each one consisting of two quartz fiber
sampling calorimeters, are placed at a distance of 114 m relative to the IP in both directions
along the beam axis and are used to detect spectator nucleons.
The results presented in this article are based on data samples collected during the
Pb-Pb data taking periods of the LHC in the years 2010 and 2011. In the case of the
2011 data sample the Minimum Bias (MB) Level-0 (L0) trigger condition was defined by
the coincidence of signals in both V0 detectors along with a valid bunch crossing trigger.
For the 2010 data sample, in addition, the detection of at least two hits in the ITS was
required. Both MB trigger definitions lead to trigger efficiencies larger than 95% for inelastic Pb-Pb collisions. Electromagnetic interactions were rejected by the Level-1 (L1)
trigger, which required a minimum energy deposition in the ZDC by spectator neutrons.
The beam-induced background was further reduced during the offline analysis by selecting
events according to the relative timing of signals in V0 and ZDC. The offline centrality
selection is done using the sum of the two V0 signal amplitudes. By fitting the corresponding distribution with the results of Glauber model simulations, the average number
inel for a
of participants hNpart i and the average nuclear overlap function hTAA i = hNcoll i/σNN
given centrality class can be determined as described in [43]. Here, hNcoll i is the average
inel the inelastic nucleon-nucleon cross
number of binary nucleon-nucleon collisions and σNN
section. The numerical values for hNpart i, hNcoll i, and hTAA i are tabulated in [12].
The 2010 data sample consists of 1.5 × 107 events, taken with the corresponding MB
trigger. The 2011 event sample was enriched with central and semi-central Pb-Pb collisions
by using thresholds on the V0 multiplicity at the L0 trigger. From the latter data set we
analyzed 1.9 × 107 central (0–10% of the centrality distribution) and 1.7 × 107 semi-central
(10–50%) events. The summed 2010 and 2011 data samples correspond to an integrated
−1 [12].
luminosity of Lint = 26.4 ± 0.3(stat.)+2.1
−1.7 (syst.) µb
Measurement of the inclusive J/ψ yield. The J/ψ signal counts NJ/ψ are obtained
from the number of entries in the background subtracted invariant mass distributions in
the range 2.92 < mee < 3.16 GeV/c2 . The uncorrelated background is evaluated with a
mixed event (ME) technique. In order to achieve a good description of the background
only electrons and positrons from events with similar properties in terms of centrality,
primary vertex position, and event plane angle are combined. The ME distributions are
scaled to the same event (SE) distributions in the mass ranges 1.5 < mee < 2.5 GeV/c2
and 3.2 < mee < 4.2 GeV/c2 , so that the J/ψ signal region is excluded. The normalization
area contains the ψ(2S) signal, but its contribution is negligible and can therefore be
safely ignored. Also, contributions from the tail of the J/ψ signal shape to this mass
interval are below the percent level and will thus not significantly affect the normalization.
Figure 1 shows a comparison of the SE and ME invariant mass distributions for the 0–40%
most central Pb-Pb collisions for electron-positron pairs at mid-rapidity (|y| < 0.8) in
two pT intervals: 0–2.5 GeV/c and 2.5–6 GeV/c. The agreement between the SE and
ME distributions outside the signal region is very good and allows signal extraction with
significances larger than eight.
The J/ψ yield per MB event in a given pT interval, YJ/ψ , is obtained as
YJ/ψ (pT ) =
BRee
NJ/ψ (pT )
.
Nevts hA × i(pT )
(2.1)
Here BRee is the branching ratio for the decay J/ψ → e+ e− , Nevts the number of events,
and hA × i the phase space dependent product of acceptance A and reconstruction efficiency . The latter is calculated from Monte Carlo (MC) simulations as the ratio between
the number of reconstructed and generated MC J/ψ, which are assumed to be unpolar√
ized. In pp collisions at s = 7 TeV the J/ψ polarization has been measured and was
found to be compatible with zero at mid-rapidity (pT > 10 GeV/c) and forward rapidity
(pT > 2 GeV/c) [46–48]. In heavy-ion collisions no measurement exists, but J/ψ mesons
produced from the recombination of charm quarks in the medium are expected to be un-
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JHEP07(2015)051
in the detector material, tracks are required to have a hit in one of the SPD layers. In
addition, at least 70 out of a maximum of 159 space points reconstructed in the TPC must
be assigned to a given track, which also needs to fulfill a quality criterion of the track fit
(χ2 /ndf < 4). The tracks are required to be in the range |η| < 0.8, where the tracking and
particle identification performance of the TPC is optimal, and to have pT > 0.85 GeV/c to
improve the signal-to-background ratio in the J/ψ mass region.
Electron candidates are selected by requiring that the dE/dx measurement in the TPC
lies within a band [−1σ, +3σ] around the momentum-dependent parameterization of the
expected signal, where σ is the phase space dependent dE/dx resolution (details can be
found in [45]). The selection is asymmetric in order to minimize the contribution from
pions. To further suppress the hadron contamination, tracks that are compatible within
±4σ with the proton expectation are rejected. A side effect of this cut is that tracks below
pT = 1 GeV/c are effectively removed.
polarized.1 The MC events used for the calculation of hA × i are constructed by adding
to background events, generated with the HIJING model [49], J/ψ mesons decaying into
e+ e− pairs, whose phase space distribution is obtained from extrapolations of other measurements [50], taking into account shadowing effects as parameterized in EKS98 [51]. The
dielectron decay is simulated with the EvtGen [52] package, using the PHOTOS model [53]
to describe the influence of final state radiation. This choice, together with the simulation
of bremsstrahlung in the detector material, is mandatory for a proper description of the
low mass tail in the measured J/ψ mass distribution and ensures that the fraction of the
signal outside of the mee integration window is properly accounted for in the correction
hA × i. The propagation of the simulated particles is done by GEANT3 [54] and a full
simulation of the detector response is performed. The same reconstruction procedure and
cuts are applied to MC events and to real data. The quality of the simulation is illustrated
by the good agreement of the background-subtracted invariant mass distributions with the
1
The impact of the polarization on the acceptance was studied for extreme polarization scenarios in [44].
–5–
JHEP07(2015)051
Figure 1.
The invariant mass distributions of inclusive J/ψ at mid-rapidity (|y| < 0.8) for
√
Pb-Pb collisions (0–40% most central) at sNN = 2.76 TeV. The left panels show the interval
0 < pT < 2.5 GeV/c and the right ones 2.5 < pT < 6 GeV/c. The upper panels display the opposite
sign distributions together with the result of the mixed event procedure. In the lower panels the
background subtracted distributions are shown and compared to the simulated line shape. Also,
the signal-to-background ratio S/B and the significance of the signal are given.
MC simulation of the J/ψ signal shape, after normalizing it to the same integral as the
measured signal (see figure 1).
The analysis has been performed in two slightly different centrality intervals (0–40%
and 0–50%), where the larger one is used for the extraction of non-prompt J/ψ which
requires a higher statistics than the inclusive measurement. Also, the pT intervals have
been optimized for the different analyses. It was checked that the results for inclusive J/ψ
obtained with the two centrality binnings are in good agreement.
RAA (pT ) =
YJ/ψ (pT )
.
pp
hTAA i σJ/ψ
(pT )
(2.2)
Since no differential J/ψ measurement at mid-rapidity at low pT is available for pp collisions
√
at s = 2.76 TeV [55], the reference needed for the construction of RAA is based on an
√
interpolation of the mid-rapidity measurements by PHENIX at s = 0.2 TeV [56], CDF
√
√
at s = 1.96 TeV [57], and ALICE at s = 7 TeV [55]. The interpolated pT distribution
is obtained by fitting the following parameterization to the available data sets [50]
1
d2 σ
zT
=c
2 )n .
dσ/dy dzT dy
(1 + a2 zT
(2.3)
Here, zT is defined as pT /hpT i, a = Γ(3/2) Γ(n − 3/2)/Γ(n − 1), and c = 2(n − 1) a,
where n is the only free fit parameter. The value for hpT i (calculated in the pT range
√
0–10 GeV/c) at s = 2.76 TeV, which is needed to translate this parameterization into
dσ/dpT , is determined by interpolating between the existing hpT i measurements for pp
and pp̄ collisions [55–57]. This interpolation is done using various functional forms for the
√
s dependence to determine the systematic uncertainty. For the absolute normalization
of the parametrized spectrum, the same interpolated value dσ/dy = 4.25 ± 0.28(stat.) ±
0.43(syst.) µb as in [12] is used.
The main sources of systematic uncertainties for the pT dependent RAA of inclusive J/ψ
are the signal reconstruction procedure, the MC input kinematics, the uncertainties on the
interpolated pp reference and on the nuclear overlap function. The corresponding values
are summarized in table 1. While the first two components are uncorrelated between
the pT intervals (type II), the uncertainty due to the nuclear overlap function is fully
correlated (type I). The pp reference on the other hand introduces both uncorrelated and
correlated contributions. To determine the uncertainty related to the signal reconstruction,
the normalization range of the ME background and the size and positions of the mee bins
have been varied. All track and electron selection criteria, such as the electron inclusion
cut and the SPD hit requirement, have been relaxed and/or tightened in order to test the
stability of the result, as was performed in [12]. The value of the systematic uncertainty
is determined as the standard deviation of the distribution of all results obtained with
the listed variations. The evaluation of the uncertainties associated with the MC input
kinematics is also described in [12], while the uncertainty of the pp reference is estimated
–6–
JHEP07(2015)051
Determination of the pp reference for RAA . From the corrected J/ψ yield YJ/ψ (pT )
the nuclear modification factor RAA (pT ) is calculated as
from the differences between the cross-section values obtained with the fitting procedure
based on eq. (2.3) and the measured values used for the fit at the various energies.
Determination of hpT i and hp2T i. Since the collected Pb-Pb statistics would allow the
extraction of the J/ψ yield in a few pT intervals only, the average transverse momentum
hpT i is determined by a fit to the distribution of the hpT i of e+ e− pairs as a function of
mee . When building such a distribution, the individual e+ e− pairs are weighted by the
inverse of their acceptance times efficiency (A × )−1 , assuming that they come from the
decay of a J/ψ. The resulting hpT i distributions are fitted by the expression
hpT imeas =
1
S(mee ) hpT iJ/ψ + B(mee ) hpT iBkg .
S(mee ) + B(mee )
(2.4)
Both factors S and B depend on mee and correspond to the distribution of the J/ψ signal
and of the background. For S the same background subtracted signal distribution S(mee )
is used as for the extraction of the yield (see lower panels of figure 1), while the background
B is generated from the ME sample, as B(mee ) = cB BME (mee ). The normalization factor
is determined by fitting cB BME (mee ) to the corresponding mee distribution of e+ e− pairs in
the regions 1.5 < mee < 2.5 GeV/c2 and 3.2 < mee < 4.2 GeV/c2 , thus excluding the signal
region. For the sum S(mee )+B(mee ) in the denominator of eq. (2.4), the measured OS pair
mee distribution is used. The hpT iBkg , defined as the hpT i of the combinatorial background
pairs, is also calculated from the ME sample. This analysis is performed in three different
centrality intervals: 0–10%, 10–40%, and 40–90%. Figure 2 shows the measured hpT i of
the e+ e− pairs in the pT range 0–10 GeV/c together with the results of the fit procedure.
In addition, with an equivalent method, the mean square transverse momentum hp2T i is
also calculated for the same centrality intervals.
The systematic uncertainties of the hpT i measurement for inclusive J/ψ are mainly
determined by the signal extraction, the stability of track and electron selection criteria
and the fit procedure (see table 2). While the first two components are not correlated
between the different centrality intervals (type II), the systematic uncertainty intrinsic to
the fit procedure can affect the data points in a correlated way (type I). The uncertainties
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JHEP07(2015)051
Figure 2. The average transverse momentum hpT i of e+ e− pairs, measured for the pT range 0–
√
10 GeV/c, as a function of the invariant mass mee in centrality selected Pb-Pb collisions at sNN =
2.76 TeV. The shown uncertainties are statistical only. The background hpT i distributions and the
total fit results are also shown superimposed to the data points.
Source
Signal reconstruction
MC input kinematics
Nuclear overlap function hTAA i
pp reference
Type
II
II
I
I
II
I
II
Table 1. The correlated (type I) and uncorrelated (type II) systematic uncertainties (in percent)
on the measurement of the nuclear modification factor RAA of inclusive J/ψ for several pT intervals
√
in Pb-Pb collisions (0–40% and 0–50% most central) at sNN = 2.76 TeV.
Source
Signal extraction
Track selection
Stability of the fit procedure (from MC)
Total
Centrality
0–10% 10–40% 40–90%
2.0
1.6
4.5
3.1
3.9
13.7
2.0
2.0
2.0
2.0
2.0
2.0
3.7
4.2
14.4
type
II
II
I
I
II
Table 2. The correlated (type I) and uncorrelated (type II) systematic uncertainties (in percent)
on the measurement of the average transverse momentum hpT i of inclusive J/ψ in three centrality
√
intervals in Pb-Pb collisions at sNN = 2.76 TeV.
related to the signal extraction have been evaluated by varying the normalization range
of the ME background, the size and positions of the mee bins, the fit region and by using
in addition to the ME background a linear function for the background description. In
addition, the stability of the fit procedure was tested by modifying the approach, e.g. by
using in the fit the direct sum of S and B, instead of the OS pair mee distribution, or by
using a fit function for B, instead of ME. It was also verified by applying the above described
method to MC events, which were constructed by combining signal with background events
with a realistic S/B ratio. It turned out that the procedure allows to recover the hpT i of the
simulated J/ψ mesons within a 2% difference. This value is assumed as correlated (type I)
uncertainty. Finally, the uncertainty in the signal-to-background ratio is propagated into
the statistical uncertainty of the hpT iJ/ψ .
2.2
Non-prompt J/ψ
The candidate selection for the non-prompt J/ψ analysis includes, in addition to the previously described criteria, the condition that at least one of the two decay tracks has a hit
in the first SPD layer, in order to enhance the resolution of secondary vertices.
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JHEP07(2015)051
Total
pT range (GeV/c)
0–40%
0–50%
0–2.5 2.5–6 0–1.5 1.5–4.5 4.5–10
10.9
11.0
22.6
8.5
10.0
3.2
5.9
1.5
4.7
5.3
3.2
3.2
3.8
3.8
3.8
12.1
12.1
12.1
12.1
12.1
4.5
4.9
5.0
4.3
10.1
12.6
12.6
12.7
12.7
12.7
12.3
13.5
23.2
10.6
15.2
The non-prompt J/ψ fraction has been determined using an unbinned two-dimensional
log-likelihood fit described in detail in [37], which is performed by maximizing the quantity
ln L =
N
X
ln [fSig · FSig (x) · MSig (mee ) + (1 − fSig ) · FBkg (x) · MBkg (mee )] ,
(2.5)
where N is the total number of OS candidates in the range 2.2 < mee < 4 GeV/c2 and x is
the pseudo-proper decay length of the candidate
~ · p~T /pT ) mJ/ψ
c (L
.
pT
(2.6)
~ is the vector pointing from the primary vertex to the J/ψ decay vertex and mJ/ψ
Here L
the mass of the J/ψ taken from [58]. FSig (x) and FBkg (x) (MSig (mee ) and MBkg (mee )) are
Probability Density Functions (p.d.f.) describing the pseudo-proper decay length (invariant
mass) distribution for signal and background candidates, respectively. FSig (x) is defined as
FSig (x) = fB0 · FB (x) + (1 − fB0 ) · Fprompt (x) ,
(2.7)
where Fprompt (x) and FB (x) are the p.d.f. for prompt and non-prompt J/ψ, respectively,
and fB0 is the fraction of reconstructed J/ψ coming from beauty hadron decays
fB0 =
NJ/ψ←hB
.
NJ/ψ←hB + Nprompt
(2.8)
A correction due to different average hA × i values, in a given pT interval, for prompt
and non-prompt J/ψ, is necessary to obtain from fB0 the fraction of produced non-prompt
J/ψ, fB
−1
1 − fB0 hA × iB
fB = 1 +
.
(2.9)
fB0 hA × iprompt
The various ingredients for the determination of fB are described in the following:
Monte Carlo pT distributions and polarization assumptions. Assuming both
prompt and non-prompt J/ψ to be unpolarized, at a given pT their acceptance times
efficiency values hA × i are the same. However, the pT distributions of prompt and nonprompt J/ψ can be different, resulting in different average hA × i computed over a pT
range of finite size. Different hypotheses for the kinematical (pT ) distributions of both
prompt and non-prompt J/ψ are considered, i.e. including or excluding shadowing or suppression effects as, e.g., those predicted in references [59–61] for non-prompt J/ψ. Due to
the weak pT dependence of hA × i, the resulting uncertainty on fB is small, being ∼ 5%
at low pT and ∼ 3% in the highest pT bin, and is independent of centrality.
At a given pT , prompt and non-prompt J/ψ can have different polarization and therefore a different acceptance. However, the polarization of J/ψ from b-hadron decays is expected to be small due to the averaging effect caused by the admixture of various exclusive
B → J/ψ +X decay channels. Indeed, in more elementary colliding systems, the sizable polarization, which is observed when the polarization axis refers to the B-meson direction [62],
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JHEP07(2015)051
x=
is strongly smeared when calculated with respect to the direction of the daughter J/ψ [63],
as observed by CDF [64]. The central values of the fraction of non-prompt J/ψ are evaluated with eq. (2.9) assuming unpolarized prompt J/ψ and a polarization of non-prompt
J/ψ as predicted by EVTGEN [52]. The assumption of a null polarization for non-prompt
J/ψ results in a relative decrease of fB by only 1% at high pT (4.5–10 GeV/c) and 3%
at low pT (1.5–4.5 GeV/c). The relative variations of fB expected in extreme scenarios
for the polarization of prompt J/ψ was studied in [44]. The uncertainties related to the
polarization of prompt and non-prompt J/ψ are are not further propagated to the results.
P.d.f. for non-prompt J/ψ: FB (x). The shape of the x distribution of non-prompt
J/ψ is estimated by using PYTHIA 6.4.21 [67] in the Perugia-0 tune [68] to generate beauty
√
hadrons at s = 2.76 TeV, and the EvtGen package [52] to describe their decays. The systematic uncertainty related to this shape is estimated by assuming a softer pT distribution
for the non-prompt J/ψ which is obtained by adding the suppression effects as predicted
√
in [61] and a harder one taken from the same PYTHIA event generator at s = 7 TeV
instead of 2.76 TeV. The resulting systematic uncertainty is within 3–4%.
P.d.f. for the background: FBkg (x). The main difference of the analysis presented in
this section, with respect to previous work on pp collisions [37], concerns the description
of FBkg (x). In this analysis such a function includes an extra symmetric exponential tail
(∝ e−|x|/λsym ) [57] and depends on the invariant mass and the pT of the dielectron pair. It
is determined, for each centrality class, by a fit to the data in three pT regions (1.5–3, 3–4.5,
4.5–10 GeV/c) and in four invariant mass regions on the side-bands of the J/ψ mass peak
(2.2–2.6, 2.6–2.8, 3.16–3.5, 3.5–4 GeV/c2 , labelled with the indices 1, 2, 3 and 4, respectively), for a total of 3 × 4 combinations. The background function in the invariant mass
region 2.8–3.16 GeV/c2 and in each of the three pT ranges are obtained by an interpolation
procedure as the weighted combination of the p.d.f. determined in the other four invariant
mass regions. The weights are chosen inversely proportional to the absolute difference (or
its square) between the mean of the invariant mass distribution in the given mass interval
– 10 –
JHEP07(2015)051
P.d.f. for prompt J/ψ: Fprompt (x). The x distribution Fprompt (x) for prompt J/ψ,
which decay at the primary vertex, coincides with the resolution function R(x), which describes the accuracy by which x can be reconstructed. It also enters in the p.d.f. describing
the x distributions of non-prompt J/ψ, FB (x), and of the background candidates, FBkg (x).
The determination of R(x) is based on the same MC data sample as used for the inclusive
J/ψ analysis (see section 2.1). The systematic uncertainty on R(x) was estimated with a
MC approach by propagating the maximum observed discrepancies of the track parameters (space and momentum variables) between data and MC to the x variable [45, 65, 66]
and was found to be at most 10%. To propagate this systematic uncertainty to the final
results the fits are repeated after modifying in the log-likelihood function the resolution to
(1/(1 + δ)) × R (x/(1 + δ)). In this expression δ parameterizes the relative variation of the
RMS of the resolution function and is varied between −10% and +10%. The systematic
uncertainty due to the resolution function is smaller in the highest pT bin, because of the
better resolution in the x variable and the higher values of the signal-to-background ratio.
and that in the interpolated region
FBkg interp (x) =
4
X
wi FBkg i (x);
wi ∝ |hmee ii − hmee iinterp |−n
(n = 1 or 2). (2.10)
i=1
P.d.f. for the invariant mass distribution of the signal: MSig (mee ). The shape
of the invariant mass distribution for the signal is determined by the same MC simulations
described in section 2.1. The influence of detector material budget is studied with dedicated
MC simulations, where the material budget is varied within its uncertainty (±6%) [69]. The
resulting contribution to the systematic uncertainty on fB slightly increases for central
events, and ranges from 2 to 4%.
P.d.f. for the invariant mass distribution of the background: MBkg (mee ). The
shape of the invariant mass distribution for the background candidates is determined from
ME pairs. The related systematic uncertainty on fB is evaluated using the like-sign distribution, instead of the ME one. The uncertainty increases at higher centrality and in the
lowest pT interval due to the decrease of the S/B ratio.
As an example in figure 3 the projections of the best fit function for n = 1 and w1 =
w4 = 0 are shown superimposed to the invariant mass (upper panel) and x (lower panel)
distributions of the candidates in the centrality range 10–50% for 1.5 < pT < 10 GeV/c.
A summary of the systematic uncertainties on the determination of the non-prompt
J/ψ fraction is provided in table 3 for the three centrality intervals in the integrated pT
range and, in the two pT ranges where the results will be given, for the most central
collisions (0–10%).
The value of fB is determined in two pT bins (1.5–4.5 and 4.5–10 GeV/c) for the 0–
50% centrality range and in three centrality classes (0–10%, 10–40% and 40–90%) for 1.5 <
pT < 10 GeV/c. The fB measurements are then combined with the nuclear modification
factors of inclusive J/ψ to get the non-prompt and prompt J/ψ RAA
non−prompt J/ψ
RAA
=
fBPb−Pb incl.
RAA
fBpp
J/ψ
,
prompt J/ψ
RAA
=
1 − fBPb−Pb incl.
RAA
1 − fBpp
J/ψ
. (2.11)
√
pp interpolation. The value of fB in pp collision at s = 2.76 TeV, fBpp , is needed to
compute the RAA for prompt and non-prompt J/ψ mesons, see eq. (2.11). It is determined
by an interpolation procedure. Therefore, a fit is performed to the existing measurements of
– 11 –
JHEP07(2015)051
Optionally, only the two adjacent mass regions can be considered in the interpolation
procedure, corresponding to the condition w1 = w4 = 0. The central value of fB has been
determined as the average of the values obtained with the different assumptions (n = 1 or
n = 2, with or without the condition w1 = w4 = 0). The RMS of the distributions of the
relative variations obtained for fB is used to define the systematic uncertainty. It becomes
larger for central events and in the lowest pT interval, where the signal-to-background ratio
S/B is lower. This approach allows to cope with the much lower S/B ratio in Pb-Pb than
in pp collisions.
√
fB as a function of pT in mid-rapidity pp collisions at s = 7 TeV (ALICE [37], ATLAS [35]
and CMS [70]). The function used to fit the data is chosen as
, phenom.
FONLL
dσJ/ψ
dσJ/ψ←h
B
model
fB
(pT ) =
,
(2.12)
dpT
dpT
which is the ratio of the differential cross section for non-prompt J/ψ obtained by an
implementation of pQCD calculations at fixed order with next-to leading-log resummation
(FONLL) [71] to that for inclusive J/ψ, parameterized by the phenomenological function
defined in eq. (2.3). A similar fit is then performed to the CDF results [57] in pp̄ collisions
√
√
at s = 1.96 TeV. Finally, the fBpp (pT ) value at s = 2.76 TeV is determined by an
energy interpolation, which gives fBpp = 0.122 ± 0.010 in the integrated pT range 1.5–
– 12 –
JHEP07(2015)051
Figure 3.
The invariant mass (upper panel) and pseudo-proper decay length (lower panel)
distributions for e+ e− pairs with pT > 1.5 GeV/c in Pb-Pb collisions in the centrality interval 10–
√
50% at sNN = 2.76 TeV. The projections of the maximum likelihood fit used to extract fB are
superimposed to the data.
1.5 < pT < 10 GeV/c
Centr. 0–10%
Type
Centr. Centr. Centr.
pT
pT
0–10% 10–40% 40–90% 1.5–4.5 GeV/c 4.5–10 GeV/c
Resolution function
23
15
10
28
12
I
P.d.f. for non-prompt J/ψ
4
3
3
4
3
I
P.d.f. for the background
22
15
5
23
19
II
MC pT distribution
5
5
5
5
3
I
P.d.f. for the invariant mass of signal
5
3
2
5
3
I
P.d.f. for the invariant mass of background
7
5
3
7
5
II
Total
34
23
13
38
24
Source
10 GeV/c. The quoted uncertainty includes: (i) a component from the fit procedure, which
depends on the uncertainties of both data and FONLL predictions; (ii) the systematic
uncertainty due to the energy interpolation, which has been estimated by considering
√
different functional forms of the s dependency (linear, exponential and power law); (iii)
an additional systematic uncertainty, which has been obtained by repeating the whole
fitting procedure after excluding, one at a time, the data samples used for the fB fit in pp
√
collisions at s = 7 TeV.
3
Results
Figure 4 shows the hpT i of inclusive J/ψ for the three analyzed centrality intervals. The
numerical values for hpT i are summarized in table 4. As a reference, the hpT i in pp collisions
at the same centre-of-mass energy, as determined by the interpolation method described in
section 2.1, is also presented. The hpT i for Pb-Pb collisions is significantly smaller than that
for pp collisions. Such a behaviour is not observed at smaller centre-of-mass energies (see
left panel of figure 4), for which no significant system size dependence of hpT i is seen. This
might indicate the onset of processes which either deplete the high pT region or enhance
the J/ψ production at low pT in heavy-ion collisions at the LHC. The latter effect would
be expected as a consequence of a significant contribution from cc̄ coalescence.
It has been suggested [77] that the observable rAA = hp2T iAA /hp2T ipp should be particularly sensitive to medium modifications affecting the J/ψ transverse momentum distribu√
tions. The measured hp2T i values for Pb-Pb collisions at sNN = 2.76 TeV are summarized
in table 4. The corresponding rAA values as a function of hNpart i are shown in figure 5
and are found to be significantly below unity. This is in contrast to results from lower
√
centre-of-mass energies, where either values consistent with unity (PHENIX at sNN =
√
0.2 TeV [9, 72]) or around 1.5 (NA50 at sNN = 17.3 GeV [73]) were obtained (see left
panel of figure 5). The measured hNpart i dependences of hpT i and rAA are compared with
a transport model for inclusive J/ψ by Zhao et al. [75, 76] in the right panels of figures 4
and 5. This model includes regeneration and dissociation processes, based on in-medium
– 13 –
JHEP07(2015)051
Table 3. Systematic uncertainties (in percent) on the measurement of the fraction fB of J/ψ
from the decay of beauty hadrons, for different centrality intervals in the transverse momentum
range 1.5 < pT < 10 GeV/c, and in the two pT intervals for the most central collisions. The
contributions which are fully correlated between the different centrality classes are denoted as type
I, the uncorrelated ones as type II.
Centrality
0–10%
10–40%
40–90%
pp
hpT i (GeV/c)
2.23 ± 0.10 ± 0.08
2.01 ± 0.12 ± 0.08
2.02 ± 0.19 ± 0.29
2.54 ± 0.02 ± 0.01
hp2T i (GeV 2 /c2 )
5.50 ± 0.58 ± 0.25
4.97 ± 0.65 ± 0.34
5.15 ± 1.05 ± 1.23
9.07 ± 0.15 ± 0.07
Table 4. The numerical values of hpT i and hp2T i calculated in the range 0 < pT < 10 GeV/c for
the three analyzed centrality intervals in Pb-Pb collisions (the first uncertainty is the statistical
and the second is the uncorrelated systematic (type II), the correlated uncertainty has a value of
2%, see table 2). The values for pp collisions obtained by the interpolation procedure are given as
a reference.
J/ψ spectral functions, throughout the evolution of a thermally expanding fireball. It also
incorporates nuclear shadowing by reducing the input charm cross section by a factor of
up to 1/3, with a centrality dependence as estimated in [78]. There is a fair agreement
between our hpT i results and the model calculation, while the rAA is not described by this
prediction. Our hpT i and rAA results are also compared with the calculations by Zhou
et al. [74]. These calculations are also based on a transport approach and incorporate
dissociation and regeneration of J/ψ and heavier charmonia, as well as nuclear shadowing
– 14 –
JHEP07(2015)051
Figure 4. The average transverse momentum hpT i of inclusive J/ψ measured at mid-rapidity
(|y| < 0.8) in centrality selected Pb-Pb collisions (filled circles) and pp collisions (open circles) at
√
sNN = 2.76 TeV as a function of the number of participants hNpart i. The uncorrelated systematic
uncertainties (type II) are depicted by the open boxes. Left panel: a comparison to results obtained
√
by the PHENIX collaboration for Au-Au and Cu-Cu collisions at sNN = 0.2 TeV [9, 72] (open
√
and filled diamonds) and by the NA50 collaboration for Pb-Pb collisions at sNN = 17.3 GeV [73]
(crosses). The hpT i values are calculated for NA50 and PHENIX in the pT interval 0–5 GeV/c, while
for ALICE the pT interval is 0–10 GeV/c. Right panel: hpT i is compared to theory predictions by
Zhou et al. [74] and Zhao et al. [75, 76] for the pT interval 0–10 GeV/c.
according to EKS98 [51]. While the most central data point is matched by the prediction,
it does not describe the evolution of rAA towards peripheral collisions. It must be noted
that our results from Pb-Pb collisions at forward rapidity [79] exhibit a continuous decrease
of hpT i and rAA from peripheral towards central events and are thus closer to the theory
predictions, while the behaviour of mid-rapidity Pb-Pb results is more compatible with a
flat hNpart i dependence.
The RAA of inclusive J/ψ in three pT intervals is shown in figure 6 along with the
results by the CMS collaboration for the interval 6.5 < pT < 30 GeV/c [13], both in 0–40%
most central Pb-Pb collisions. The corresponding numerical values are 0.82 ± 0.11(stat.) ±
0.10(syst.) for the interval 0 < pT < 2.5 GeV/c and 0.58 ± 0.06(stat.) ± 0.08(syst.) for
2.5 < pT < 6 GeV/c, where the systematic uncertainties quoted here are the uncorrelated
(type II) ones, as listed in table 1. The data point for 4.5 < pT < 10 GeV/c corresponds
to the RAA value given in table 5 (centrality range 0–50%). Table 5 also contains the RAA
values for prompt J/ψ, which are numerically identical to the ones for inclusive J/ψ. The
inclusive RAA values below pT = 6 GeV/c are significantly higher than those measured at
higher pT , corresponding to a decrease of RAA with increasing pT , while the high pT data
point is close to the CMS measurement. This pT dependence is similar to the one observed
at forward rapidity [12], and is in clear contrast to the pT dependence measured at lower
– 15 –
JHEP07(2015)051
Figure 5. The ratio rAA = hp2T iAA /hp2T ipp in the pT interval 0–10 GeV/c for inclusive J/ψ mea√
sured at mid-rapidity (|y| < 0.8) in centrality selected Pb-Pb collisions (filled circles) at sNN =
2.76 TeV as a function of the number of participants hNpart i. The uncorrelated systematic uncertainties (type II) are depicted by the open boxes, while correlated uncertainty (type I) is shown as
the filled box at unity. Left panel: a comparison to results obtained by the PHENIX collaboration
√
for Au-Au and Cu-Cu collisions at sNN = 0.2 TeV [9, 72] (filled diamonds) and by the NA50
√
collaboration for Pb-Pb collisions at sNN = 17.3 GeV [73] (crosses). The PHENIX and NA50
rAA values are calculated in the pT interval 0–5 GeV/c. Right panel: rAA is compared to theory
predictions by Zhou et al. [74] and Zhao et al. [75, 76] for the pT interval 0–10 GeV/c.
√
centre-of-mass energies by the PHENIX collaboration for sNN = 0.2 TeV [9]. Figure 6
also shows the model predictions by Zhou et al. [74]. The value of the predicted RAA is
systematically below the measurement and exhibits a pT dependence similar to the one in
the data. The prediction by Zhao et al. [75, 76] is close to our result. In both models,
the rise of RAA towards pT = 0 is due to the dominant contribution from J/ψ regeneration
via coalescence.
The fraction of non-prompt J/ψ in the pT range 1.5–10 GeV/c is shown as a function
of the number of participants for the centrality intervals 40–90% (hNpart i = 38), 10–40%
(hNpart i = 192), and 0–10% (hNpart i = 356) in the left panel of figure 7. Within uncertainties, no centrality dependence is observed. The pT dependence of fB (centrality: 0–50%)
is shown in the right panel of figure 7 and compared with the measurements by CMS in
the centrality interval 0–100% and pT > 6.5 GeV/c (for the numerical values see table 5).
Our results at low transverse momenta extend the CMS measurements in Pb-Pb collisions
√
towards lower pT . Also shown are results at mid-rapidity in pp at s = 7 TeV (ALICE [37],
√
ATLAS [35] and CMS [70]) and in pp̄ collisions at s = 1.96 TeV (CDF [57]). Considering
the ALICE and CMS results in Pb-Pb collisions together, a similar pT dependence as in pp
– 16 –
JHEP07(2015)051
Figure 6.
The nuclear modification factor RAA of inclusive J/ψ, measured at mid-rapidity
√
(|y| < 0.8) in Pb-Pb collisions (0–40% most central) at sNN = 2.76 TeV, as a function of transverse
momentum pT . The filled symbols are placed at the measured pT for the given interval. Since for
the data point in 4.5 < pT < 10 GeV/c (open symbol, 0–50% most central) hpT i is not available due
to the limited statistics, it is plotted at the centre of the pT interval. The uncorrelated systematic
uncertainties (type II) are depicted by the open boxes, while the correlated uncertainties (type I)
are shown as the filled boxes at unity. The data are compared to corresponding results by PHENIX
√
for Au-Au collisions (0–40% most central) at sNN = 0.2 TeV [9], by CMS for Pb-Pb collisions
√
(0–40% most central) at sNN = 2.76 TeV [13], and to predictions by the model of Zhou et al. [74]
and Zhao et al. [75, 76].
pT (GeV/c)
fB (%)
RAA (inclusive J/ψ) RAA (prompt J/ψ) RAA (non-prompt J/ψ)
0.0–1.5
–
0.89±0.20±0.21
–
–
1.5–4.5
10.7±4.8±2.5
0.76±0.09±0.08
0.76±0.10±0.08
0.73±0.34±0.20
4.5–10.0
17.0±6.1±2.2
0.38±0.07±0.06
0.38±0.07±0.06
0.37±0.15±0.09
Table 5. The numerical values on the fraction of J/ψ from beauty hadron decays fB at midrapidity and the nuclear modification factors RAA of inclusive, prompt and non-prompt J/ψ for
√
Pb-Pb collisions at s = 2.76 TeV. These results correspond to the centrality interval 0–50%. The
first uncertainty is statistical and the second uncorrelated systematic (type II).
is observed. However, this similarity could be coincidental, being due to a compensation of
the medium effects on the prompt component (J/ψ dissociation and recombination) and
on the non-prompt part (b-quark energy loss).
In figure 8 the nuclear modification factor for non-prompt J/ψ for 1.5 < pT <
4.5 GeV/c and 4.5 < pT < 10 GeV/c is shown together with the result by CMS for
6.5 < pT < 30 GeV/c [13] and with theoretical model predictions [30, 31, 59–61, 80–85].
One should note that the centrality ranges are not the same for ALICE (0–50%) and CMS
(0–20% and 20–100%). However, the results obtained by CMS for these two centrality bins
are compatible with each other, and also compatible with our measurement in the high
pT interval (4.5 < pT < 10 GeV/c). The model by Uphoff et al. [61] follows a partonic
transport approach based on the Boltzmann equation, which allows interactions among all
partons. It does not include radiative processes for heavy quarks. The calculation has been
– 17 –
JHEP07(2015)051
Figure 7. The fraction of J/ψ from beauty hadron decays fB at mid-rapidity measured in the
√
pT interval 1.5 < pT < 10 GeV/c for centrality selected Pb-Pb collisions at sNN = 2.76 TeV (left).
√
√
The pT dependence of fB at mid-rapidity for Pb-Pb ( sNN = 2.76 TeV, |yJ/ψ | < 0.8) and pp ( s =
7 TeV, |yJ/ψ | < 0.9) [37] collisions is compared with measurements by CDF (|yJ/ψ | < 0.6) [57],
ATLAS (|yJ/ψ | < 0.75) [35], and CMS (|yJ/ψ | < 0.9) [13, 70] (right).
performed for a fixed impact parameter b = 5 fm. In the model of Alberico et al. [59, 60] the
propagation of the heavy quarks in the medium is described by the relativistic Langevin
equation. The predicted pT dependence of RAA is strongly influenced by the choice of
transport coefficients. Two values are considered, either as provided by a perturbative
calculation (hard thermal loop approach) or extracted from lattice-QCD simulations. The
calculations have been provided for the centrality range 0–50%. A transport approach,
which is based on a strong-coupling scheme, is employed in the model of He et al. [80]. The
transport is implemented using non-perturbative interactions for heavy quarks and mesons
through the QGP, hadronization and hadronic phases of a nuclear collision. In particular, the elastic heavy-quark scattering in the QGP is evaluated within a thermodynamic
T-matrix approach, by generating resonances close to the critical temperature that can in
turn recombine into B mesons, followed by hadronic diffusion using effective hadronic scattering amplitudes. The hydrodynamic evolution of the system is quantitatively constrained
by the measured transverse momentum distributions and elliptic flow of light hadrons. Radiative processes, which should improve the description at high pT , are not included in this
approach. The calculations have been performed in the centrality range 0–50%. The model
of Vitev et al. [30, 31] assumes the existence of open heavy flavour bound-state solutions
in the QGP in the vicinity of the critical temperature. A description of beauty quark
quenching is combined with B meson inelastic breakup processes. Furthermore, modified
– 18 –
JHEP07(2015)051
Figure 8.
The nuclear modification factor RAA at mid-rapidity (|y| < 0.8) for non-prompt
√
J/ψ in Pb-Pb collisions at sNN = 2.76 TeV as a function of transverse momentum pT . The
ALICE measurement corresponds to the 0–50% centrality range and to the pT intervals 1.5 <
pT < 4.5 GeV/c and 4.5 < pT < 10 GeV/c. The uncorrelated systematic uncertainties (type II) are
depicted by the open boxes, while the correlated uncertainties (type I) are shown as filled boxes
at unity. Results by CMS for higher pT in the centrality range 0–20% and 20–100% [13] are also
shown (the two points have been slightly displaced horizontally for better visibility). The data
are compared to theoretical predictions at mid-rapidity (see text for details). In the right panel,
the ALICE result in the pT interval 4.5 < pT < 10 GeV/c is compared to theoretical predictions
integrated over the same pT range.
4
Conclusions
√
A study of J/ψ production at mid-rapidity in Pb-Pb collisions at sNN = 2.76 TeV has
been presented. A reduction of the inclusive J/ψ hpT i is observed in Pb-Pb collisions in
comparison to pp. The ratio rAA = hp2T iAA /hp2T ipp is found to be significantly below unity,
corresponding to a medium-induced change in the shape of the pT spectra. The nuclear
modification factor RAA depends on pT . It is around 0.8 for pT < 2.5 GeV/c and reaches,
at higher pT , almost the same level of suppression as observed at RHIC energies at low pT .
These observations might be indicative of a sizable contribution of charm quark coalescence
to the J/ψ production at low pT . Transport models including this additional component
are able to qualitatively describe the features seen in the data.
The fraction of J/ψ from beauty hadron decays is determined as a function of centrality
and pT . No significant centrality dependence is observed. By combining this measurement
with the inclusive J/ψ results the RAA of non-prompt J/ψ is obtained in the region 1.5 <
pT < 10 GeV/c, thus extending the coverage of CMS to the low pT region. The nuclear
modification in the region 4.5 < pT < 10 GeV/c is found to be stronger than predicted by
most of the models.
– 19 –
JHEP07(2015)051
beauty parton distribution functions and beauty fragmentation functions in a co-moving
plasma are implemented in this calculation. The prediction is shown for a fixed centrality,
corresponding to hNpart i = 200, a value very close to the average number of participants in
the centrality range 0–50%. In the model, a sizable fraction of the suppression is ascribed to
the inelastic break-up processes (collisional dissociation), as can be deduced from figure 8
by comparing the full model prediction with and without the contribution of this specific
process. The model of Djordjevic [81], shown in figure 8 for the centrality range 0–50%,
uses a formalism that takes into account finite size dynamical QCD medium with finite
magnetic mass effects and running coupling. In the WHDG model [82] (centrality range
0–50%) the energy loss is computed using perturbative QCD and considering both elastic
and inelastic partonic collisions and path length fluctuations. The approach of Aichelin
et al. [83, 84] includes a contribution of radiative gluon emission in the interaction of heavy
quarks with light quarks, which are considered as dynamical scattering centers. In this
model the relative contribution to the energy loss by radiative processes, as compared to
collisional ones, is influenced by introducing a finite gluon mass. The results of the model
shown in figure 8, which are obtained for the centrality range 0–50%, correspond to either
a pure collisional scenario or a combination of collisional and radiative energy loss. Finally,
in the model of Horowitz and Gyulassy [85], also applied to the centrality interval 0–50%,
the string inspired AdS/CFT gravity-gauge theory correspondence [86, 87] is applied to the
case of heavy quark energy loss. In the right hand inset of figure 8, the ALICE RAA value,
integrated over the range 4.5 < pT < 10 GeV/c, is compared to theoretical predictions computed in the same pT range. Most of the models predict a larger value of RAA than observed
in the measurement. However, more precise data are needed to discriminate among the
different models. The next LHC run will provide increased statistics for this measurement.
Acknowledgments
– 20 –
JHEP07(2015)051
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: State Committee of Science, World Federation of Scientists (WFS) and Swiss
Fonds Kidagan, Armenia, Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico
(CNPq), Financiadora de Estudos e Projetos (FINEP), Fundação de Amparo à Pesquisa do
Estado de São Paulo (FAPESP); National Natural Science Foundation of China (NSFC),
the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology of
China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural
Science Research Council, the Carlsberg Foundation and the Danish National Research
Foundation; The European Research Council under the European Community’s Seventh
Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French
CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ and CEA,
France; German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie
(BMBF) and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian Orszagos Tudomanyos Kutatasi Alappgrammok (OTKA) and National Office for Research and Technology (NKTH); Department
of Atomic Energy and Department of Science and Technology of the Government of India;
Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi - Museo Storico della Fisica
e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research
Foundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion General de Asuntos del Personal Academico(DGAPA), México, Amerique Latine
Formation academique – European Commission(ALFA-EC) and the EPLANET Program
(European Particle Physics Latin American Network) Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); National Science Centre,
Poland; Ministry of National Education/Institute for Atomic Physics and Consiliul National al Cercetǎrii Ştiintifice – Executive Agency for Higher Education Research Development and Innovation Funding (CNCS-UEFISCDI) – Romania; Ministry of Education and
Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of
Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia; Department of Science and
Technology, South Africa; Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure shared between Europe and Latin America (EELA),
Ministerio de Economı́a y Competitividad (MINECO) of Spain, Xunta de Galicia (Consellerı́a de Educación), Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergı́a, Cuba, and IAEA (International Atomic Energy Agency); Swedish
Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council
(STFC); The United States Department of Energy, the United States National Science
Foundation, the State of Texas, and the State of Ohio; Ministry of Science, Education and
Sports of Croatia and Unity through Knowledge Fund, Croatia. Council of Scientific and
Industrial Research (CSIR), New Delhi, India
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A. Herghelegiu78 , G. Herrera Corral11 , B.A. Hess35 , K.F. Hetland38 , T.E. Hilden46 ,
H. Hillemanns36 , B. Hippolyte55 , P. Hristov36 , M. Huang18 , T.J. Humanic20 , N. Hussain45 ,
T. Hussain19 , D. Hutter43 , D.S. Hwang21 , R. Ilkaev99 , I. Ilkiv77 , M. Inaba128 , C. Ionita36 ,
M. Ippolitov76 ,100 , M. Irfan19 , M. Ivanov97 , V. Ivanov85 , V. Izucheev112 , P.M. Jacobs74 ,
S. Jadlovska115 , C. Jahnke120 , H.J. Jang68 , M.A. Janik134 , P.H.S.Y. Jayarathna122 , C. Jena30 ,
S. Jena122 , R.T. Jimenez Bustamante97 , P.G. Jones102 , H. Jung44 , A. Jusko102 , P. Kalinak59 ,
A. Kalweit36 , J. Kamin53 , J.H. Kang138 , V. Kaplin76 , S. Kar132 , A. Karasu Uysal69 ,
O. Karavichev56 , T. Karavicheva56 , E. Karpechev56 , U. Kebschull52 , R. Keidel139 ,
D.L.D. Keijdener57 , M. Keil36 , K.H. Khan16 , M.M. Khan19 , P. Khan101 , S.A. Khan132 ,
A. Khanzadeev85 , Y. Kharlov112 , B. Kileng38 , B. Kim138 , D.W. Kim68 ,44 , D.J. Kim123 ,
H. Kim138 , J.S. Kim44 , M. Kim44 , M. Kim138 , S. Kim21 , T. Kim138 , S. Kirsch43 , I. Kisel43 ,
S. Kiselev58 , A. Kisiel134 , G. Kiss136 , J.L. Klay6 , C. Klein53 , J. Klein93 , C. Klein-Bösing54 ,
A. Kluge36 , M.L. Knichel93 , A.G. Knospe118 , T. Kobayashi128 , C. Kobdaj114 , M. Kofarago36 ,
T. Kollegger97 ,43 , A. Kolojvari131 , V. Kondratiev131 , N. Kondratyeva76 , E. Kondratyuk112 ,
A. Konevskikh56 , M. Kopcik115 , C. Kouzinopoulos36 , O. Kovalenko77 , V. Kovalenko131 ,
M. Kowalski117 , S. Kox71 , G. Koyithatta Meethaleveedu48 , J. Kral123 , I. Králik59 ,
A. Kravčáková41 , M. Krelina40 , M. Kretz43 , M. Krivda102 ,59 , F. Krizek83 , E. Kryshen36 ,
M. Krzewicki43 , A.M. Kubera20 , V. Kučera83 , T. Kugathasan36 , C. Kuhn55 , P.G. Kuijer81 ,
I. Kulakov43 , J. Kumar48 , L. Kumar79 ,87 , P. Kurashvili77 , A. Kurepin56 , A.B. Kurepin56 ,
A. Kuryakin99 , S. Kushpil83 , M.J. Kweon50 , Y. Kwon138 , S.L. La Pointe111 , P. La Rocca29 ,
C. Lagana Fernandes120 , I. Lakomov36 , R. Langoy42 , C. Lara52 , A. Lardeux15 , A. Lattuca27 ,
E. Laudi36 , R. Lea26 , L. Leardini93 , G.R. Lee102 , S. Lee138 , I. Legrand36 , R.C. Lemmon82 ,
V. Lenti104 , E. Leogrande57 , I. León Monzón119 , M. Leoncino27 , P. Lévai136 , S. Li7 ,70 ,
X. Li14 , J. Lien42 , R. Lietava102 , S. Lindal22 , V. Lindenstruth43 , C. Lippmann97 , M.A. Lisa20 ,
H.M. Ljunggren34 , D.F. Lodato57 , P.I. Loenne18 , V.R. Loggins135 , V. Loginov76 , C. Loizides74 ,
X. Lopez70 , E. López Torres9 , A. Lowe136 , P. Luettig53 , M. Lunardon30 , G. Luparello26 ,
P.H.F.N.D. Luz120 , A. Maevskaya56 , M. Mager36 , S. Mahajan90 , S.M. Mahmood22 ,
A. Maire55 , R.D. Majka137 , M. Malaev85 , I. Maldonado Cervantes63 , L. Malinina66 ,
D. Mal’Kevich58 , P. Malzacher97 , A. Mamonov99 , L. Manceau111 , V. Manko100 , F. Manso70 ,
V. Manzari104 ,36 , M. Marchisone27 , J. Mareš60 , G.V. Margagliotti26 , A. Margotti105 ,
J. Margutti57 , A. Marı́n97 , C. Markert118 , M. Marquard53 , N.A. Martin97 , J. Martin
Blanco113 , P. Martinengo36 , M.I. Martı́nez2 , G. Martı́nez Garcı́a113 , M. Martinez Pedreira36 ,
Y. Martynov3 , A. Mas120 , S. Masciocchi97 , M. Masera27 , A. Masoni106 , L. Massacrier113 ,
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A. Mastroserio33 , H. Masui128 , A. Matyja117 , C. Mayer117 , J. Mazer125 , M.A. Mazzoni109 ,
D. Mcdonald122 , F. Meddi24 , A. Menchaca-Rocha64 , E. Meninno31 , J. Mercado Pérez93 ,
M. Meres39 , Y. Miake128 , M.M. Mieskolainen46 , K. Mikhaylov58 ,66 , L. Milano36 ,
J. Milosevic22 ,133 , L.M. Minervini23 ,104 , A. Mischke57 , A.N. Mishra49 , D. Miśkowiec97 ,
J. Mitra132 , C.M. Mitu62 , N. Mohammadi57 , B. Mohanty79 ,132 , L. Molnar55 , L. Montaño
Zetina11 , E. Montes10 , M. Morando30 , D.A. Moreira De Godoy54 ,113 , S. Moretto30 ,
A. Morreale113 , A. Morsch36 , V. Muccifora72 , E. Mudnic116 , D. Mühlheim54 , S. Muhuri132 ,
M. Mukherjee132 , J.D. Mulligan137 , M.G. Munhoz120 , S. Murray65 , L. Musa36 , J. Musinsky59 ,
B.K. Nandi48 , R. Nania105 , E. Nappi104 , M.U. Naru16 , C. Nattrass125 , K. Nayak79 ,
T.K. Nayak132 , S. Nazarenko99 , A. Nedosekin58 , L. Nellen63 , F. Ng122 , M. Nicassio97 ,
M. Niculescu62 ,36 , J. Niedziela36 , B.S. Nielsen80 , S. Nikolaev100 , S. Nikulin100 , V. Nikulin85 ,
F. Noferini12 ,105 , P. Nomokonov66 , G. Nooren57 , J. Norman124 , A. Nyanin100 , J. Nystrand18 ,
H. Oeschler93 , S. Oh137 , S.K. Oh67 , A. Ohlson36 , A. Okatan69 , T. Okubo47 , L. Olah136 ,
J. Oleniacz134 , A.C. Oliveira Da Silva120 , M.H. Oliver137 , J. Onderwaater97 , C. Oppedisano111 ,
A. Ortiz Velasquez63 , A. Oskarsson34 , J. Otwinowski117 , K. Oyama93 , M. Ozdemir53 ,
Y. Pachmayer93 , P. Pagano31 , G. Paić63 , C. Pajares17 , S.K. Pal132 , J. Pan135 , A.K. Pandey48 ,
D. Pant48 , P. Papcun115 , V. Papikyan1 , G.S. Pappalardo107 , P. Pareek49 , W.J. Park97 ,
S. Parmar87 , A. Passfeld54 , V. Paticchio104 , R.N. Patra132 , B. Paul101 , T. Peitzmann57 ,
H. Pereira Da Costa15 , E. Pereira De Oliveira Filho120 , D. Peresunko76 ,100 , C.E. Pérez Lara81 ,
V. Peskov53 , Y. Pestov5 , V. Petráček40 , V. Petrov112 , M. Petrovici78 , C. Petta29 , S. Piano110 ,
M. Pikna39 , P. Pillot113 , O. Pinazza105 ,36 , L. Pinsky122 , D.B. Piyarathna122 , M. Ploskoń74 ,
M. Planinic129 , J. Pluta134 , S. Pochybova136 , P.L.M. Podesta-Lerma119 , M.G. Poghosyan86 ,
B. Polichtchouk112 , N. Poljak129 , W. Poonsawat114 , A. Pop78 , S. Porteboeuf-Houssais70 ,
J. Porter74 , J. Pospisil83 , S.K. Prasad4 , R. Preghenella105 ,36 , F. Prino111 , C.A. Pruneau135 ,
I. Pshenichnov56 , M. Puccio111 , G. Puddu25 , P. Pujahari135 , V. Punin99 , J. Putschke135 ,
H. Qvigstad22 , A. Rachevski110 , S. Raha4 , S. Rajput90 , J. Rak123 , A. Rakotozafindrabe15 ,
L. Ramello32 , R. Raniwala91 , S. Raniwala91 , S.S. Räsänen46 , B.T. Rascanu53 , D. Rathee87 ,
K.F. Read125 , J.S. Real71 , K. Redlich77 , R.J. Reed135 , A. Rehman18 , P. Reichelt53 ,
F. Reidt93 ,36 , X. Ren7 , R. Renfordt53 , A.R. Reolon72 , A. Reshetin56 , F. Rettig43 ,
J.-P. Revol12 , K. Reygers93 , V. Riabov85 , R.A. Ricci73 , T. Richert34 , M. Richter22 ,
P. Riedler36 , W. Riegler36 , F. Riggi29 , C. Ristea62 , A. Rivetti111 , E. Rocco57 , M. Rodrı́guez
Cahuantzi2 , A. Rodriguez Manso81 , K. Røed22 , E. Rogochaya66 , D. Rohr43 , D. Röhrich18 ,
R. Romita124 , F. Ronchetti72 , L. Ronflette113 , P. Rosnet70 , A. Rossi36 ,30 , F. Roukoutakis88 ,
A. Roy49 , C. Roy55 , P. Roy101 , A.J. Rubio Montero10 , R. Rui26 , R. Russo27 , E. Ryabinkin100 ,
Y. Ryabov85 , A. Rybicki117 , S. Sadovsky112 , K. Šafařı́k36 , B. Sahlmuller53 , P. Sahoo49 ,
R. Sahoo49 , S. Sahoo61 , P.K. Sahu61 , J. Saini132 , S. Sakai72 , M.A. Saleh135 , C.A. Salgado17 ,
J. Salzwedel20 , S. Sambyal90 , V. Samsonov85 , X. Sanchez Castro55 , L. Šándor59 ,
A. Sandoval64 , M. Sano128 , G. Santagati29 , D. Sarkar132 , E. Scapparone105 , F. Scarlassara30 ,
R.P. Scharenberg95 , C. Schiaua78 , R. Schicker93 , C. Schmidt97 , H.R. Schmidt35 ,
S. Schuchmann53 , J. Schukraft36 , M. Schulc40 , T. Schuster137 , Y. Schutz113 ,36 , K. Schwarz97 ,
K. Schweda97 , G. Scioli28 , E. Scomparin111 , R. Scott125 , K.S. Seeder120 , J.E. Seger86 ,
Y. Sekiguchi127 , D. Sekihata47 , I. Selyuzhenkov97 , K. Senosi65 , J. Seo96 ,67 , E. Serradilla64 ,10 ,
A. Sevcenco62 , A. Shabanov56 , A. Shabetai113 , O. Shadura3 , R. Shahoyan36 , A. Shangaraev112 ,
A. Sharma90 , N. Sharma61 ,125 , K. Shigaki47 , K. Shtejer9 ,27 , Y. Sibiriak100 , S. Siddhanta106 ,
K.M. Sielewicz36 , T. Siemiarczuk77 , D. Silvermyr84 ,34 , C. Silvestre71 , G. Simatovic129 ,
G. Simonetti36 , R. Singaraju132 , R. Singh79 , S. Singha79 ,132 , V. Singhal132 , B.C. Sinha132 ,
T. Sinha101 , B. Sitar39 , M. Sitta32 , T.B. Skaali22 , M. Slupecki123 , N. Smirnov137 ,
R.J.M. Snellings57 , T.W. Snellman123 , C. Søgaard34 , R. Soltz75 , J. Song96 , M. Song138 ,
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Deceased
Also at: University of Kansas, Lawrence, Kansas, United States
A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan,
Armenia
Benemérita Universidad Autónoma de Puebla, Puebla, Mexico
Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine
Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science
(CAPSS), Kolkata, India
Budker Institute for Nuclear Physics, Novosibirsk, Russia
California Polytechnic State University, San Luis Obispo, California, United States
Central China Normal University, Wuhan, China
Centre de Calcul de l’IN2P3, Villeurbanne, France
Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba
Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico
Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Rome, Italy
Chicago State University, Chicago, Illinois, USA
China Institute of Atomic Energy, Beijing, China
– 29 –
JHEP07(2015)051
Z. Song7 , F. Soramel30 , S. Sorensen125 , M. Spacek40 , E. Spiriti72 , I. Sputowska117 ,
M. Spyropoulou-Stassinaki88 , B.K. Srivastava95 , J. Stachel93 , I. Stan62 , G. Stefanek77 ,
M. Steinpreis20 , E. Stenlund34 , G. Steyn65 , J.H. Stiller93 , D. Stocco113 , P. Strmen39 ,
A.A.P. Suaide120 , T. Sugitate47 , C. Suire51 , M. Suleymanov16 , R. Sultanov58 , M. Šumbera83 ,
T.J.M. Symons74 , A. Szabo39 , A. Szanto de Toledo120 , i , I. Szarka39 , A. Szczepankiewicz36 ,
M. Szymanski134 , J. Takahashi121 , N. Tanaka128 , M.A. Tangaro33 , J.D. Tapia Takaki, ii,51 ,
A. Tarantola Peloni53 , M. Tarhini51 , M. Tariq19 , M.G. Tarzila78 , A. Tauro36 , G. Tejeda
Muñoz2 , A. Telesca36 , K. Terasaki127 , C. Terrevoli30 ,25 , B. Teyssier130 , J. Thäder74 ,97 ,
D. Thomas118 , R. Tieulent130 , A.R. Timmins122 , A. Toia53 , S. Trogolo111 , V. Trubnikov3 ,
W.H. Trzaska123 , T. Tsuji127 , A. Tumkin99 , R. Turrisi108 , T.S. Tveter22 , K. Ullaland18 ,
A. Uras130 , G.L. Usai25 , A. Utrobicic129 , M. Vajzer83 , M. Vala59 , L. Valencia Palomo70 ,
S. Vallero27 , J. Van Der Maarel57 , J.W. Van Hoorne36 , M. van Leeuwen57 , T. Vanat83 ,
P. Vande Vyvre36 , D. Varga136 , A. Vargas2 , M. Vargyas123 , R. Varma48 , M. Vasileiou88 ,
A. Vasiliev100 , A. Vauthier71 , V. Vechernin131 , A.M. Veen57 , M. Veldhoen57 , A. Velure18 ,
M. Venaruzzo73 , E. Vercellin27 , S. Vergara Limón2 , R. Vernet8 , M. Verweij135 , L. Vickovic116 ,
G. Viesti30 , i , J. Viinikainen123 , Z. Vilakazi126 , O. Villalobos Baillie102 , A. Vinogradov100 ,
L. Vinogradov131 , Y. Vinogradov99 , T. Virgili31 , V. Vislavicius34 , Y.P. Viyogi132 ,
A. Vodopyanov66 , M.A. Völkl93 , K. Voloshin58 , S.A. Voloshin135 , G. Volpe136 ,36 , B. von
Haller36 , I. Vorobyev92 ,37 , D. Vranic97 ,36 , J. Vrláková41 , B. Vulpescu70 , A. Vyushin99 ,
B. Wagner18 , J. Wagner97 , H. Wang57 , M. Wang7 ,113 , Y. Wang93 , D. Watanabe128 ,
Y. Watanabe127 , M. Weber36 , S.G. Weber97 , J.P. Wessels54 , U. Westerhoff54 , J. Wiechula35 ,
J. Wikne22 , M. Wilde54 , G. Wilk77 , J. Wilkinson93 , M.C.S. Williams105 , B. Windelband93 ,
M. Winn93 , C.G. Yaldo135 , H. Yang57 , P. Yang7 , S. Yano47 , Z. Yin7 , H. Yokoyama128 ,
I.-K. Yoo96 , V. Yurchenko3 , I. Yushmanov100 , A. Zaborowska134 , V. Zaccolo80 , A. Zaman16 ,
C. Zampolli105 , H.J.C. Zanoli120 , S. Zaporozhets66 , N. Zardoshti102 , A. Zarochentsev131 ,
P. Závada60 , N. Zaviyalov99 , H. Zbroszczyk134 , I.S. Zgura62 , M. Zhalov85 , H. Zhang18 ,7 ,
X. Zhang74 , Y. Zhang7 , C. Zhao22 , N. Zhigareva58 , D. Zhou7 , Y. Zhou80 ,57 , Z. Zhou18 ,
H. Zhu18 ,7 , J. Zhu113 ,7 , X. Zhu7 , A. Zichichi12 ,28 , A. Zimmermann93 ,
M.B. Zimmermann54 ,36 , G. Zinovjev3 , M. Zyzak43
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Commissariat à l’Energie Atomique, IRFU, Saclay, France
COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan
Departamento de Fı́sica de Partı́culas and IGFAE, Universidad de Santiago de Compostela,
Santiago de Compostela, Spain
Department of Physics and Technology, University of Bergen, Bergen, Norway
Department of Physics, Aligarh Muslim University, Aligarh, India
Department of Physics, Ohio State University, Columbus, Ohio, United States
Department of Physics, Sejong University, Seoul, South Korea
Department of Physics, University of Oslo, Oslo, Norway
Dipartimento di Elettrotecnica ed Elettronica del Politecnico, Bari, Italy
Dipartimento di Fisica dell’Università ’La Sapienza’ and Sezione INFN Rome, Italy
Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy
Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy
Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy
Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Bologna, Italy
Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Catania, Italy
Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Padova, Italy
Dipartimento di Fisica ‘E.R. Caianiello’ dell’Università and Gruppo Collegato INFN, Salerno, Italy
Dipartimento di Scienze e Innovazione Tecnologica dell’Università del Piemonte Orientale and
Gruppo Collegato INFN, Alessandria, Italy
Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy
Division of Experimental High Energy Physics, University of Lund, Lund, Sweden
Eberhard Karls Universität Tübingen, Tübingen, Germany
European Organization for Nuclear Research (CERN), Geneva, Switzerland
Excellence Cluster Universe, Technische Universität München, Munich, Germany
Faculty of Engineering, Bergen University College, Bergen, Norway
Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia
Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague,
Prague, Czech Republic
Faculty of Science, P.J. Šafárik University, Košice, Slovakia
Faculty of Technology, Buskerud and Vestfold University College, Vestfold, Norway
Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt,
Frankfurt, Germany
Gangneung-Wonju National University, Gangneung, South Korea
Gauhati University, Department of Physics, Guwahati, India
Helsinki Institute of Physics (HIP), Helsinki, Finland
Hiroshima University, Hiroshima, Japan
Indian Institute of Technology Bombay (IIT), Mumbai, India
Indian Institute of Technology Indore, Indore (IITI), India
Inha University, Incheon, South Korea
Institut de Physique Nucléaire d’Orsay (IPNO), Université Paris-Sud, CNRS-IN2P3, Orsay, France
Institut für Informatik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany
Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany
Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, Münster, Germany
Institut Pluridisciplinaire Hubert Curien (IPHC), Université de Strasbourg, CNRS-IN2P3,
Strasbourg, France
Institute for Nuclear Research, Academy of Sciences, Moscow, Russia
Institute for Subatomic Physics of Utrecht University, Utrecht, Netherlands
Institute for Theoretical and Experimental Physics, Moscow, Russia
Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia
Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
Institute of Physics, Bhubaneswar, India
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JHEP07(2015)051
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Institute of Space Science (ISS), Bucharest, Romania
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico
Instituto de Fı́sica, Universidad Nacional Autónoma de México, Mexico City, Mexico
iThemba LABS, National Research Foundation, Somerset West, South Africa
Joint Institute for Nuclear Research (JINR), Dubna, Russia
Konkuk University, Seoul, South Korea
Korea Institute of Science and Technology Information, Daejeon, South Korea
KTO Karatay University, Konya, Turkey
Laboratoire de Physique Corpusculaire (LPC), Clermont Université, Université Blaise Pascal,
CNRS–IN2P3, Clermont-Ferrand, France
Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3,
Grenoble, France
Laboratori Nazionali di Frascati, INFN, Frascati, Italy
Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy
Lawrence Berkeley National Laboratory, Berkeley, California, United States
Lawrence Livermore National Laboratory, Livermore, California, United States
Moscow Engineering Physics Institute, Moscow, Russia
National Centre for Nuclear Studies, Warsaw, Poland
National Institute for Physics and Nuclear Engineering, Bucharest, Romania
National Institute of Science Education and Research, Bhubaneswar, India
Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands
Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom
Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Řež u Prahy, Czech Republic
Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Petersburg Nuclear Physics Institute, Gatchina, Russia
Physics Department, Creighton University, Omaha, Nebraska, United States
Physics Department, Panjab University, Chandigarh, India
Physics Department, University of Athens, Athens, Greece
Physics Department, University of Cape Town, Cape Town, South Africa
Physics Department, University of Jammu, Jammu, India
Physics Department, University of Rajasthan, Jaipur, India
Physik Department, Technische Universität München, Munich, Germany
Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
Politecnico di Torino, Turin, Italy
Purdue University, West Lafayette, Indiana, United States
Pusan National University, Pusan, South Korea
Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für
Schwerionenforschung, Darmstadt, Germany
Rudjer Bošković Institute, Zagreb, Croatia
Russian Federal Nuclear Center (VNIIEF), Sarov, Russia
Russian Research Centre Kurchatov Institute, Moscow, Russia
Saha Institute of Nuclear Physics, Kolkata, India
School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom
Sección Fı́sica, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru
Sezione INFN, Bari, Italy
Sezione INFN, Bologna, Italy
Sezione INFN, Cagliari, Italy
Sezione INFN, Catania, Italy
Sezione INFN, Padova, Italy
Sezione INFN, Rome, Italy
Sezione INFN, Trieste, Italy
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JHEP07(2015)051
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Sezione INFN, Turin, Italy
SSC IHEP of NRC Kurchatov institute, Protvino, Russia
SUBATECH, Ecole des Mines de Nantes, Université de Nantes, CNRS-IN2P3, Nantes, France
Suranaree University of Technology, Nakhon Ratchasima, Thailand
Technical University of Košice, Košice, Slovakia
Technical University of Split FESB, Split, Croatia
The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow,
Poland
The University of Texas at Austin, Physics Department, Austin, Texas, USA
Universidad Autónoma de Sinaloa, Culiacán, Mexico
Universidade de São Paulo (USP), São Paulo, Brazil
Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil
University of Houston, Houston, Texas, United States
University of Jyväskylä, Jyväskylä, Finland
University of Liverpool, Liverpool, United Kingdom
University of Tennessee, Knoxville, Tennessee, United States
University of the Witwatersrand, Johannesburg, South Africa
University of Tokyo, Tokyo, Japan
University of Tsukuba, Tsukuba, Japan
University of Zagreb, Zagreb, Croatia
Université de Lyon, Université Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France
V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia
Variable Energy Cyclotron Centre, Kolkata, India
Vinča Institute of Nuclear Sciences, Belgrade, Serbia
Warsaw University of Technology, Warsaw, Poland
Wayne State University, Detroit, Michigan, United States
Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary
Yale University, New Haven, Connecticut, United States
Yonsei University, Seoul, South Korea
Zentrum für Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms,
Germany