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Nuclear absorption and anomalous $Jpsi$ suppression in Pb+Pb collisions


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Nuclear absorption and anomalous J/ψ suppression in Pb+Pb collisions

arXiv:nucl-th/0307115v1 31 Jul 2003

A. K. Chaudhuri
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Variable Energy Cyclotron Centre 1-AF, Bidhan Nagar, Kolkata-700 064 We discuss J/ψ suppression in a QCD based nuclear absorption model. Centrality dependence of J/ψ suppression in S+U and in Pb+Pb collisions are explained in the model. However, the model fails to explain the centrality dependence of ψ′ suppression. ψ′ suppression in S+U or in Pb+Pb collisions require additional suppression. Additional suppression of ψ′, due to hadronic comovers or due to QGP formation could not be distinguished in Pb+Pb collisions. We then show that the centrality dependence of the ratio, ψ′ over J/ψ, could possibly distinguish two scenario (e.g. QGP or hadronic comover) at RHIC energy. 1. INTRODUCTION J/ψ suppression is recognized as one of the promising signal of the decon?nement phase transition. Due to screening of color force, binding of c? pair into a J/ψ meson will be c hindered, leading to the so called J/ψ suppression in heavy ion collisions [1]. NA50 collaboration [2] observed anomalous suppression (suppression beyond ’conventional’ nuclear absorption) in 158 GeV/c Pb+Pb collisions. Several authors have explained the ’anomalous J/ψ suppression’ with or without the assumption of QGP formation [3–7]. In [7] it was shown that a QCD based nuclear absorption model could explain the anomalous J/ψ suppression in Pb+Pb collisions. The model also explain the centrality dependence of pT broadening of J/ψ in Pb+Pb collisions [8]. Recently, in QM2002, NA50 collaboration presented the preliminary analysis of 2000 Pb+Pb run [9]. They also presented the analJ/ψN ysis of high statistics pA data [10], implying J/ψ-nucleon absorption cross section, σabs =4.4 ± 1mb, much less than the earlier estimate of 6-7 mb. The latest NA50 data [9,10] are also well explained in the QCD based nuclear absorption model [11]. Here, after a brief description of the QCD based nuclear absorption model, we have presented our recent analysis of NA38/NA50 data on the centrality dependence of J/ψ and ψ′ suppression in S+U and in Pb+Pb collisions. Details of the analysis could be found in [12]. 2. J/ψ SUPPRESSION IN QCD BASED NUCLEAR ABSORPTION MODEL Details of QCD based nuclear absorption can be found in [6,7]. J/ψ production is assumed to be two step process, (i) production of c? pair, perturbatively calculable and (ii) c formation of J/ψ meson, intrinsically non-perturbative process, which is parameterized.

2 In pA/AA collisions, the produced pair interact with nuclear medium, which increases the relative 4-square momentum of the pair. Some of the pairs can gain enough momentum to cross the threshold for open charm production. Charmonium production is then reduced in AA collisions. Relevant parameter, gain in the 4-square momentum per unit length (ε2 =0.185 GeV 2 /f m) was obtained from a ?t to the high statistics pA data on the total J/ψ cross section [10]. The other parameter of the model, B?? σNN , was obtained from a DY NN ?t to NA38 S+U data [13]. The same value was used for the latest NA50 Pb+Pb data [9]. In Fig.1a and 1b, our model calculations are compared with NA38/NA50 data on J/ψ suppression. Data are well explained. It can be seen that there is no scope for additional suppression either due to comover interaction or due to QGP formation.
σ
J/ψ

35 30 25 20 15 0 0.5 20 40 60 80 100 0.6 (a) S+U

B??σ(J/Ψ)/σ(DY)

B??σ(J/ψ)/σ(DY)

30 20 10 0 0 40

(b)Pb+Pb

80

120

B'??σ(Ψ')/σ(DY)

Β'??σ(ψ')/s(DY)

0.4 0.3 0.2 0.1 0.0 0 20 40

(c) S+U

(d) Pb+Pb 0.4

0.2

0.0 60 80 100 0 40 80 120

ET (GeV)

ET (GeV)

Figure 1. Centrality dependence of the ratio, J/ψ over Drell-Yan and ψ′ over Drell-Yan, in S+U and Pb+Pb collisions. The solid lines are the QCD based nuclear absorption model calculation. The dashed and dash-dotted lines, for ψ′ suppression, are obtained with nuclear+comover and nuclear+QGP suppression respectively. 3. ψ′ SUPPRESSION It is experimentally observed that in pA collisions, A-dependence of ψ′ suppression is the same as for J/ψ[10,14]. The phenomena is explained in the color octet model [15], where c? attach with a collinear gluon to neutralise its colour. The pre-resonance (c?g) c c state then transforms in to a J/ψ or a ψ′. In pA collisions, nuclear medium sees only the pre-resonance state leading to similar A-dependence for J/ψ and ψ′. In the QCD based nuclear absorption model, nuclear suppression of J/ψ and ψ′ is due to the same

3 mechanism, i.e. gain in the relative 4-square momentum of the c? pair. The parameter c relevant for the A-dependence, the 4-square momentum gain per unit length ε2 , should be same, regardless of the ?nal state, J/ψ or ψ′. Naturally, J/ψ and ψ′ should show similar A-dependence. In Fig.1c and 1d, NA50/NA38 data [2,13] on the centrality dependence of the ratio, ψ′ over DY, are shown. The solid lines are the ?t, obtained in the QCD based nuclear absorption model. Clearly, the model predict less absorption than the data exhibit. Thus in AA collisions, additional suppression mechanism is operative for ψ′’s, which is absent in pA collisions. The additional suppression of ψ′ could be due QGP formation following a decon?nement phase transition or due to hadronic comovers (unlike in pA collisions, large number of secondaries are produced in AA collisions). To take into account the suppression due to QGP formation, following Blaizot et al. [3], we introduce an additional suppression factor (SQGP ) such that above a threshold density, nc , all the ψ′ are dissolved. Threshold density (nc ) is obtained from a ?t to the ψ′ suppression data [12]. Similarly, to account for the comover suppression, suppression factor due to comover-ψ′ interaction (Sco ) was introduced. We calculate Sco following [16]. Sco involve a parameter, ψ′-comover absorption cross section (σco ), which again is obtained from a ?t to the experimental ψ′ data.

0.02 Pb+Pb S+U

0.01

0.00 0 50 100 150 ET (GeV) 200 250

Figure 2. ET dependence of the ratio, ψ′ over J/ψ, in 200 GeV/c S+U and in 158 GeV/c Pb+Pb collisions. The solid and the dashed lines are the model calculations, for Pb+Pb collisions with nuclear+comover and nuclear+QGP suppression, respectively. The dash-dot and the dash-dot-dot lines are the predictions for the ratio at RHIC energy, with nuclear+comover and nuclear+QGP suppression respectively.

σ(ψ')/σ(J/ψ) ψ σ ψ

In Fig.1c and 1d, the dashed line is the ratio obtained with nuclear+comover suppression, with σco =8 mb. The comover scenario ?ts the ET dependence of ψ′ in S+U and in Pb+Pb collisions reasonably well. The QGP scenario, as expected, fails to explain the S+U data but describe the Pb+Pb data well, with the threshold density, nc =2.8 f m?2 . However, as nuclear plus comover suppression also explain the data, it is not possible to conclude positively about the formation of QGP from the ET dependence of ψ′ suppression. 4. CENTRALITY DEPENDENCE OF ψ′ OVER J/ψ Gupta and Satz [17] proposed the ratio of ψ′ over J/ψ, as a signal of QGP. The proposal follows from the simple observation that in a QGP both the J/ψ and ψ′ will be melted. Consequently, the ratio σ(ψ′)/σ(J/ψ) will remain constant with ET . Otherwise, the ratio

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σ(ψ′) will continually fall with ET , as ψ′ are more suppressed than J/ψ. The experimental σ(J/ψ) in S+U and in Pb+Pb collisions are shown in Fig.2 . For S+U collisions, the ratio fall continuously with ET . QGP is not formed in the collisions. For the Pb+Pb collisions, the ratio shows a tendency of saturation beyond 70 GeV. In Fig.2, the solid and dashed lines are the ratio for Pb+Pb collisions in the nuclear+comover and nuclear+QGP scenario. We note that even at large ET , di?erence between two model calculations are small. Even if there is a phase transition, it will be di?cult to reach de?nite conclusion at SPS energy. At RHIC, the situation is better. At RHIC hard scattering (proportional to number of binary collisions) can occur. Our prediction, with 37% hard scattering [18] for the ratio is shown in Fig.2. With nuclear+comover absorption, the ratio decreases continually with ET . In contrast, with nuclear+QGP suppression, the ratio remain constant for ET > 70 GeV. The di?erence, between the two model predictions, is also large.

5. SUMMARY AND CONCLUSIONS To conclude, we have analyzed the centrality dependence of J/ψ and ψ′ suppression in S+U and in Pb+Pb collisions. It was shown that while the J/ψ suppression is well explained in the QCD based nuclear absorption model, the model could not explain the centrality dependence of ψ′ suppression. ψ′’s require additional suppression, either due to QGP formation or due to comovers, two scenarios could not be distinguished. We then show that the ET dependence of the ratio of ψ′ over J/ψ could possibly signal the decon?ning phase transition at RHIC energy. REFERENCES 1. T. Matsui and H. Satz, Phys. Lett. B178(1986) 416. 2. NA50 collaboration, M. C. Abreu et al. Phys. Lett. B 477 (2000) 28, Nucl. Phys.A638(1998) 261c. 3. J. P. Blaizot, P. M. Dinh and J.Y. Ollitrault, Phys. Rev. Lett. 85 (2000) 4012. 4. A. Capella, E. G. Ferreiro and A. B. Kaidalov, Phys. Rev. Lett. 85 (2000) 2080. 5. A. K. Chaudhuri, Phys. Rev. C64 (2001) 054903, Phys. Lett. B527 (2002)80, Phys.Rev. C66 (2002)021902. 6. J. Qiu, J. P. Vary and X. Zhang, Phys.Rev. Lett.88 (2002) 232301. 7. A. K. Chaudhuri, hep-ph/0109141, Phys. Rev. Lett.88(2002) 232302. 8. A. K. Chaudhuri, nucl-th/0209012, Phys.Rev.C(in press). 9. L. Ramello et al., in Quark Matter 2002, Nantes, France (unpublished). 10. P. Cortese et al., in Quark Matter 2002, Nantes, France (unpublished). 11. A. K. Chaudhuri, nucl-th/0302044, Phys.Rev. C(in press). 12. A. K. Chaudhuri, nucl-th/0303030. 13. NA38 collaboration, C. Baglin et al., Phys. Lett. B345(1995) 617. 14. D. M. Alde et al., E777 Collab., Phys.Rev. Lett.66 (1991) 133. 15. G. T. Bodwin, E. Braaten and G.P. Lepage, Phys.Rev.D51 (1995) 1125; E. Braaten and S. Fleming, Phys.Rev.Lett.74 (1995) 3327. 16. Sean Gavin and R. Vogt, hep-ph/9606460, Phys. Rev. Lett. 78 (1997)1006. 17. S. Gupta and H. Satz, Phys.Lett.B283 (1992) 439. 18. D. Kharzeev and M. Nardi, Phys. Lett.B507 (2001) 121.


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