Suppression of psi(2S) production in p-Pb collisions at sqrt(sNN) = 5.02 TeV

JHEP12(2014)073

Received_{: May 30, 2014} Revised_{: October 14, 2014} Accepted_{: November 17, 2014} Published: December 10, 2014

Suppression of ψ(2S) production in p-Pb collisions at

√s

= 5.02

TeV

The ALICE collaboration

E-mail: ALICE-publications@cern.ch

Abstract: _{The ALICE Collaboration has studied the inclusive production of the}

char-monium state ψ(2S) in proton-lead (p-Pb) collisions at the nucleon-nucleon centre of mass

energy √sNN = 5.02 TeV at the CERN LHC. The measurement was performed at forward

(2.03 < ycms < 3.53) and backward (−4.46 < ycms < −2.96) centre of mass rapidities,

studying the decays into muon pairs. In this paper, we present the inclusive production

cross sections σ_ψ(2S), both integrated and as a function of the transverse momentum pT,

for the two ycms domains. The results are compared to those obtained for the 1S vector

state (J/ψ), by showing the ratios between the production cross sections, as well as the

double ratios [σ_ψ(2S)/σ_J/ψ]pPb/[σψ(2S)/σJ/ψ]pp between p-Pb and proton-proton collisions.

Finally, the nuclear modification factor for inclusive ψ(2S) is evaluated and compared to the measurement of the same quantity for J/ψ and to theoretical models including parton shadowing and coherent energy loss mechanisms. The results show a significantly larger suppression of the ψ(2S) compared to that measured for J/ψ and to models. These obser-vations represent a clear indication for sizeable final state effects on ψ(2S) production.

Keywords: _{Charm physics, Heavy Ions}

JHEP12(2014)073

The physics of charmonia, bound states of the charm (c) and anti-charm (c) quarks, is an

extremely broad and interesting field of investigation [1]. The description of the various

states and the calculation of their production cross sections in hadronic collisions involve an interplay of perturbative and non-perturbative aspects of Quantum ChromoDynamics

(QCD) [2], which still today represent a significant challenge for theory [3]. Charmonium

states can have smaller sizes than light hadrons (down to a few tenths of a fm) and large

binding energies (> 500 MeV) [4]. These properties make charmonia a useful probe of the

hot nuclear matter created in ultrarelativistic heavy-ion collisions, which can be seen as

a plasma of deconfined quarks and gluons (QGP) (see [5] for a recent overview of QGP

studies). In particular, the cc binding can be screened by the high density of colour charges present in the QGP, leading to a suppression of the yields of charmonia in high-energy nuclear collisions compared to the corresponding production rates in elementary pp

collisions at the same energy [6]. In the so-called “sequential suppression” scenario, the

melting of a bound cc state occurs when the temperature of the hot medium exceeds a

threshold dissociation temperature [7,8], which depends on the binding energy of the state

and can be calculated in lattice QCD [9]. At LHC energies, where the number of produced

cc pairs is large, this suppression effect can be partly counterbalanced by charmonium “regeneration” processes due to the recombination of charm quarks that occurs as the

system cools and hadrons form [10–12].

Among the charmonium states, the strongly bound S-wave J/ψ and the weakly bound radially excited ψ(2S) have received most attention in the context of QGP studies. Both decay to lepton pairs with a non-negligible branching ratio (5.93% and 0.77%,

respec-tively, for the µ+_µ−

channel [13]). The results obtained by the NA50 collaboration at the

CERN SPS showed a significant suppression of the J/ψ production in Pb-Pb collisions at

√_s

NN = 17 GeV [14] and a comparatively larger suppression of the ψ(2S) [15], in

quali-tative agreement with sequential suppression models. However, the same experiment also detected a significant suppression of both states (although not as strong as in Pb-Pb) in

proton-nucleus (p-A) collisions [16], where no QGP formation was expected. The same

ob-servation was made by other fixed-target experiments studying p-A collisions at Fermilab

(E866 [17]) and HERA (HERA-B [18]). It was indeed realized that the charmonium yields

are also sensitive to the presence of cold nuclear matter (CNM) in the target nucleus, and

various mechanisms (nuclear parton shadowing [19], cc break-up via interaction with

nucle-ons [20–22], initial/final state energy loss [23]) were taken into account in order to describe

experimental observations. In particular, these experiments observed a stronger suppres-sion for ψ(2S) relative to J/ψ at central rapidity, while at forward rapidity no difference was found within uncertainties. This feature of the results was interpreted in terms of pair break-up: at central rapidity the time spent by the cc state in the nuclear medium (crossing

time) is typically larger than the formation time of the resonances (∼ 0.1 fm/c [24, 25]),

so that the loosely bound ψ(2S) can be more easily dissociated than the J/ψ. Conversely, in forward production the crossing time is smaller than the formation time and the influ-ence of the nucleus on the pre-hadronic state is the same, independent of the particular

JHEP12(2014)073

More generally, the study of charmonia in p-A collisions can be used as a tool for a quantitative investigation of the aforementioned processes, relevant in the context of studies of the strong interaction. Therefore, measurements at high energies are important to test our understanding of the various mechanisms. In particular, the pair break-up cross sections discussed above are expected to be strongly reduced due to the increasingly shorter time spent by the cc pair in CNM. On the other hand, the other effects listed above (shadowing, energy loss) are not expected to depend on the final quantum numbers of the charmonium states. In such a situation, a similar suppression for the two charmonium states should be observed in high-energy p-A collisions.

In the context of comparative studies between the resonances, the PHENIX experiment at RHIC has recently published results on the ψ(2S) suppression at central rapidity for d-Au

collisions at √sNN = 200 GeV [27], by studying the nuclear modification factor R_dAuψ(2S) =

dN_dAuψ(2S)/dy/(Ncoll×dNppψ(2S)/dy), which corresponds to the ratio of the production yields in

d-Au and pp at the same energy, normalized by the number of nucleon-nucleon collisions in

d-Au. The ratio of the nuclear modification factors Rψ(2S)_dAu /R_dAuJ/ψ is found to be smaller than

1, and strongly decreasing from peripheral to central d-Au events. The observation of a ψ(2S) suppression stronger than that of the J/ψ is in contrast to the expectation of a similar suppression as described above. Data from the LHC can be useful to shed further light on

this observation, as nuclear crossing times [25] may be as low as 10−4

fm/c for charmonium production at forward rapidity, implying a negligible influence of pair break-up processes and, in more general terms, to test our understanding of charmonium propagation in CNM. In this Letter, we present the first measurement of inclusive ψ(2S) production in

√_s

NN = 5.02 TeV p-Pb collisions at the LHC, carried out by the ALICE Collaboration, and

we compare the results with those for J/ψ. The resonances were measured in the dimuon

decay channel using the Muon Spectrometer (MS) [28], which covers the pseudorapidity

range −4 < ηlab < −2.5. The other detectors involved in this analysis are: (i) the two

innermost layers of the Inner Tracking System (Silicon Pixel Detectors, SPD), used for

the determination of the primary vertex of the interaction and covering |ηlab| < 2.0 (first

layer) and |ηlab| < 1.4 (second layer) [29]; (ii) the two VZERO scintillator hodoscopes, used

mainly for triggering purposes and covering −3.7 < ηlab < −1.7 and 2.8 < ηlab < 5.1 [30];

(iii) the Zero Degree Calorimeters (ZDC), at 112.5 m from the interaction point [31], used

to remove collisions outside the nominal timing of the LHC bunches. Details of the ALICE

experimental setup are provided elsewhere [32].

Due to the LHC design, the colliding beams have different energies per nucleon (Ep=

4 TeV, EPb= 1.58 · APbTeV, where APb= 208 is the mass number of the Pb nucleus). As

a consequence, the centre of mass of the nucleon-nucleon collision is shifted by ∆y = 0.465 with respect to the laboratory frame in the direction of the proton beam. Data were taken in two configurations, by inverting the sense of the orbits of the two beams. In this way, both

forward (2.03 < ycms < 3.53) and backward (−4.46 < ycms< −2.96) centre of mass

rapidi-ties were covered, with the positive rapidity defined by the direction of the proton beam. We refer to the two data samples as p-Pb and Pb-p respectively. The integrated

luminosi-ties for the two data samples are LpPb_{int = 5.01 ±0.19 nb}−1

and LPbp_{int = 5.81 ±0.20 nb}−1

JHEP12(2014)073

Data were collected with a dimuon trigger, defined as the coincidence of the minimum-bias (MB) condition with the detection of two opposite-sign muon candidates in the trigger system of the MS. The MB condition is a coincidence between signals in the two VZERO

hodoscopes and has > 99% efficiency for non-single diffractive events [34]. For the muon

candidates, a transverse momentum pT,µ = 0.5 GeV/c trigger threshold is applied. The

ef-fect of this threshold is not sharp, and the single muon trigger efficiency reaches its plateau

value (∼ 96%) for pT,µ ∼ 1.5 GeV/c. The offline event selection, the muon reconstruction

and identification criteria and the kinematic cuts applied at the single and dimuon

lev-els are identical to those described in [35]. In addition, a cut on the transverse distance

from the primary vertex of each of the reconstructed muon tracks, weighted with its

mo-mentum (pDCA), was performed. Tracks with pDCA > 6 × σpDCA were rejected. The

quantity σpDCA is the pDCA resolution, which is obtained from data, taking into account

the resolution on track momentum and slope [36]. Such a track cut reduces the background

continuum by a few percent without affecting the resonances.

The extraction of the resonance signals is carried out by means of a fit to the dimuon

invariant mass spectrum, as illustrated in figure1 for the two rapidity ranges under study.

The J/ψ and ψ(2S) line shapes are described either by Crystal Ball (CB) functions [37],

with asymmetric tails on both sides of the peak, or by pseudo-Gaussian functions [38].

The parameters of the resonance shapes are obtained by means of a Monte-Carlo (MC) simulation. Pure J/ψ and ψ(2S) signal samples are generated, and then tracked and re-constructed in the experimental setup with the same procedure applied to real data. The choice of the MC kinematic distributions of charmonia is discussed below when introducing the acceptance calculation. Due to the large signal to background ratio (S/B) in the J/ψ mass region and in order to account for small deviations of the mass (∼0.1%) and width (∼10%) between MC and data, the corresponding parameters are left free in the fit. For the ψ(2S), due to the less favourable S/B, the mass and widths are constrained by those for the J/ψ using the following relations, which involve the corresponding MC quantities:

mψ(2S) = mJ/ψ+ (mMC_ψ(2S)− mMC_J/ψ) and σψ(2S)= σJ/ψ· (σMC_ψ(2S)/σ_J/ψMC). Alternative values of

the ψ(2S) mass resolution have also been tested, allowing the ratio (σ_ψ(2S)MC /σMC_J/ψ) to vary

within 10% [36]. Finally, the parameters of the asymmetric tails, which can hardly be

con-strained by the data, are kept fixed to their MC values. Additional sets of tails, obtained

from the MC, but sampling the ycmsand pTphase space, have also been tested. The

depen-dence of the extracted J/ψ and ψ(2S) yields on the variation of the tails and on the ψ(2S) mass resolution is included in the systematic uncertainty on the signal extraction. The background continuum under the resonances is parameterized by empirical shapes, using a polynomial times an exponential function or a Gaussian having a width increasing with mass. In order to assess the systematic uncertainty on signal extraction, fits with various combinations of the signal and background shapes are performed, and the start/end point of the fit range is also varied. The raw ψ(2S) yields and their statistical uncertainty is finally obtained as the average of the results of the various fits performed, while the systematic uncertainty is calculated as the root-mean-square (RMS) of their distribution. This results

JHEP12(2014)073

Counts per 50 MeV/

2 10 3 10 4 10 /ndf = 1.33 2 χ = 5.02 TeV NN s ALICE, p-Pb > 0 T p < 3.53, cms y 2.03 < ) 2 c (GeV/ -µ + µ m 2 2.5 3 3.5 4 4.5 5 2_c

Counts per 50 MeV/

2 10 3 10 4 10 /ndf = 1.39 2 χ = 5.02 TeV NN s ALICE, p-Pb > 0 T p < -2.96, cms y -4.46 <

Figure 1. Opposite-sign dimuon invariant mass spectra for the p-Pb (left) and Pb-p (right) data samples, together with the result of a fit. For the fits shown here, Crystal Ball functions (shown as dashed lines) and a variable-width Gaussian have been used for the resonances and the background, respectively. The χ2_{/ndf refers to the goodness of the signal and background}

combined fit in the displayed mass range.

statistical and the second is systematic. The ψ(2S) mass resolution extracted from the fits

is ∼70 MeV/c2_{. As a cross-check, an alternative approach for signal extraction, based on}

event counting, was also tested. More precisely, after fitting the invariant mass distribu-tion and subtracting the background contribudistribu-tion, the number of ψ(2S) was obtained by

integrating the background subtracted spectrum in the region 3.5 < mµµ < 3.8 GeV/c2.

Corrections, based on the signal fitting functions, were applied to the measured number of counts to account for the fraction of ψ(2S) outside of the integration region (∼15%) and for the number of J/ψ falling inside the ψ(2S) mass range (∼8%). The results were found to be

stable within 1% with respect to 0.1 GeV/c2 variations of the integration region. The

num-ber of J/ψ and ψ(2S) extracted in this way are also in excellent agreement (i.e., well within

the systematic uncertainties) with respect to the N_pPbψ(2S) and N_Pbpψ(2S) values quoted above.

The acceptance times efficiency values (A × ǫ) for the ψ(2S) were evaluated using

MC simulations in a similar way as detailed in [35] for the J/ψ. The input pT

distribu-tions were obtained from those used for the J/ψ [35_{], scaled such that hp}Tiψ(2S)pPb,5.02 TeV =

hpTiJ/ψ_{pPb,5.02 TeV} × (hpTi_{pp,7 TeV}ψ(2S) /hpTiJ/ψ_{pp,7 TeV}), and using the √s = 7 TeV pp values from

LHCb [39, 40] obtained in the slightly larger range 2 < ycms < 4.5. The input y

distri-butions were obtained from those used for the J/ψ assuming a scaling of the widths with

ymaxψ(2S)/yJ/ψmax, where yimax= log(√s/mi) is the maximum rapidity for the resonance i at the

√

s value under study. An unpolarized distribution for the ψ(2S) was assumed, according

to the results obtained in pp collisions at √s = 7 TeV by the CMS and LHCb

experi-ments [41, 42]. The systematic uncertainty for the ψ(2S) acceptance was calculated as

the maximum spread of the values obtained by assuming as alternative input distributions those used for the J/ψ itself and amounts to 1.8% (2.5%) for p-Pb (Pb-p).

The efficiency of the tracking and trigger detectors of the MS was taken into account in the MC simulations by means of a map of dead channels (tracking) and by building effi-ciency tables for the detector elements (trigger). The evolution of the detector performance

JHEP12(2014)073

throughout the data taking was followed in the MC, by generating a number of events which is proportional to the run-by-run number of dimuon triggers, in order to properly weight the detector conditions over the entire data taking. The systematic uncertainties on the effi-ciencies were obtained with algorithms based on real data, with the same procedure adopted

in [35], and they are identical for J/ψ and ψ(2S). A small uncertainty related to the

effi-ciency of the matching between tracking and triggering information was also included [35].

The pT-integrated A × ǫ values for ψ(2S) production, obtained with this procedure,

are 0.270 ± 0.014 (p-Pb) and 0.184 ± 0.013 (Pb-p), where the lower value for Pb-p is mainly due to a smaller detector efficiency in the corresponding data taking period, related to a worse detector performance. The quoted uncertainties are systematic and are obtained as the quadratic sum of the uncertainties on MC input, tracking, triggering and matching efficiencies. The statistical uncertainties are negligible.

The cross section times the branching ratio B.R.(ψ(2S) → µµ) for inclusive ψ(2S) production in p-Pb collisions (and similarly for Pb-p) is:

B.R._ψ(2S)→µ+_µ−· σ

ψ(2S)

pPb =

Ncor_{ψ(2S)→µµ}

LpPb_int (1)

where N_{ψ(2S)→µµ}cor _{is the number of ψ(2S) corrected for A × ǫ, and L}pPb_int is the integrated

luminosity, calculated as NMB/σ_pPbMB. NMB is the number of MB events, obtained as the

number of dimuon triggers divided by the probability of having a triggered dimuon in a MB

event. The NMB numerical values and uncertainties are the same as those quoted in [35].

The cross sections for the occurrence of the MB condition, σMB

pPb, are measured in a vdM

scan [33_{] to be 2.09 ± 0.07 b for the p-Pb configuration and 2.12 ± 0.07 b for the Pb-p one.}

The luminosity is also independently determined by means of a second luminosity signal,

as described in [33]. The two measurements differ by at most 1% throughout the whole

data-taking period and such a value is quadratically added to the luminosity uncertainty. The ψ(2S) cross section values are:

B.R. · σψ(2S)_pPb (2.03 < ycms<3.53) = 0.791 ± 0.096(stat.) ± 0.091(syst.uncorr.) ± 0.013(syst.corr.) µb B.R. · σ_Pbpψ(2S)(−4.46 < ycms< −2.96) = 0.653 ± 0.104(stat.) ± 0.080(syst.uncorr.) ± 0.010(syst.corr.) µb

The systematic uncertainties for the ψ(2S) cross section measurement are obtained as

the quadratic sum of the various contributions listed in table 1. The splitting between

uncorrelated and correlated sources is also summarized there. The corresponding values

for the J/ψ can be found in [35].

The study of the cross section ratio between ψ(2S) and J/ψ, and the comparison of this ratio between different systems, offers a powerful tool to investigate nuclear effects on charmonium production. In addition, several systematic uncertainties cancel, or are significantly reduced, when studying such ratios. In particular, in the present analysis, the tracking, trigger and matching efficiencies, as well as the normalization-related quantities, cancel out. For the MC input, the fraction of the uncertainty related to the choice of the

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Table 1. Systematic uncertainties (in percent) affecting the measurement of inclusive ψ(2S) cross sections. The Lint uncertainties are splitted in two components, respectively uncorrelated and

correlated between p-Pb and Pb-p, as detailed in [33]. All the other uncertainties are uncorre-lated between forward and backward rapidity. Uncertainties refer to pT-integrated quantities and,

where they depend on pT, the corresponding maximum and minimum values are also quoted. The

efficiency-related uncertainties refer to muon pairs.

(2%) uncertainty for p-Pb (Pb-p) is assigned to this source. Finally, the uncertainty on signal extraction is considered as uncorrelated between J/ψ and ψ(2S), and its value for the cross section ratios amounts to 10% for both p-Pb and Pb-p. The resulting values are:

B.R.ψ(2S)→µ+_µ−σψ(2S) B.R._J/ψ→µ+_µ−σJ/ψ (2.03 < ycms< 3.53) = 0.0154 ± 0.0019(stat.) ± 0.0015(syst.) B.R._ψ(2S)→µ+_µ−σ ψ(2S) B.R._J/ψ→µ+_µ−σJ/ψ (−4.46 < ycms < −2.96) = 0.0116 ± 0.0018(stat.) ± 0.0011(syst.)

In figure 2 we compare these ratios with the corresponding ALICE results for pp

collisions [36], obtained in slightly different centre of mass energy and rapidity regions,√s

= 7 TeV, 2.5 < |y| < 4, as no LHC pp results are available in the same kinematic conditions of proton-nucleus collisions. The pp ratios are significantly higher than those for p-Pb and Pb-p, which are compatible within uncertainties.

The double ratio [σ_ψ(2S)/σ_J/ψ]pPb/[σψ(2S)/σJ/ψ]pp is a useful quantity to directly

com-pare the relative suppression of the two states between various experiments. For this analysis, since the collision energy and the y-coverage of the p-Pb (Pb-p) and pp

mea-surements are different, we have estimated the possible dependence of the σψ(2S)_/σJ/ψ _vs

√

s and y in pp collisions. We start from the empirical observation that this ratio is very

similar at collider energies over a rather broad range of y and √s. In particular, from

the LHCb data (√s = 7 TeV, 2 < y < 4.5) [39, 40] one gets 2.11% for the inclusive ratio

integrated over pT, while the corresponding value from CDF data (pp at √s = 1.96 TeV,

|y| < 0.6) [43] is 2.05%, i.e., only 3% smaller (the latter quantity was obtained by

ex-trapolating the CDF ψ(2S) measurement to pT = 0 with the phenomenological function

f (pT) = (pT)/[1 + (pT/a)2]b) [44]. The LHCb result can be extrapolated to central rapidity

at√s = 7 TeV, assuming a Gaussian y-distribution for both resonances, with the width of

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= 7 TeV (open symbol: reflected around

s pp = 5.02 TeV NN s p-Pb

Figure 2. The cross section ratios B.R.ψ(2S)→µ+_µ−σ

ψ(2S)_/B.R.

J/ψ→µ+_µ−σ

J/ψ _{for p-Pb and Pb-p}

collisions, compared with the corresponding pp results at √s = 7 TeV [36]. The horizontal bars correspond to the width of the rapidity regions under study. The vertical error bars represent statistical uncertainties, the boxes correspond to systematic uncertainties.

assuming a scaling of the widths with yψ(2S)max /ymaxJ/ψ. The effect of this rescaling is small,

lead-ing to a 3% increase of the ratio. The central-rapidity ratio σψ(2S)/σJ/ψ at√s = 5.02 TeV

is then obtained by means of an interpolation between the CDF and LHCb-rescaled values,

assuming a linear dependence of the ratio vs√s. Finally, one can extrapolate the ratio to

the p-Pb and Pb-p rapidity ranges by using for the J/ψ the Gaussian shape obtained with

the interpolation procedure described in [45] and for the ψ(2S) the corresponding shape

scaled with ymaxψ(2S)/yJ/ψmax. The difference between the measured value of σψ(2S)/σJ/ψ for√s

= 7 TeV, 2 < ycms < 4.5 and the results of the interpolation procedure to √s = 5.02 TeV,

2.03 < ycms< 3.53 (−4.46 < ycms < −2.96) is -1.6% (-3.7%). When calculating the double

ratio [σψ(2S)/σJ/ψ]pPb/[σψ(2S)/σJ/ψ]pp, we choose to use for pp the measured value at √s

= 7 TeV, 2.5 < ycms < 4 [36] (rather than the interpolated one at √s = 5.02 TeV) and to

include a 8% systematic uncertainty on this quantity, i.e., about twice the maximum dif-ference between the measured values of the ratio in pp and the results of the interpolation procedure. A similar uncertainty would be obtained using as an input for the calculation,

instead of the LHCb data, the more recent pp result from ALICE on σ_ψ(2S)/σ_J/ψ [36].

The values of the double ratio are shown in figure 3, where they are also compared

with the corresponding results obtained by the PHENIX experiment at √sNN = 200 GeV,

for |y| < 0.35 [27]. When forming the double ratio, the systematic uncertainties on the pp

ratio, including the 8% contribution described in the previous paragraph, are considered as correlated between forward and backward rapidity, while the other systematic uncertainties are treated as uncorrelated. The dominating contributions to the systematic uncertainty come from the signal extraction and from the interpolation procedure used for the pp cross

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Figure 3. Double ratios [σψ(2S)/σJ/ψ]pPb/[σψ(2S)/σJ/ψ]pp for p-Pb and Pb-p collisions, compared

to the corresponding PHENIX result at √sNN= 200 GeV [27]. The horizontal bars correspond to

the width of the rapidity regions under study. For ALICE, the vertical error bars correspond to statistical uncertainties, the boxes to uncorrelated systematic uncertainties, and the shaded areas to correlated uncertainties. For PHENIX, the various sources of systematic uncertainties were combined in quadrature.

section. The ALICE results show that, compared to pp, the ψ(2S) is more suppressed than the J/ψ to a 2.3σ (4.1σ) level in p-Pb (Pb-p). The PHENIX result shows a similar feature, at a 1.3σ level.

The suppression of charmonium states with respect to the corresponding pp yield

can be quantified using the nuclear modification factor. For ψ(2S), Rψ(2S)_pPb is obtained by

combining RJ/ψ_pPb [35] with the double ratio evaluated above:

Rψ(2S)_pPb = R_pPbJ/ψ _·σ ψ(2S) pPb σJ/ψ_pPb · σppJ/ψ σppψ(2S) (2)

In figure 4, Rψ(2S)_pPb is shown and compared with RJ/ψ_pPb. For the double ratios, the

difference in the √s and y domains between p-Pb and pp is taken into account by the

inclusion of the 8% systematic uncertainty described above. The other quoted uncertainties

combine those from RJ/ψ_pPb [35] with those for the double ratio, avoiding a double counting

of the J/ψ related uncertainties. Figure 4 indicates that the ψ(2S) suppression is much

stronger than for the J/ψ and reaches a factor ∼2 with respect to pp. The results are

compared with theoretical calculations including either nuclear shadowing only [46, 47]

or coherent energy loss, with or without a shadowing contribution [48]. For the former

mechanism, the values correspond to calculations performed for the J/ψ. However, due to the relatively similar kinematic distributions of gluons that produce the cc pair which will then hadronize to a J/ψ or a ψ(2S), the shadowing effects are expected to be the same,

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EPS09 NLO (Vogt)

/fm (Arleo et al.) 2 =0.075 GeV 0 q ELoss with /fm (Arleo et al.) 2 =0.055 GeV 0 q

EPS09 NLO + ELoss with

Figure 4. The nuclear modification factor for ψ(2S), compared to the corresponding quantity for J/ψ [35]. The horizontal bars correspond to the width of the rapidity regions under study. The vertical error bars correspond to statistical uncertainties, the boxes to uncorrelated systematic uncertainties, and the shaded areas to partially correlated uncertainties. The filled box on the right, centered on RpPb= 1, shows uncertainties that are fully correlated between J/ψ and ψ(2S). Model

calculations tuned on J/ψ, and including nuclear shadowing [46, 47] and coherent energy loss [48] are also shown. The corresponding calculations for ψ(2S) produce identical values for the coherent energy loss mechanisms and a 2-3% larger result for nuclear shadowing and therefore are not shown.

within 2-3% [49, 50], for the two charmonium states. No sensitivity to the final quantum

numbers of the charmonium state is expected for coherent energy loss [51], implying that

the calculations shown in figure4are valid for both resonances. As a consequence, all three

models would predict an almost identical suppression for the ψ(2S) and the J/ψ over the full rapidity range, with negligible theoretical uncertainties. This prediction is in strong disagreement with our data and clearly indicates that other mechanisms must be invoked in order to describe the ψ(2S) suppression in proton-nucleus collisions.

The break-up cross section of the final state resonance due to interactions with CNM is expected to depend on the binding energy of the charmonium and such a mechanism would be a natural explanation for the larger suppression of ψ(2S). However, this process becomes

relevant only if the charmonium formation time τf is smaller than the time τcspent by the cc

pair inside the nucleus. One can evaluate the average proper time τcspent in CNM as τc=

hLi/(βzγ) [25], where hLi is the average length of nuclear matter crossed by the pair, which

can be calculated in the framework of the Glauber model [52], βz= tanh yrestcc is the velocity

of the cc along the beam direction in the nucleus rest frame, and γ = Ecc/mcc. For cc pairs

in the charmonium mass range emitted at pT = 0 in the forward acceptance, one gets

τc ∼ 10−4fm/c, while the corresponding value at backward rapidity is τc ∼ 7 · 10−2fm/c.

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2

c

Counts per 50 MeV/

2 10 3 10 4 10 < 2 GeV/c T p 0 < = 5.02 TeV NN s ALICE, p-Pb <3.53 cms y 2.03< /ndf = 1.04 2 χ 2 3 4 < 3 GeV/c T p 2 < /ndf = 1.21 2 χ 2 3 4 < 5 GeV/c T p 3 < /ndf = 1.39 2 χ 2 3 4 < 8 GeV/c T p 5 < /ndf = 1.04 2 χ 2.5 3 3.5 4 4.5 2 10 3 10 4 10 0 < p_T < 2 GeV/c <-2.96 cms y -4.46< /ndf = 1.10 2 χ 2.5 3 3.5 4 4.5 2 3 4 2 < pT < 3 GeV/c /ndf = 1.26 2 χ 2.5 3 3.5 4 4.5 2 3 4 3 < pT < 5 GeV/c /ndf = 1.17 2 χ ) 2 c (GeV/ -µ + µ m 2.5 3 3.5 4 4.5 2 3 4 5 < pT < 8 GeV/c /ndf = 1.18 2 χ

Figure 5. Opposite-sign dimuon invariant mass spectra, in bins of transverse momentum, for the p-Pb and Pb-p data samples. For the fits shown here, Crystal Ball functions (shown as dashed lines) and a variable-width Gaussian have been used for the resonances and the background, respectively. The χ2_{/ndf refers to the goodness of the signal and background combined fit in the}

displayed mass range.

situation, no break-up effects depending on the final charmonium state should be expected

at forward rapidity, and even for backward production one has at most τf ∼ τcwhich would

hardly accomodate the strong difference observed between ψ(2S) and J/ψ suppression. As a consequence, other final state effects should be considered, including the interaction of the cc pair with the final state hadronic system created in the proton-nucleus collision.

The sizeable ψ(2S) statistics collected in proton-nucleus collisions allows for a

differ-ential study of the various observables as a function of pT, in the range 0 < pT < 8 GeV/c.

We have chosen a transverse momentum binning which leads to similar relative statistical

uncertainties in each bin over the pT range covered. The analysis is carried out with the

same procedure adopted for the integrated data samples. In particular, the systematic

uncertainties are evaluated differentially in pT, and their range is also reported in table1.

In figure 5 the invariant mass spectra for the various pT bins are shown, together with

the result of the fits. In figure 6 the differential cross sections at forward and backward

rapidity are presented. The systematic uncertainties on signal extraction, MC input and

efficiencies are considered as bin-to-bin uncorrelated. The Lint uncertainties are correlated

between the various pT bins and partially correlated between p-Pb and Pb-p.

In figure 7 we present the pT dependence of the double ratio

[σ_ψ(2S)/σ_J/ψ]pPb/[σψ(2S)/σJ/ψ]pp, with the p-Pb J/ψ cross sections taken from [35]

and the pp values from [36]. As for the integrated double ratio, the systematic

uncertain-ties related to efficiencies and to normalizations cancel out for both proton-nucleus and pp, while the uncertainties on signal extraction and Monte-Carlo input are considered

JHEP12(2014)073

Figure 6_{. The ψ(2S) differential cross sections B.R.·d}2σ/dydpTfor p-Pb and Pb-p collisions. The

horizontal bars correspond to the width of the transverse momentum bins. The vertical error bars correspond to the statistical uncertainties, the boxes to uncorrelated systematic uncertainties and the shaded areas to pT-correlated uncertainties. A global 1.6% uncertainty applies to both p-Pb and

Pb-p results. The points corresponding to negative y are slightly shifted in pTto improve visibility.

two systems is correlated as a function of pT, while the uncertainties on the ratio in pp

collisions are correlated, for each pT bin, between forward and backward rapidity.

Finally, in figure 8the pT dependence of the ψ(2S) nuclear modification factor,

calcu-lated using eq. (2), is presented and compared with the corresponding result for J/ψ [53].

The uncertainties are obtained with the procedure used in figure 4, and the results are

compared to the same models quoted there.

Within uncertainties, no pT dependence of the double ratio can be seen, and

conse-quently as a function of transverse momentum Rψ(2S)_pPb has qualitatively a similar shape as

that exhibited by RJ/ψ_pPb, but systematically characterized by smaller values. Theoretical

models, which in this case also yield the same prediction for J/ψ and ψ(2S), are in fair agreement with J/ψ results, but clearly overestimate the ψ(2S) nuclear modification factor values.

It is interesting to note that different values of transverse momentum for the

reso-nances correspond to different τc, with the crossing times decreasing with increasing pT.

In particular, for backward production, τc varies by about a factor 2, between ∼0.07 (at

pT = 0) and ∼0.03 fm/c (at pT= 8 GeV/c). As a consequence, a larger fraction of cc pairs

may form the final resonance state inside CNM at low pT, and one might expect smaller

values of the double ratio in that transverse momentum region due to the weaker binding

energy of ψ(2S). Although the results shown in figure7could be suggestive of such a trend,

JHEP12(2014)073

Figure 7. The double ratio [σψ(2S)/σJ/ψ]pPb/[σψ(2S)/σJ/ψ]pp for p-Pb and Pb-p collisions, as a

function of pT. The horizontal bars correspond to the width of the transverse momentum bins. The

vertical error bars correspond to the statistical uncertainties, the boxes to uncorrelated systematic uncertainties and the shaded areas to correlated uncertainties. The points corresponding to negative y are slightly shifted in pTto improve visibility.

) c (GeV/ T p 0 1 2 3 4 5 6 7 8 pPb R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 /fm (Arleo et al.) 2 =0.055 GeV 0 q EPS09 NLO + ELoss with

/fm (Arleo et al.) 2 =0.075 GeV 0 q ELoss with EPS09 NLO (Vogt)

< 3.53 cms y = 5.02 TeV, 2.03 < NN s ALICE, p-Pb ψ J/ (2S) ψ ) c (GeV/ T p 0 1 2 3 4 5 6 7 8 pPb R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 /fm (Arleo et al.) 2 =0.055 GeV 0 q EPS09 NLO + ELoss with

/fm (Arleo et al.) 2 =0.075 GeV 0 q ELoss with EPS09 NLO (Vogt)

< -2.96 cms y = 5.02 TeV, -4.46 < NN s ALICE, p-Pb ψ J/ (2S) ψ

Figure 8. The nuclear modification factor for ψ(2S), compared to the corresponding quantity for J/ψ [53], as a function of pT. Plots correspond to p-Pb (left) and Pb-p (right) collisions. The

horizontal bars correspond to the width of the transverse momentum bins. The vertical error bars correspond to statistical uncertainties, the boxes to uncorrelated systematic uncertainties, and the shaded areas to partially correlated uncertainties. The filled box on the right, centered at RpPb = 1, shows uncertainties that are fully correlated between J/ψ and ψ(2S). For details on

JHEP12(2014)073

In summary, we have presented results on inclusive ψ(2S) production in proton-nucleus collisions at the LHC. Measurements were performed with the ALICE Muon Spectrometer

in the p-going (2.03 < ycms < 3.53) and Pb-going (−4.46 < ycms < −2.96) directions,

and the production cross sections, the double ratios with respect to the J/ψ in p-Pb and pp and the nuclear modification factors were estimated. The results show that ψ(2S) is

significantly more suppressed than J/ψ in both rapidity regions, and that no pTdependence

of this effect is found within uncertainties. This observation implies that initial state nuclear effects alone cannot account for the modification of the ψ(2S) yields, as also confirmed by

the poor agreement of the ψ(2S) RpPb with models based on shadowing and/or energy

loss. Final state effects, such as the pair break-up by interactions with cold nuclear matter, might in principle lead to the observed effect, but the extremely short crossing times for the cc pair, in particular at forward rapidity, make such an explanation unlikely. Consequently, other final state effects should be considered, including the interaction of the cc pair with the final state hadronic system created in the proton-nucleus collision.

Acknowledgments

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 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 Cient´ıfico e Tecnol´ogico (CNPq), Financiadora de Estudos

e Projetos (FINEP), Funda¸c˜ao de Amparo `a Pesquisa do Estado de S˜ao 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 BMBF and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian 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); CONACYT, DGAPA, M´exico, 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); Polish Ministry of Science and Higher Education;

Na-JHEP12(2014)073

tional Authority for Scientific Research - NASR (Autoritatea Nat¸ional˘a pentru Cercetare

S¸tiint¸ific˘a - ANCS); 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; CIEMAT, EELA, Ministerio de Econom´ıa y Competitividad (MINECO) of Spain, Xunta de Galicia (Conseller´ıa de Educaci´on), CEADEN, Cubaenerg´ıa, 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.

Open Access. This article is distributed under the terms of the Creative Commons

Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in

any medium, provided the original author(s) and source are credited.

References

[1] N. Brambilla et al., Heavy quarkonium: progress, puzzles and opportunities, Eur. Phys. J. C 71 (2011) 1534[arXiv:1010.5827] [INSPIRE].

[2] G.T. Bodwin, E. Braaten and G.P. Lepage, Rigorous QCD analysis of inclusive annihilation and production of heavy quarkonium,Phys. Rev. D 51 (1995) 1125[Erratum ibid. D 55 (1997) 5853] [hep-ph/9407339] [INSPIRE].

[3] M. Butenschoen and B.A. Kniehl, Reconciling J/ψ production at HERA, RHIC, Tevatron and LHC with NRQCD factorization at next-to-leading order,

Phys. Rev. Lett. 106 (2011) 022003[arXiv:1009.5662] [INSPIRE].

[4] E. Eichten, K. Gottfried, T. Kinoshita, K.D. Lane and T.-M. Yan, Charmonium: comparison with experiment,Phys. Rev. D 21 (1980) 203 [INSPIRE].

[5] Proceedings of the QM 2012 Int. Conf.,Nucl. Phys. A 904-905 (2013). [6] T. Matsui and H. Satz, J/ψ suppression by quark-gluon plasma formation,

Phys. Lett. B 178 (1986) 416[INSPIRE].

[7] F. Karsch and H. Satz, The spectral analysis of strongly interacting matter, Z. Phys. C 51 (1991) 209[INSPIRE].

[8] S. Digal, P. Petreczky and H. Satz, Quarkonium feed down and sequential suppression, Phys. Rev. D 64 (2001) 094015[hep-ph/0106017] [INSPIRE].

[9] H.T. Ding et al., Charmonium properties in hot quenched lattice QCD, Phys. Rev. D 86 (2012) 014509[arXiv:1204.4945] [INSPIRE].

[10] P. Braun-Munzinger and J. Stachel, (Non)thermal aspects of charmonium production and a new look at J/ψ suppression,Phys. Lett. B 490 (2000) 196[nucl-th/0007059] [INSPIRE]. [11] R.L. Thews, M. Schroedter and J. Rafelski, Enhanced J/ψ production in deconfined quark

JHEP12(2014)073

heavy quarks in ultra-relativistic nucleus-nucleus collisions,Nucl. Phys. A 789 (2007) 334 [nucl-th/0611023] [INSPIRE].

[13] Particle Data Group collaboration, J. Beringer et al., Review of particle physics (RPP), Phys. Rev. D 86 (2012) 010001[INSPIRE].

[14] NA50 collaboration, B. Alessandro et al., A new measurement of J/ψ suppression in Pb-Pb collisions at 158 GeV per nucleon,Eur. Phys. J. C 39 (2005) 335[hep-ex/0412036]

[INSPIRE].

[15] NA50 collaboration, B. Alessandro et al., ψ′

production in Pb-Pb collisions at 158 GeV/nucleon,Eur. Phys. J. C 49 (2007) 559[nucl-ex/0612013] [INSPIRE]. [16] NA50 collaboration, B. Alessandro et al., J/ψ and ψ′

production and their normal nuclear absorption in proton-nucleus collisions at 400 GeV,Eur. Phys. J. C 48 (2006) 329

[nucl-ex/0612012] [INSPIRE].

[17] NuSea collaboration, M.J. Leitch et al., Measurement of J/ψ and ψ′

suppression in p-A collisions at 800 GeV/c,Phys. Rev. Lett. 84 (2000) 3256[nucl-ex/9909007] [INSPIRE]. [18] HERA-B collaboration, I. Abt et al., A measurement of the ψ′

to J/ψ production ratio in 920 GeV proton-nucleus interactions,Eur. Phys. J. C 49 (2007) 545[hep-ex/0607046]

[INSPIRE].

[19] K.J. Eskola, H. Paukkunen and C.A. Salgado, EP S09: a new generation of NLO and LO nuclear parton distribution functions,JHEP 04 (2009) 065[arXiv:0902.4154] [INSPIRE]. [20] R. Vogt, Are the J/ψ and χcA dependencies the same?, Nucl. Phys. A 700 (2002) 539

[hep-ph/0107045] [INSPIRE].

[21] B.Z. Kopeliovich and B.G. Zakharov, Quantum effects and color transparency in charmonium photoproduction on nuclei,Phys. Rev. D 44 (1991) 3466[INSPIRE].

[22] D.C. McGlinchey, A.D. Frawley and R. Vogt, Impact parameter dependence of the nuclear modification of J/ψ production in d+Au collisions at √sNN= 200 GeV,

Phys. Rev. C 87 (2013) 054910[arXiv:1208.2667] [INSPIRE].

[23] F. Arleo and S. Peigne, J/ψ suppression in p-A collisions from parton energy loss in cold QCD matter,Phys. Rev. Lett. 109 (2012) 122301[arXiv:1204.4609] [INSPIRE].

[24] F. Arleo, P.B. Gossiaux, T. Gousset and J. Aichelin, Charmonium suppression in p-A collisions,Phys. Rev. C 61 (2000) 054906[hep-ph/9907286] [INSPIRE].

[25] D.C. McGlinchey, A.D. Frawley and R. Vogt, Impact parameter dependence of the nuclear modification of J/ψ production in d+Au collisions at √sNN= 200 GeV,

Phys. Rev. C 87 (2013) 054910[arXiv:1208.2667] [INSPIRE].

[26] R. Vogt, The xF dependence of ψ and Drell-Yan production,Phys. Rev. C 61 (2000) 035203

[hep-ph/9907317] [INSPIRE].

[27] PHENIX collaboration, A. Adare et al., Nuclear modification of ψ′

, χc and J/ψ production

in d+Au collisions at √sNN= 200 GeV,Phys. Rev. Lett. 111 (2013) 202301

[arXiv:1305.5516] [INSPIRE].

[28] ALICE collaboration, Rapidity and transverse momentum dependence of inclusive J/ψ production in pp collisions at√s = 7 TeV,Phys. Lett. B 704 (2011) 442[Erratum ibid. B 718(2012) 692] [arXiv:1105.0380] [INSPIRE].

JHEP12(2014)073

tracks,2010 JINST 5 P03003[arXiv:1001.0502] [INSPIRE].

[30] ALICE collaboration, Performance of the ALICE VZERO system,2013 JINST 8 P10016 [arXiv:1306.3130] [INSPIRE].

[31] ALICE collaboration, Measurement of the cross section for electromagnetic dissociation with neutron emission in Pb-Pb collisions at √sNN= 2.76 TeV,

Phys. Rev. Lett. 109 (2012) 252302[arXiv:1203.2436] [INSPIRE].

[32] ALICE collaboration, The ALICE experiment at the CERN LHC, 2008 JINST 3 S08002

[INSPIRE].

[33] ALICE collaboration, Measurement of visible cross sections in proton-lead collisions at √_s

NN= 5.02 TeV in van der Meer scans with the ALICE detector,2014 JINST 9 P11003

[arXiv:1405.1849] [INSPIRE].

[34] ALICE collaboration, Pseudorapidity density of charged particles p-Pb collisions at √_s

NN= 5.02 TeV,Phys. Rev. Lett. 110 (2013) 032301[arXiv:1210.3615] [INSPIRE].

[35] ALICE collaboration, J/ψ production and nuclear effects in p-Pb collisions at √_s

NN= 5.02 TeV,JHEP 02 (2014) 073[arXiv:1308.6726] [INSPIRE].

[36] ALICE collaboration, Measurement of quarkonium production at forward rapidity in pp collisions at√s = 7 TeV,Eur. Phys. J. C 74 (2014) 2974[arXiv:1403.3648] [INSPIRE]. [37] J. Gaiser, Line shape and resolution, in Charmonium spectroscdpy from radiative decays of

the J/ψ and ψ′

,SLAC-255, SLAC, Stanford U.S.A. (1982), pg. 177 [INSPIRE]. [38] R. Shahoyan, J/ψ and ψ′

production in 450 GeV pA interactions and its dependence on the rapidity and XF, Ph.D. thesis,http://www.cern.ch/NA50/theses/ruben.ps.gz, Instituto

Superior T´ecnico, Lisbon Portugal (2001).

[39] LHCb collaboration, Measurement of J/ψ polarization in pp collisions at √s = 7 TeV, Eur. Phys. J. C 73 (2013) 2631[arXiv:1307.6379] [INSPIRE].

[40] LHCb collaboration, Measurement of ψ(2S) meson production in pp collisions at √

s = 7 TeV,Eur. Phys. J. C 72 (2012) 2100[arXiv:1204.1258] [INSPIRE].

[41] CMS collaboration, Measurement of the prompt J/ψ and ψ(2S) polarizations in pp collisions at√s = 7 TeV,Phys. Lett. B 727 (2013) 381[arXiv:1307.6070] [INSPIRE].

[42] LHCb collaboration, Measurement of ψ(2S) polarisation in pp collisions at√s = 7 TeV, Eur. Phys. J. C 74 (2014) 2872[arXiv:1403.1339] [INSPIRE].

[43] CDF collaboration, T. Aaltonen et al., Production of ψ(2S) mesons in p¯p collisions at 1.96 TeV,Phys. Rev. D 80 (2009) 031103[arXiv:0905.1982] [INSPIRE].

[44] PHENIX collaboration, A. Adare et al., J/ψ production versus transverse momentum and rapidity in p+_{p collisions at}√_{s = 200 GeV,Phys. Rev. Lett. 98 (2007) 232002}

[hep-ex/0611020] [INSPIRE].

[45] ALICE and LHCb collaborations, Reference pp cross-sections for J/ψ studies in proton-lead collisions at √sNN= 5.02 TeV and comparisons between ALICE and LHCb results,

ALICE-PUBLIC-2013-002, CERN, Geneva Switzerland (2013) [LHCb-CONF-2013-013]. [46] J.L. Albacete et al., Predictions for p+Pb collisions at √sNN= 5 TeV,

Int. J. Mod. Phys. E 22 (2013) 1330007[arXiv:1301.3395] [INSPIRE]. [47] R. Vogt, private communication.

JHEP12(2014)073

loss in cold QCD matter,JHEP 03 (2013) 122[arXiv:1212.0434] [INSPIRE]. [49] E.G. Ferreiro, F. Fleuret, J.P. Lansberg and A. Rakotozafindrabe, J/ψ and ψ′

production in proton(deuteron)-nucleus collisions: lessons from RHIC for the proton-lead LHC run, J. Phys. Conf. Ser. 422 (2013) 012018[arXiv:1211.4749] [INSPIRE].

[50] E.G. Ferreiro, private communication. [51] F. Arleo, private communication.

[52] M.L. Miller, K. Reygers, S.J. Sanders and P. Steinberg, Glauber modeling in high energy nuclear collisions,Ann. Rev. Nucl. Part. Sci. 57 (2007) 205[nucl-ex/0701025] [INSPIRE]. [53] ALICE collaboration, B. Abelev et al., Rapidity and transverse momentum dependence of

the inclusive J/ψ nuclear modification factor in p-Pb collisions at √sNN= 5.02 TeV, in

JHEP12(2014)073

The ALICE collaboration

B. Abelev69_{, J. Adam}37_{, D. Adamov´a}77_{, M.M. Aggarwal}81_{, M. Agnello}105,88_{, A. Agostinelli}26_,

N. Agrawal44_{, Z. Ahammed}124_{, N. Ahmad}18_{, I. Ahmed}15_{, S.U. Ahn}62_{, S.A. Ahn}62_{, I. Aimo}105,88_,

S. Aiola129_{, M. Ajaz}15_{, A. Akindinov}53_{, S.N. Alam}124_{, D. Aleksandrov}94_{, B. Alessandro}105_,

D. Alexandre96_{, A. Alici}12,99_{, A. Alkin}3_{, J. Alme}35_{, T. Alt}39_{, S. Altinpinar}17_{, I. Altsybeev}123_,

C. Alves Garcia Prado113_{, C. Andrei}72_{, A. Andronic}91_{, V. Anguelov}87_{, J. Anielski}49_{, T. Antiˇci´c}92_,

F. Antinori102_{, P. Antonioli}99_{, L. Aphecetche}107_{, H. Appelsh¨auser}48_{, S. Arcelli}26_{, N. Armesto}16_,

R. Arnaldi105_{, T. Aronsson}129_{, I.C. Arsene}91_{, M. Arslandok}48_{, A. Augustinus}34_{, R. Averbeck}91_,

T.C. Awes78_{, M.D. Azmi}83_{, M. Bach}39_{, A. Badal`a}101_{, Y.W. Baek}40,64_{, S. Bagnasco}105_,

R. Bailhache48_{, R. Bala}84_{, A. Baldisseri}14_{, F. Baltasar Dos Santos Pedrosa}34_{, R.C. Baral}56_,

R. Barbera27_{, F. Barile}31_{, G.G. Barnaf¨oldi}128_{, L.S. Barnby}96_{, V. Barret}64_{, J. Bartke}110_,

M. Basile26_{, N. Bastid}64_{, S. Basu}124_{, B. Bathen}49_{, G. Batigne}107_{, A. Batista Camejo}64_,

B. Batyunya61_{, P.C. Batzing}21_{, C. Baumann}48_{, I.G. Bearden}74_{, H. Beck}48_{, C. Bedda}88_,

N.K. Behera44_{, I. Belikov}50_{, F. Bellini}26_{, R. Bellwied}115_{, E. Belmont-Moreno}59_{, R. Belmont III}127_,

V. Belyaev70_{, G. Bencedi}128_{, S. Beole}25_{, I. Berceanu}72_{, A. Bercuci}72_{, Y. Berdnikov}II,79_,

D. Berenyi128_{, M.E. Berger}86_{, R.A. Bertens}52_{, D. Berzano}25_{, L. Betev}34_{, A. Bhasin}84_{, I.R. Bhat}84_,

A.K. Bhati81_{, B. Bhattacharjee}41_{, J. Bhom}120_{, L. Bianchi}25_{, N. Bianchi}66_{, C. Bianchin}52_,

J. Bielˇc´ık37_{, J. Bielˇc´ıkov´a}77_{, A. Bilandzic}74_{, S. Bjelogrlic}52_{, F. Blanco}10_{, D. Blau}94_{, C. Blume}48_,

F. Bock68,87_{, A. Bogdanov}70_{, H. Bøggild}74_{, M. Bogolyubsky}106_{, F.V. B¨ohmer}86_{, L. Boldizs´ar}128_,

M. Bombara38_{, J. Book}48_{, H. Borel}14_{, A. Borissov}127,90_{, F. Boss´u}60_{, M. Botje}75_{, E. Botta}25_,

S. B¨ottger47_{, P. Braun-Munzinger}91_{, M. Bregant}113_{, T. Breitner}47_{, T.A. Broker}48_,

T.A. Browning89_{, M. Broz}37_{, E. Bruna}105_{, G.E. Bruno}31_{, D. Budnikov}93_{, H. Buesching}48_,

S. Bufalino105_{, P. Buncic}34_{, O. Busch}87_{, Z. Buthelezi}60_{, D. Caffarri}28_{, X. Cai}7_{, H. Caines}129_,

L. Calero Diaz66_{, A. Caliva}52_{, E. Calvo Villar}97_{, P. Camerini}24_{, F. Carena}34_{, W. Carena}34_,

J. Castillo Castellanos14_{, E.A.R. Casula}23_{, V. Catanescu}72_{, C. Cavicchioli}34_,

C. Ceballos Sanchez9_{, J. Cepila}37_{, P. Cerello}105_{, B. Chang}116_{, S. Chapeland}34_{, J.L. Charvet}14_,

S. Chattopadhyay124_{, S. Chattopadhyay}95_{, V. Chelnokov}3_{, M. Cherney}80_{, C. Cheshkov}122_,

B. Cheynis122_{, V. Chibante Barroso}34_{, D.D. Chinellato}115_{, P. Chochula}34_{, M. Chojnacki}74_,

S. Choudhury124_{, P. Christakoglou}75_{, C.H. Christensen}74_{, P. Christiansen}32_{, T. Chujo}120_,

S.U. Chung90_{, C. Cicalo}100_{, L. Cifarelli}26,12_{, F. Cindolo}99_{, J. Cleymans}83_{, F. Colamaria}31_,

D. Colella31_{, A. Collu}23_{, M. Colocci}26_{, G. Conesa Balbastre}65_{, Z. Conesa del Valle}46_,

M.E. Connors129_{, J.G. Contreras}11_{, T.M. Cormier}127_{, Y. Corrales Morales}25_{, P. Cortese}30_,

I. Cort´es Maldonado2_{, M.R. Cosentino}113_{, F. Costa}34_{, P. Crochet}64_{, R. Cruz Albino}11_,

E. Cuautle58_{, L. Cunqueiro}66_{, A. Dainese}102_{, R. Dang}7_{, A. Danu}57_{, D. Das}95_{, I. Das}46_{, K. Das}95_,

S. Das4_{, A. Dash}114_{, S. Dash}44_{, S. De}124_{, H. Delagrange}I,107_{, A. Deloff}71_{, E. D´enes}128_,

G. D’Erasmo31_{, A. De Caro}29,12_{, G. de Cataldo}98_{, J. de Cuveland}39_{, A. De Falco}23_,

D. De Gruttola29,12_{, N. De Marco}105_{, S. De Pasquale}29_{, R. de Rooij}52_{, M.A. Diaz Corchero}10_,

T. Dietel49_{, P. Dillenseger}48_{, R. Divi`a}34_{, D. Di Bari}31_{, S. Di Liberto}103_{, A. Di Mauro}34_,

P. Di Nezza66_{, Ø. Djuvsland}17_{, A. Dobrin}52_{, T. Dobrowolski}71_{, D. Domenicis Gimenez}113_,

B. D¨onigus48, O. Dordic21, S. Dørheim86, A.K. Dubey124, A. Dubla52, L. Ducroux122, P. Dupieux64_{, A.K. Dutta Majumdar}95_{, T. E. Hilden}42_{, R.J. Ehlers}129_{, D. Elia}98_{, H. Engel}47_,

B. Erazmus34,107_{, H.A. Erdal}35_{, D. Eschweiler}39_{, B. Espagnon}46_{, M. Esposito}34_{, M. Estienne}107_,

S. Esumi120_{, D. Evans}96_{, S. Evdokimov}106_{, D. Fabris}102_{, J. Faivre}65_{, D. Falchieri}26_{, A. Fantoni}66_,

M. Fasel87_{, D. Fehlker}17_{, L. Feldkamp}49_{, D. Felea}57_{, A. Feliciello}105_{, G. Feofilov}123_{, J. Ferencei}77_,

A. Fern´andez T´ellez2_{, E.G. Ferreiro}16_{, A. Ferretti}25_{, A. Festanti}28_{, J. Figiel}110_,

M.A.S. Figueredo117_{, S. Filchagin}93_{, D. Finogeev}51_{, F.M. Fionda}31_{, E.M. Fiore}31_{, E. Floratos}82_,

JHEP12(2014)073

A.M. Gago97_{, M. Gallio}25_{, D.R. Gangadharan}19_{, P. Ganoti}78_{, C. Garabatos}91_{, E. Garcia-Solis}13_,

C. Gargiulo34_{, I. Garishvili}69_{, J. Gerhard}39_{, M. Germain}107_{, A. Gheata}34_{, M. Gheata}34,57_,

B. Ghidini31_{, P. Ghosh}124_{, S.K. Ghosh}4_{, P. Gianotti}66_{, P. Giubellino}34_{, E. Gladysz-Dziadus}110_,

P. Gl¨assel87_{, A. Gomez Ramirez}47_{, P. Gonz´alez-Zamora}10_{, S. Gorbunov}39_{, L. G¨orlich}110_,

S. Gotovac109_{, L.K. Graczykowski}126_{, A. Grelli}52_{, A. Grigoras}34_{, C. Grigoras}34_{, V. Grigoriev}70_,

A. Grigoryan1_{, S. Grigoryan}61_{, B. Grinyov}3_{, N. Grion}104_{, J.F. Grosse-Oetringhaus}34_,

J.-Y. Grossiord122_{, R. Grosso}34_{, F. Guber}51_{, R. Guernane}65_{, B. Guerzoni}26_{, M. Guilbaud}122_,

K. Gulbrandsen74_{, H. Gulkanyan}1_{, M. Gumbo}83_{, T. Gunji}119_{, A. Gupta}84_{, R. Gupta}84_,

K. H. Khan15_{, R. Haake}49_{, Ø. Haaland}17_{, C. Hadjidakis}46_{, M. Haiduc}57_{, H. Hamagaki}119_,

G. Hamar128_{, L.D. Hanratty}96_{, A. Hansen}74_{, J.W. Harris}129_{, H. Hartmann}39_{, A. Harton}13_,

D. Hatzifotiadou99_{, S. Hayashi}119_{, S.T. Heckel}48_{, M. Heide}49_{, H. Helstrup}35_{, A. Herghelegiu}72_,

G. Herrera Corral11_{, B.A. Hess}33_{, K.F. Hetland}35_{, B. Hippolyte}50_{, J. Hladky}55_{, P. Hristov}34_,

M. Huang17_{, T.J. Humanic}19_{, N. Hussain}41_{, D. Hutter}39_{, D.S. Hwang}20_{, R. Ilkaev}93_{, I. Ilkiv}71_,

M. Inaba120_{, G.M. Innocenti}25_{, C. Ionita}34_{, M. Ippolitov}94_{, M. Irfan}18_{, M. Ivanov}91_{, V. Ivanov}79_,

A. Jacho lkowski27_{, P.M. Jacobs}68_{, C. Jahnke}113_{, H.J. Jang}62_{, M.A. Janik}126_,

P.H.S.Y. Jayarathna115_{, C. Jena}28_{, S. Jena}115_{, R.T. Jimenez Bustamante}58_{, P.G. Jones}96_,

H. Jung40_{, A. Jusko}96_{, V. Kadyshevskiy}61_{, S. Kalcher}39_{, P. Kalinak}54_{, A. Kalweit}34_{, J. Kamin}48_,

J.H. Kang130_{, V. Kaplin}70_{, S. Kar}124_{, A. Karasu Uysal}63_{, O. Karavichev}51_{, T. Karavicheva}51_,

E. Karpechev51_{, U. Kebschull}47_{, R. Keidel}131_{, D.L.D. Keijdener}52_{, M. Keil SVN}34_,

M.M. KhanIII,18_{, P. Khan}95_{, S.A. Khan}124_{, A. Khanzadeev}79_{, Y. Kharlov}106_{, B. Kileng}35_,

B. Kim130_{, D.W. Kim}62,40_{, D.J. Kim}116_{, J.S. Kim}40_{, M. Kim}40_{, M. Kim}130_{, S. Kim}20_{, T. Kim}130_,

S. Kirsch39_{, I. Kisel}39_{, S. Kiselev}53_{, A. Kisiel}126_{, G. Kiss}128_{, J.L. Klay}6_{, J. Klein}87_,

C. Klein-B¨osing49_{, A. Kluge}34_{, M.L. Knichel}91_{, A.G. Knospe}111_{, C. Kobdaj}34,108_{, M. Kofarago}34_,

M.K. K¨ohler91_{, T. Kollegger}39_{, A. Kolojvari}123_{, V. Kondratiev}123_{, N. Kondratyeva}70_,

A. Konevskikh51_{, V. Kovalenko}123_{, M. Kowalski}110_{, S. Kox}65_{, G. Koyithatta Meethaleveedu}44_,

J. Kral116, I. Kr´alik54, F. Kramer48, A. Kravˇc´akov´a38, M. Krelina37, M. Kretz39, M. Krivda96,54, F. Krizek77_{, E. Kryshen}34_{, M. Krzewicki}91_{, V. Kuˇcera}77_{, Y. Kucheriaev}I,94_{, T. Kugathasan}34_,

C. Kuhn50_{, P.G. Kuijer}75_{, I. Kulakov}48_{, J. Kumar}44_{, P. Kurashvili}71_{, A. Kurepin}51_,

A.B. Kurepin51_{, A. Kuryakin}93_{, S. Kushpil}77_{, M.J. Kweon}87_{, Y. Kwon}130_{, P. Ladron de}

Guevara58_{, C. Lagana Fernandes}113_{, I. Lakomov}46_{, R. Langoy}125_{, C. Lara}47_{, A. Lardeux}107_,

A. Lattuca25_{, S.L. La Pointe}52_{, P. La Rocca}27_{, R. Lea}24_{, L. Leardini}87_{, G.R. Lee}96_{, I. Legrand}34_,

J. Lehnert48_{, R.C. Lemmon}76_{, V. Lenti}98_{, E. Leogrande}52_{, M. Leoncino}25_{, I. Le´on Monz´on}112_,

P. L´evai128_{, S. Li}64,7_{, J. Lien}125_{, R. Lietava}96_{, S. Lindal}21_{, V. Lindenstruth}39_{, C. Lippmann}91_,

M.A. Lisa19_{, H.M. Ljunggren}32_{, D.F. Lodato}52_{, P.I. Loenne}17_{, V.R. Loggins}127_{, V. Loginov}70_,

D. Lohner87_{, C. Loizides}68_{, X. Lopez}64_{, E. L´opez Torres}9_{, X.-G. Lu}87_{, P. Luettig}48_,

M. Lunardon28_{, G. Luparello}52_{, R. Ma}129_{, A. Maevskaya}51_{, M. Mager}34_{, D.P. Mahapatra}56_,

S.M. Mahmood21, A. Maire87, R.D. Majka129, M. Malaev79, I. Maldonado Cervantes58,

L. MalininaIV,61_{, D. Mal’Kevich}53_{, P. Malzacher}91_{, A. Mamonov}93_{, L. Manceau}105_{, V. Manko}94_,

F. Manso64_{, V. Manzari}98_{, M. Marchisone}64,25_{, J. Mareˇs}55_{, G.V. Margagliotti}24_{, A. Margotti}99_,

A. Mar´ın91_{, C. Markert}111_{, M. Marquard}48_{, I. Martashvili}118_{, N.A. Martin}91_{, P. Martinengo}34_,

M.I. Mart´ınez2_{, G. Mart´ınez Garc´ıa}107_{, J. Martin Blanco}107_{, Y. Martynov}3_{, A. Mas}107_,

S. Masciocchi91_{, M. Masera}25_{, A. Masoni}100_{, L. Massacrier}107_{, A. Mastroserio}31_{, A. Matyja}110_,

C. Mayer110_{, J. Mazer}118_{, M.A. Mazzoni}103_{, F. Meddi}22_{, A. Menchaca-Rocha}59_,

J. Mercado P´erez87_{, M. Meres}36_{, Y. Miake}120_{, K. Mikhaylov}61,53_{, L. Milano}34_{, J. Milosevic}V,21_,

A. Mischke52_{, A.N. Mishra}45_{, D. Mi´skowiec}91_{, J. Mitra}124_{, C.M. Mitu}57_{, J. Mlynarz}127_,

N. Mohammadi52_{, B. Mohanty}73,124_{, L. Molnar}50_{, L. Monta˜no Zetina}11_{, E. Montes}10_,