We study J/psi suppression in AB collisions assuming that the charmonium states evolve from small, color transparent configurations. Their interaction with nucleons and nonequilibrated, secondary hadrons is simulated us- ing the microscopic model UrQMD. The Drell-Yan lepton pair yield and the J/psi /Drell-Yan ratio are calculated as a function of the neutral transverse en- ergy in Pb+Pb collisions at 160 GeV and found to be in reasonable agreement with existing data

Bass, Steffen A.

Bleicher, Marcus

Frankfurt, Leonid

Gerland, Lars

Greiner, Walter

Spieles, Christian

Strikman, Mark

Stöcker, Horst

Vogt, Ramona

English

Hochschulschriftenserver - Universität Frankfurt am Main

arXiv:hep-ph/9810486 v1   26 Oct 1998J/ψ suppression in heavy ion collisions – interplay of hard andsoft QCD processesC. Spieles1∗†, R. Vogt1,2, L. Gerland3, S.A. Bass4∗, M. Bleicher3, L. Frankfurt3,5∗,M. Strikman6, H. St¨ ocker3, W. Greiner31 Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA2 Physics Department, University of California at Davis, Davis, CA 95616, USA3 Institut f¨ ur Theoretische Physik, J. W. Goethe-Universit¨ at, D-60054 Frankfurt a.M., Germany4 Department of Physics, Duke University, Durham, N.C. 27708-0305, USA5 School of Physics and Astronomy, Tel Aviv University, 69978 Ramat Aviv, Tel Aviv, Israel6 Department of Physics, Pennsylvania State University, University Park, PA 16802, USAAbstractWe study J/ψ suppression in AB collisions assuming that the charmoniumstates evolve from small, color transparent conﬁgurations. Their interac-tion with nucleons and nonequilibrated, secondary hadrons is simulated us-ing the microscopic model UrQMD. The Drell-Yan lepton pair yield and theJ/ψ/Drell-Yan ratio are calculated as a function of the neutral transverse en-ergy in Pb+Pb collisions at 160 GeV and found to be in reasonable agreementwith existing data.∗Supported by the Alexander v. Humboldt Foundation†Email: cspieles@lbl.gov1I. INTRODUCTIONExperimental data on the AB and ET dependence of J/ψ-meson production from theCERN SPS [1–4] exhibit tantalizing evidence for J/ψ suppression when compared to hardQCD production. Do the J/ψ data in Pb+Pb-collisions indicate the creation of a deconﬁnedphase of strongly interacting matter, i.e. a quark-gluon plasma (QGP) [5,6]? Such QGPscenarios generally rely on the experimental observation of deviations from the predictionsof models of hadronic suppression. Simple analytical models of J/ψ suppression have usednuclear absorption alone [5] or absorption by nucleons and comoving secondaries [7] to ﬁtall but the Pb+Pb data. These models generally assume a single, ﬁxed nucleon absorptioncross section for all charmonium states and that the comover interaction rate, assuming aBjorken scaling expansion, is essentially thermal. (For theoretical reviews, see Refs. [8–10].)1Charmonium ﬁnal state interactions with mesons and baryons are simulated using theUltrarelativistic Quantum Molecular Dynamics, UrQMD [15], a microscopic hadronic trans-port model. The charmonium nucleon cross sections employed here are calculated in anonrelativistic potential model [16] since at SPS energies, charmonium-nucleon interactionsare predominantly nonperturbative. The charmonium precursors are not eigenstates of theQCD Hamiltonian, therefore their eﬀective sizes and interaction cross sections may vary fromtheir production to the ﬁnal state formation [16]. Thus the charmonium absorption crosssections are assumed to expand linearly with time until their asymptotic value is reached.Since the produced particles in the collision have a non-thermal distribution [17], comoverinteractions can rather eﬀectively dissociate the J/ψ. Because of abundant high mass res-onances, most of the meson induced J/ψ dissociation processes are exothermic [17]. Theyaccount for an important fraction of the observed suppression, larger in Pb+Pb collisionsthan the nuclear absorption alone.II. THE MODELWe apply perturbative QCD to the production of charmonium states by simulatingnucleus-nucleus collisions in the impulse approximation. The nuclear dependence of partondistribution functions is neglected. The resulting space-time distribution of charmoniumproduction points is inserted into the evolving hadronic environment calculated with UrQMD[17] since the rare quarkonium production processes are small perturbations on the heavyion collision2. Our model is thus designed to account for partonic and hadronic aspects ofthe charmonium dynamics. The space-time distributions of hard and soft processes, i.e.1Recently, there have been several attempts to build models of charmonium production and ab-sorption by means of microscopic hadronic transport simulations [11–14]. Based on rather diﬀerentmodel treatments of the charmonium dynamics and the interconnection of hard and soft processesit is claimed in all these studies that conventional hadronic scenarios are consistent with the Pb+Pbdata.2Since we consider only exclusive charmonium production, the QCD factorization theorem isinapplicable.2in particular charmonium production and absorption, may overlap. Thus, in our model, acharmonium state produced in a hard process can be dissociated by the interaction with acomoving hadron before this state leaves the nuclear environment and before all other hardproduction processes are completed (in contrast to Ref. [14]). The probability for such anevent, however, is reduced due to the initially small absorption cross sections and the ﬁniteformation times of comoving mesons according to the string model (see below). To avoiddouble counting, interactions of c¯ c states with produced hadrons in individual NN collisionsare excluded. In AB collisions, all hard NN collisions can contribute to the c¯ c productionwhile all soft NN collisions can contribute to the hadronic environment in which the c¯ c statemay be dissociated. The error imposed by this concept — inherent to all microscopic andanalytical models of comover absorption — is estimated to be very small.The charmonium states are distributed according to their assumed production probabilitytimes their decay probability to J/ψ’s. Thus 40% of the ﬁnal states are χ’s, 55% are J/ψ’s,and 5% are ψ′s [18]. According to the spin degeneracy, 1/3 of the χ’s are χc10 states and2/3 are χc11 states. Their momenta are assigned according to the parametrization [19],EdσdMdp3 ∼ (1 − xF)3.55 exp(−pT 2.08GeV−1) .The rescattering cross sections for X(c¯ c)+B, assuming B ≡ N, are taken from Ref. [16]:σ(J/ψN) = 3.6 mb, σ(ψ′N) = 20 mb, σ(χc10N) = 6.8 mb, and σ(χc11N) = 15.9 mb.Charmonium-meson cross sections (X(c¯ c) + π, X(c¯ c) + ρ, etc.) are reduced by a factorof 2/3 from the corresponding baryon values. All baryon and meson collisions above therespective dissociation thresholds are assumed to break up the charmonium state. Universaland energy independent cross sections are employed, ignoring any charmonium-meson reso-nances, perhaps too crude an assumption. From phase space arguments one can infer thatthe J/ψ dissociation cross sections with π’s should be suppressed close to threshold whileit should be enhanced for exothermic channels. However, we have found that, during theinitial stage of the J/ψ comover collisions, the average interaction energy, < E >≈ 5 GeV,is far above threshold [17]. There are no calculations of J/ψ dissociation cross sections withmesons other than π’s and ρ’s [10]. In a thermal comover scenario the density of heaviermesons is suppressed by the Boltzmann factor. According to the UrQMD simulations, how-ever, these mesons dominate the comover absorption [17]. In a forthcoming paper [20] theinﬂuence of the energy dependence of the comover interactions will be studied further.The cross sections correspond to the geometrical transverse radii riT =qσiπ of the char-monium states. We use σi to estimate the respective formation times τiF of the charmoniumstates by choosing τiF = riT/c. During these formation times the cross sections increaselinearly with t [16], starting from 0 at t = 0.Here it is important that we also take into account the formation time of comovingmesons (on average, τF ≈ 1 fm/c). Particles produced by string fragmentation are notallowed to interact with other hadrons – in particular with a charmonium state – withintheir formation time. However, leading hadrons are allowed to interact with a reduced crosssection even within their formation time. The reduction factor is 1/2 for mesons whichcontain a leading constituent quark from an incident nucleon and 2/3 for baryons whichcontain a leading diquark.3For this study, we have slightly modiﬁed the angular distributions of meson-baryon inter-action in the UrQMD 1.0 model since their strong forward peak underpredicts the total pro-duced transverse energy [21]. The model now reproduces the ET spectra in S(200 GeV)+Auand Pb(160 GeV)+Pb collisions measured by NA35 and NA49, respectively [22]. Neitherthe amount of baryon stopping nor the rapidity distribution of negatively charged parti-cles which have been shown to agree with experimental pp and AB interactions [15] aresigniﬁcantly aﬀected by this modiﬁcation.III. RESULTS AND DISCUSSIONFigure 1 shows the calculated number of Drell-Yan muon pairs, proportional to the num-ber of hard collisions, in Pb+Pb collisions as a function of the produced neutral transverseenergy within 1.1 < η < 2.3. The NA50 data [23] have been included with the abscissarescaled to reﬂect the latest change in data [4] which indicates an ≈ 20 % shift in the abso-lute ET scale from previous publications [3,23]. We are aware that the new analysis does notimply a simple overall rescaling of the old data points. However, in order to reasonably com-pare the gross features of the experimental dimuon ET spectrum with our model calculation,we have multiplied all ET-values of the data by 0.8. This factor was obtained by comparingthe ET − EZDC contour from Quark Matter ’97 [23] and the Moriond ’98 [4] proceedings.The agreement between the model and the rescaled data is to be expected since the NA49ET distribution [24] is described correctly [22]3. The additional simulation is a simple andwell understood model of hard scattering processes in nucleus-nucleus collisions.Figure 2 shows the J/ψ production cross section according to UrQMD calculations forseveral projectile-target combinations (p(450 GeV)+C, p(200 GeV)+Cu, p(200 GeV)+W,p(200 GeV)+U, S(200 GeV)+U, and Pb(160 GeV)+Pb) in comparison to experimentaldata [3]. The results of the calculations are normalized to the experimental cross sectionin p(200 GeV)+p interactions. The 450 GeV and 160 GeV simulations are rescaled toplab = 200 GeV with the parametrization of Ref. [26], as done by NA50 [2,3]. Consideringonly nuclear dissociation results in a far smaller J/ψ suppression than seen in the data, notonly for Pb+Pb collisions but also for S+U and even pA reactions. Note that the system-atics of nuclear absorption shown in Fig. 2 does not reﬂect a universal straight line as inGlauber calculations with constant absorption cross sections. By taking the nonequilibriumcharmonium-meson interactions into account, good agreement with the data is obtained.However, a strong dependence on parameters such as the charmonium and comover forma-tion times and the dissociation cross sections remains to be studied in detail [20].Due to the linear expansion of the charmonium cross sections with time, the J/ψ and3However, the agreement between the model and the NA38 experiment becomes poor for S+Ucollisions. The UrQMD calculation appears to overestimate the neutral transverse energy byabout 25% in the range 1.7 < η < 4.1 [1,25] although the calculated ET spectrum of the similarS+Au system agrees well with the NA35 data [24]. The UrQMD calculation thus indicates aninconsistency between the ET measurements by NA35, NA49 and NA50 on the one hand andNA38 on the other [22].4ψ′ cross sections are similar in the very early stages, leading to a weak A-dependence ofthe ψ′/J/ψ ratio in pA collisions at central rapidities, apparently consistent with the data.However, for a deeper understanding of this ratio quantum interference eﬀects [27] as wellas refeeding processes, πJ/ψ → ψ′π [28,29], must be considered. The ψ′/J/ψ ratio will bestudied further later [20].Figure 3 shows the J/ψ to Drell-Yan ratio as a function of ET for Pb+Pb interactionsat 160 GeV compared to the NA50 data [4]. The normalization of Bµµσ(J/ψ)/σ(DY) = 46in pp interactions at 200 GeV has been ﬁt to S+U data within a geometrical model [5]. Theapplication of this value to our analysis is not arbitrary: the model of Ref. [5] renders theidentical ET-integrated J/ψ survival probability, S = 0.49, as the UrQMD calculation forthis system. An additional factor of 1.25 [8] has been applied to the Pb+Pb calculation inorder to account for the lower energy, 160 GeV, since the J/ψ and Drell-Yan cross sectionshave diﬀerent energy and isospin dependencies. The gross features of the ET dependenceof the J/ψ to Drell-Yan ratio are reasonably well described by the model calculation. Nodiscontinuities in the shape of the ratio as a function of ET are predicted by the simulation.IV. CONCLUSIONWe have examined charmonium production and absorption processes in pA and ABcollisions at SPS energies. The microscopic simulation of hard processes in the impulseapproximation and the hadronic transport description of AB collisions with the UrQMDmodel simultaneously provide reasonable ET dependencies of the Drell-Yan rates as well asbaryon and meson rapidity distributions. We have modelled J/ψ absorption according tothe scenario described in Ref. [16]. Cross sections evolving from color transparent smallconﬁgurations to asymptotic states derived from quantum diﬀusion and the dynamical χpolarization (color ﬁltering) are taken into account.The calculated J/ψ production cross sections for minimum bias pA, S+U and Pb+Pbcollisions agree with experiment. Dissociation by nonequilibrium comovers accounts forabout half of the total absorption in S+U and Pb+Pb reactions. The contribution of theinteraction with comovers in pA reactions is small but not negligible. The suppressionof charmonium states is sensitive to the comover momentum distributions. The eﬀectivedissociation by comovers seems to indicate a nonequilibrated hadronic environment. Theobserved ET dependence of the J/ψ to Drell-Yan ratio in Pb+Pb collisions is reproducedby the model. The calculated result is smooth, without abrupt discontinuities, in agree-ment with new high statistics data [30]. We conclude that within our model, the data oncharmonium cross sections at the SPS can be explained without invoking exotic mechanisms.5FIGURES0 20 40 60 80 100 120 140ET (GeV)5100251012510225dN/dET(1/GeV)FIG. 1. Number of Drell-Yan pairs in Pb+Pb interactions as a function of the neutral trans-verse energy within 1.1 < η < 2.3. The calculation is normalized to the data. Shown is theUrQMD result and experimental data from NA50 [23] with the ET of the data rescaled by 0.8.The modiﬁcation is motivated by a comparison of the recently published ET −EZDC contour plot[4] with the previously published analysis [3,23].6100 101 102 103 104 105AB456789100234B(J/)/(AB)(nb/nucleon)nuclear onlytotalFIG. 2. J/ψ-production cross sections times dimuon branching ratio in the kinematical domain0 < ycm < 1 and |cosθCS| < 0.5, and rescaled, if necessary, to plab = 200 GeV as a function ofAB. The data (open triangles) are from [3]. Open circles denote the production cross sections ifonly nuclear absorption is considered.70 20 40 60 80 100 120 140ET (GeV)05101520253035404550B(J/)/(DY)FIG. 3. The ratio of J/ψ to Drell-Yan production as a function of ET for Pb+Pb at 160 GeV.The experimental data are from Ref. [4]. The normalization factor, from pp interactions at 200 GeV,Bµµσ(J/ψ)/σ(DY) = 46 is taken from Ref. [5]. This value, however, has been indirectly determinedin the framework of a diﬀerent model. An additional factor of 1.25 [8] has been applied to thePb+Pb calculation in order to account for the lower energy. Note that no scaling factor has beenapplied to the x-axis for either the calculations or the data.8REFERENCES[1] C. Baglin et al. (NA38 Collab.), Phys. Lett. B251 (1990) 472; Phys. Lett. B270 (1991)105; Phys. Let t. B345 (1995) 617.[2] M. Gonin et al. (NA50 Collab.), Nucl. Phys. A610 (1 996) 404c.[3] M.C. Abreu et al. (NA50 Collab.), Phys. Lett. B410 (1997) 327, 337.[4] A. Romana (NA50 Collab.), in Proceedings of the XXXIIIrd Rencontres de Moriond,March 1998, Les Arcs, France.[5] D. Kharzeev, C. Lourenco, M. Nardi and H. Satz, Z. Phys. C74 (1997) 307.[6] C.-Y. Wong, Presented at the Workshop on Quarkonium Production in RelativisticNuclear Collisions, Seattle, May 1998, hep-ph/9809497.[7] R. Vogt, Phys. Lett. B430 (1998) 15.[8] R. Vogt, LBNL-41758, Phys. Rep. in press.[9] D. Kharzeev, in Proceedings of Quark Matter ’97, Tsukuba, Japan, nucl-th/9802037.[10] B. M¨ uller, DUKE-TH-98-145, Talk given at the CERN Heavy Ion Forum, June 1997,nucl-th/9806023.[11] W. Cassing and C. M. Ko, Phys. Lett. B396 (1997) 39.[12] W. Cassing and E.L. Bratkovskaya, Nucl. Phys. A623 (1997) 570.[13] J. Geiss, C. Greiner, E.L. Bratkovskaya, W. Cassing and U. Mosel, nucl-th/9803008.[14] D.E. Kahana and S.H. Kahana, nucl-th/9808025.[15] S.A. Bass et al., Prog. Part. Nucl. Phys. 41 (1998) 225, nucl-th/9803035.[16] L. Gerland, L. Frankfurt, M. Strikman, H. St¨ ocker and W. Greiner, Phys. Rev. Lett.81 (1998) 762.[17] C. Spieles, R. Vogt, L. Gerland, S.A. Bass, M. Bleicher, H. St¨ ocker and W. Greiner,LBNL-42280, hep-ph/9809441.[18] R. Gavai, D. Kharzeev, H. Satz, G.A. Schuler, K. Sridhar and R. Vogt, Int. J. Mod. Phys.A10 (1995) 3043.[19] R. Vogt, Atomic Data and Nuclear Data Tables 50 (1992) 343.[20] C. Spieles, R. Vogt, S.A. Bass, M. Bleicher and H. St¨ ocker, in preparation.[21] M. Bleicher, C. Spieles, C. Ernst, L. Gerland, S. Soﬀ, H. St¨ ocker, W. Greiner andS.A. Bass, hep-ph/9803346, subm. to Phys. Lett. B.[22] C. Spieles, R. Vogt, S.A. Bass, M. Bleicher, H. St¨ ocker, LBNL-42380.[23] L. Ramello (NA50 Collab.), in Proceedings of Quark Matter ’97, Tsukuba, Japan, 1997.[24] T. Alber et al., Phys. Rev. Lett. 75 (1995) 3814.[25] A. Borhani (NA38 Collab.), Ph.D. thesis, Ecole Polytechnique, Palaiseau (1996).[26] G.A. Schuler, CERN Preprint, CERN-TH-7170-94, hep-ph/9403387.[27] L. Frankfurt and M. Strikman, Prog. Part. Nucl. Phys. 27 (1991) 135,J. H¨ ufner and B.Z. Kopeliovich, Phys. Rev. Lett. 76 (1996) 192.[28] H. Sorge, E. Shuryak and I. Zahed, Phys. Rev. Lett. 79 (1997) 2775.[29] J.-W. Chen and M. Savage, Phys. Rev. D57 (1998) 2837.[30] L. Kluberg (NA50 Collab.), Proceedings of the Conference on Heavy Ion Collisions fromNuclear to Quark Matter, September 1998, Erice, Italy.9

J/psi suppression in heavy ion collisions - interplay of hard and soft QCD processes

J/psi suppression in heavy ion collisions - interplay of hard and soft QCD processes

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