Forward rapidity ψ(2S) meson production in pp, p-Pb and Pb-Pb collisions with ALICE at the LHC

The ALICE Collaboration has studied the inclusive ψ(2S) meson production in pp, p-Pb and Pb-Pb collisions at the CERN LHC. The ψ(2S) is detected through its decay to a muon pair, using the forward Muon Spectrometer, which covers the pseudo-rapidity range –4 < η < -2.5. The ψ(2S) production cross sections in pp collisions are presented as a function of rapidity (y) and transverse momentum (pT). In p-Pb collisions, ψ(2S) results are compared to the J/ψ ones by the ratio of their production cross sections as a function of rapidity, transverse momentum and event activity. The ψ(2S) nuclear modification factor, RpA, is also discussed. The results show a ψ(2S) suppression compared to the one observed for the J/ψ meson and are not described by theoretical models including cold nuclear matter effects as nuclear shadowing and energy loss. Finally, the preliminary results of ψ(2S) meson production in Pb-Pb collisions are shown in two pT ranges as a function of the collision centrality.


Introduction
The study of charmonia (bound states of c andc quarks), in different collision systems, is the object of intense theoretical and experimental investigations [1]. Proton-proton (pp) collisions are fundamental to evaluate the production cross section and to test production models. In proton-nucleus (p-A) collisions, several initial and final state effects, related to the presence of cold nuclear matter (shadowing, energy loss and nuclear absorption) can influence the observed charmonium yields [2,3]. Finally, in nucleus-nucleus (A-A) collisions, a deconfined phase of quarks and gluons (QGP) is expected to play an important role on the charmonium production [4]. Among the charmonium the ψ(2S) meson is receiving a lot of attention since it is more weakly-bound than the J/ψ and intriguing results have been already obtained at lower collision energies [5]. ALICE data can improve the understanding of ψ(2S) production in hadronic collisions.

ALICE detector and data samples
The ALICE detector consists of a central barrel dedicated to particle tracking and identification (in the pseudo-rapidity range of |η| < 0.9) and a forward spectrometer for the detection of muons (in the interval of −4 < η < −2.5). More details about the experimental setup can be found in [6]. Charmonium states are detected in the dimuon decay channel using the Muon Spectrometer. The pp analysis is performed in the rapidity interval of −4 < y lab < −2.5 using a data sample obtained at the center of mass energy of √ s=7 TeV and corresponding to an integrated luminosity of L pp int = 1.35 ± 0.07 pb −1 . In p-Pb collisions data have been collected at √ s N N =5.02 TeV in two configurations with inverted beam directions, with the following rapidity coverages: −4.46 < y cms < −2.96 (L Pbp int = 5.81 ± 0.18 nb −1 , Pb-going direction) at backward rapidity and 2.03 < y cms < 3.53 (L pPb int = 5.01 ± 0.19 nb −1 , p-going direction) at forward rapidity. Finally, the ψ(2S) production in Pb-Pb collisions is studied at √ s N N =2.76 TeV (L PbPb int = 68.8 ± 0.9 µb −1 ) in the rapidity region of −4 < y lab < −2.5.

Results
The ψ(2S) cross section is obtained as: , the number of reconstructed ψ(2S), is divided by the branching ratio BR µ + µ − , the detector mean acceptance times efficiency A and finally normalized to the integrated luminosity L int .

pp collisions
The results in pp collisions [7] are shown in Fig.1: the p T -differential cross section is compared to LHCb results [8]. A good agreement is observed between the two experiments (small differences are visible at low p T , but the comparison is not trivial because of the different rapidity coverage of the two detectors). The p T differential results are compared to LHCb measurements [8].

p-Pb collisions
The cross section ratio between the tightly bound J/ψ and the loosely bound ψ(2S) charmonium states, B.R. ψ(2S)→µ + µ − σ ψ(2S) /B.R. J/ψ→µ + µ − σ J/ψ is shown in the left panel of Fig.2. These ratios are significantly lower than the ones in pp, both at forward and backward rapidity, pointing to a bigger ψ(2S) suppression (compared to the J/ψ) in p-Pb collisions than in pp. The double ratios together with that of PHENIX, Fig.2. These results indicate thate the ψ(2S) suppression is more than the J/ψ to a level of 2.1σ at forward-rapidity and 3.5σ at backward-rapidity. At midrapidity, PHENIX results [9], from √ s NN = 200 GeV d-Au collisions, are in qualitative agreement with ALICE data [10]. The nuclear modification factor R pA , i.e. the ratio of the ψ(2S) production yield in p-A to the one in pp scaled by the number of binary collisions, is another useful quantity to study the effects of nuclear matter on the ψ(2S) production. The R pA of ψ(2S) and J/ψ, are shown in Fig.3, left, in the two rapidity intervals, indicating a stronger ψ(2S) suppression than that of the J/ψ, both at backward and forward rapidity.  ALICE results are compared with theoretical predictions including shadowing only [11] or coherent energy loss, with or without a shadowing contribution [12]. These calculations correspond to the ones performed for the J/ψ: shadowing effects are expected to be similar (within 2-3%), because of the similar gluon distributions that produce the cc state, while no dependence on the final state is expected for coherent energy loss. The predictions are in disagreement with the ψ(2S) data and indicate that other final state effects should be considered to explain the observed ψ(2S) suppression. The break-up of the resonance in the nuclear medium depends on the binding energy of the charmonium states and could be considered a cause of the larger ψ(2S) suppression. However, the break-up is relevant only if the charmonium formation time τ f is smaller than the time τ c spent by the cc pair in the nucleus. Estimates for τ f [13] are in the range 0.05-0.15 fm/c, while τ c = L /(β z γ) [14] (where L is the average length of nuclear matter crossed by the pair, β z = tanhy rest cc and γ = E cc /m cc ) is about 10 −4 fm/c at forward rapidity and about 7 · 10 −2 fm/c at backward rapidity. In this situation, the strong ψ(2S) suppression cannot be explained in terms of the cc pair break-up (expecially at backward rapidity where the difference between the J/ψ and ψ(2S) R pA is bigger). Finally, the double ratio [σ ψ(2S) /σ J/ψ ] pPb /[σ ψ(2S) /σ J/ψ ] pp is presented as a function of the event activity (i.e. the event multiplicity based on a measurement from the Zero Degree Calorimeters) in the two rapidity intervals (see Fig.3, right panel). When compared to the J/ψ, the ψ(2S) is more suppressed with increasing event activity, in particular at backward rapidity. This could be another hint of final state effects that can affect the ψ(2S) production, in particular at backward rapidity.

Pb-Pb collisions
The double ratio [σ ψ(2S) /σ J/ψ ] PbPb /[σ ψ(2S) /σ J/ψ ] pp has been studied by ALICE as a function of the collision centrality in two p T intervals (see Fig.4). In the interval 0 < p T < 3 GeV/c, the ψ(2S) signal can be extracted in three centrality classes, while, in the interval 3 < p T < 8 GeV/c the upper limit at 95% confidence level is shown for the most central collisions. ALICE results are compared with the CMS double ratios presented in two p T intervals corresponding to two different rapidity ranges. However, the large statistical and systematic uncertainties of the ALICE results prevent a firm conclusion on the ψ(2S) behaviour in Pb-Pb and the comparison with the CMS values [15] is not straightforward, given also the different kinematic coverage.  [15], in two p T intervals corresponding to two different rapidity coverages, are also shown.

Conclusions
In summary, ALICE collaboration has studied the ψ(2S) production in pp, p-Pb and Pb-Pb collisions. In pp collisions the ψ(2S) production cross sections have been obtained as a function of p T and y, and are in good agreement with the LHCb measurements. In p-Pb collisions the ψ(2S) is more suppressed than the J/ψ at both forward and backward rapidity. Theoretical models based on shadowing and/or energy loss are in disagreement with data and the break-up of the cc pair can hardly explain the strong ψ(2S) suppression, indicating that other final state effects are required. Finally, preliminary results in Pb-Pb collisions have been shown: large uncertainities prevent to make definitive conclusions.