The recently published experimental dependence of the J/psi suppression pattern in Pb+Pb collisions at the CERN SPS on the energy of zero degree calorimeter EZDC are analyzed. It is found that the data obtained within the minimum bias analysis (using theoretical Drell-Yan ) are at variance with the previously published experimental dependence of the same quantity on the transversal energy of neutral hadrons ET . The discrepancy is related to the moderate centrality region: 100 << Np << 200 (Np is the number of nucleon participants). This could result from systematic experimental errors in the minimum bias sample. A possible source of the errors may be contamination of the minimum bias sample by o -target interactions. The data obtained within the standard analysis (using measured Drell-Yan multiplicity) are found to be much less sensitive to the contamination

Greiner, Walter

Kostyuk, Andriy P.

Stöcker, Horst

English

Hochschulschriftenserver - Universität Frankfurt am Main

arXiv:nucl-ex/0207018 v1   28 Jul 2002Comment to ‘The dependence of the anomalous J/ψ suppressionon the number of participant nucleons’A.P. Kostyuka,b, H. St¨ ockera, and W. Greineraa Institut f¨ ur Theoretische Physik, Universit¨ at Frankfurt, Germanyb Bogolyubov Institute for Theoretical Physics, Kyiv, Ukraine(November 15, 2005)AbstractThe recently published experimental dependence of the J/ψ suppression pat-tern in Pb+Pb collisions at the CERN SPS on the energy of zero degreecalorimeter EZDC are analyzed. It is found that the data obtained withinthe ‘minimum bias’ analysis (using ‘theoretical Drell-Yan’) are at variancewith the previously published experimental dependence of the same quantityon the transversal energy of neutral hadrons ET. The discrepancy is relatedto the moderate centrality region: 100 < ∼ Np < ∼ 200 (Np is the number ofnucleon participants). This could result from systematic experimental errorsin the minimum bias sample. A possible source of the errors may be con-tamination of the minimum bias sample by oﬀ-target interactions. The dataobtained within the standard analysis (using measured Drell-Yan multiplicity)are found to be much less sensitive to the contamination.Typeset using REVTEX1Recently, the NA50 collaboration published new data on the centrality dependence ofthe J/ψ suppression pattern in Pb+Pb collisions at CERN SPS [1]. The centrality of thecollisions was estimated by measuring the energy of projectile spectator nucleons EZDC bya zero degree calorimeter. The purpose of the present comment is to compare the new datawith the ones published by the NA50 collaboration previously [2–4], where the transverseenergy of produced neutral hadrons ET was used as the centrality estimator. It is shownthat the two sets of data (for brevity, we shall call them ‘EZDC-data’ and ‘ET-data’, respec-tively), having very similar qualitative behavior, seem, nevertheless, to be at variance onthe quantitative level. This may point out a presence of systematic errors in one of the datasets.In each data set, the J/ψ suppression pattern were obtained in two diﬀerent ways: bythe standard analysis and the minimum bias analysis. In the standard analysis, the ratioR of J/ψ multiplicity to the physical multiplicity of Drell-Yan pairs was calculated fromthe measured dimuon spectra. In the minimum bias analysis, the ratio R was obtained bydividing the experimentally measured ratio of the number of J/ψ events to the number ofminimum bias events by the theoretical Drell-Yan’ given by DY (ET)  =R ∞0 d2bP(ET|b) DY (b) Pint(b)R ∞0 d2bP(ET|b)Pint(b)(1)Here  DY (b)  is the average number of Drell-Yan dimuon pairs per Pb+Pb collision atimpact parameter b. Pint(b) is the probability that an interaction between two nuclei atimpact parameter b takes place. Both quantities were calculated in the Glauber approach.The ET-distribution of events at ﬁxed impact parameter b, P(ET|b), was assumed tohave the Gaussian form with the central value and dispersion given by ET(b)  = qNp(b) σET(b) = qqaNp(b) (2)Here Np(b) is the average number of nucleon participants in a Pb+Pb collision at impactfactor b calculated in the Glauber approach. The parameter values q = 0.274 GeV anda = 1.27 [5] were ﬁxed from the minimum bias transverse energy distribution [4].The EZDC-dependence of ‘theoretical Drell-Yan’ was found from the formula similar to(1). The EZDC-distribution P(EZDC|b) was assumed to be a Gaussian with the central valueand dispersion given by Eqs. (1) and (2) of Ref. [1].It is obvious that in absence of essential systematic errors, the ET- and EZDC-data shouldbe consistent with each other. If there exists a parameterization of the b-dependence of theJ/ψ multiplicity  J/ψ(b) , such that the ET dependence of R computed fromR(ET) =R ∞0 d2bP(ET|b) J/ψ(b) Pint(b) DY  ET(3)agrees with the ET-data, then the dependence of R on EZDCR(EZDC) =R ∞0 d2bP(EZDC|b) J/ψ(b) Pint(b) DY  EZDC(4)should agree with the EZDC-data without any additional parameter ﬁt. And vice versa: amodel ﬁtted to the EZDC-data should automatically agree with the ET-data.2To check, whether the two sets of the NA50 data indeed possess the above property, weused three diﬀerent models to parameterize  J/ψ(b) . Two of them, the models A and B (seebelow), were ﬁtted to the ET-data presented in Fig. 4 of Ref. [4] and then were comparedwith the ‘minimum bias’ EZDC-data from Fig.5 of Ref. [1]. The third one, the model C, wasﬁtted to the ‘minimum bias’ EZDC-data and compared with the ET-data.Model A is essentially the statistical coalescence model (SCM) [6,7], which assumesthat charmonia are created at the ﬁnal stage of the heavy ion reaction from the charmedquark-antiquark pairs produced in hard parton collisions at the initial stage. The freeparameters of the SCM (see Ref. [8] for details) are σNNc¯ c , the in-medium modiﬁed [9] crosssection of open charm production in nucleon-nucleon collisions, and η, the fraction of dimuonpairs, originating from J/ψ decays, that satisﬁes the kinematical conditions of the NA50spectrometer.The SCM is known to be applicable only for suﬃciently large systems (Np > 100). Toparameterize the data in the low ET region, we assumed a smooth transition between thenormal nuclear suppression (NNS) to the statistical coalescence model: J/ψ(b)  = exph−ζN2p(b)i J/ψ(b) NNS +n1 − exph−ζN2p(b)io J/ψ(b) SCM. (5)This introduces an additional free parameter ζ. The value of NNS cross section was ﬁxedat σabs = 6.4 mb.The drop of R(ET) at ET > ∼ 100 GeV appears in SCM due to ﬂuctuations of the ther-modynamical parameters and due to ET losses in the dimuon event sample relative to theminimum bias one [10].A good ﬁt of the ET-data (χ2/dof = 1.10) is obtained withσNNc¯ c = 3.67 µb η = 0.129 ζ = 1.04   10−4 . (6)Model B is the geometrical model of charmonium suppression [11,12]. It is assumedthat all charmonia suﬀer the normal nuclear suppression with the cross section σabs. Thenall excited charmonium states are destroyed in the region, where the nucleon participantdensity in the plane transverse to the collision axis exceeds the value n1. This suppressesthe J/ψ yield by a factor X < 1, because excited charmonia contribute to the ﬁnal J/ψmultiplicity through their decays. No charmonium state survives in the region, where theparticipant density exceeds the value n2. Fluctuations of ET result in additional suppressionat ET > ∼ 100 GeV [12]. ET losses are ignored.To make our parameterization as good as possible we treated not only n1 and n2, butalso σabs and X as free parameters. A very good ﬁt (χ2/dof = 0.95) is obtained atn1 = 2.97 fm−2 n2 = 4.05 fm−2σabs = 5.16 mb X = 0.366(7)In spite of the fact that the underlying ideas of the models A and B are very diﬀerent,the ﬁts are completely consistent with each other everywhere, except the low ET region,where only two points with large statistical errors are available (see Fig. 1).The both parameterizations are, however, in obvious disagreement with the ‘minimumbias’ EZDC-data: χ2/dof = 12.1 and χ2/dof = 11.5 for the models A and B, respectively.The strongest discrepancy is observed in the domain 100 < ∼ Np < ∼ 200 (see Fig. 2).3It must be noted that the eﬀects that were responsible for the drop of the ratio R atET > ∼ 100 GeV, the ﬂuctuations and ET losses, do not inﬂuence the EZDC dependence.Therefore, neither of the models is able to reproduce the drop of R(EZDC) at EZDC < ∼ 9TeV.Let us go the opposite way, i.e. we ﬁt the EZDC data and check, whether the ﬁt agreeswith the ET data. For this purpose, the model C will be used.Model C was proposed by NA50 collaboration in Ref. [1]. The idea is similar to themodel B, but the parameter that controls the charmonium suppression is not the nucleonparticipant density, but rather the number of participants. It is assumed that at Np > Np1,the J/ψ yield is suppressed by the factor of X1 < 1 relative to the normal nuclear suppressionvalue. At Np > Np2 the remaining J/ψ yield is additionally suppressed by the factor X2. Aminimum of χ2 is reached atNp1 = 122 Np2 = 335 σabs = 5.47mbX1 = 0.670 X2 = 0.752 (8)Although the quality of the ﬁt is not very good (χ2/dof = 2.73), the model satisfactoryreproduces the shape of the EZDC data, including the drop of the ratio R at EZDC < ∼ 9 TeV.Comparing the model C with the ET data reveals strong disagreement: χ2/dof = 13.3.The strongest discrepancy is again related to the region 100 < ∼ Np < ∼ 200 (see Fig. 1). Atlarge ET, although the drop of R at ET > ∼ 100 GeV is present on the curve, it does notreproduce the behavior of the data. Account for ET losses in the dimuon sample (similar to[13,10]) somewhat improves agreement with the data, but still the shape of the drop is notreproduced.The above consideration demonstrates that the two sets of the NA50 data on the cen-trality dependence of the J/ψ suppression pattern: the ET-dependence in Fig. 4 of Ref. [4]and the EZDC-dependence in Fig. 5 of Ref. [1] are at variance with each other. This couldmean that at least one of the two sets may contain sizable systematic errors.In our opinion, the EZDC-data obtained within the minimum bias analysis are very likelyto contain essential systematic errors because of possible contamination of the minimum biassample by oﬀ-target interactions. In the data analysis procedure of Ref. [1], empty targetruns were used to remove the contribution of the oﬀ-target interactions. This approach,however, does not take into account possible inﬂuence of target on the shape of the con-tamination. In fact, spectator fragments from an oﬀ-target collision may interact with thetarget. On the other hand, the spectator fragment from a normal collision with a targetnuclei may experience oﬀ-target interactions before they are registered by the zero degreecalorimeter. Therefore, in presence of the target, the oﬀ-target contribution may be shifteddownwards in EZDC.Indeed, the minimum bias ET distribution perfectly agrees with the Glauber model (seeFig. 1 of Ref. [4]).1 In contrast, the agreement of the EZDC minimum bias distribution seemsto be not so perfect. Unfortunately, the authors of Ref. [1] do not quote the value of χ2/dof,which characterizes the quality of the ﬁt of the minimum bias EZDC distribution (shown in1 Disagreement is observed only in the low ET region, because the eﬃciency of the target algorithmis less than unity there.4Fig.1 of Ref. [1]). Nevertheless, it is seen from the plot, that the experimental points lieslightly above the theoretical curves in the region 25 > ∼ EZDC > ∼ 18 TeV (which correspondsto 100 < ∼ Np < ∼ 200). Although the diﬀerence does not exceeds even the size of the pointsymbols, it, nevertheless, indicates a sizable discrepancy, because of the logarithmic scale ofthe vertical axis of the plot. Note, that at low EZDC, i.e. in the region of the ‘second drop’of the EZDC-data, the theoretical models are also in disagreement with the data. Therefore,the discrepancy of the two sets of data in the region 100 < ∼ Np < ∼ 200 and the drop ofthe ratio R(EZDC) at EZDC < ∼ 9 TeV may have a common origin: contamination of theminimum bias sample by oﬀ-target interactions.The standard analysis data appear to be less sensitive to the oﬀ-target contamination.In fact, the models A and B are consistent with the standard analysis EZDC-data (presentedin Fig. 3 of [1]): χ2/dof = 0.85 and χ2/dof = 0.83, respectively (see Fig. 3). No deﬁniteconclusion is possible because of the large statistical errors. The model C may be alsoconsidered to be consistent with the standard analysis data with slightly worse but stillquite satisfactory ﬁt quality: χ2/dof = 1.10. Nevertheless, no contradiction between ET andEZDC is seen in the recently presented by the NA50 collaboration standard analysis datafrom the year 2000 run, which have essentially smaller error bars [14]. This allows to concludethat the minimum bias sample seem to be more sensitive to the oﬀ-target contaminationsthan the dimuon one.The discrepancy between ET and EZDC data are unlikely to be related to interactionsof Pb nuclei and their fragments with air. In fact, comparing of the data collected in theyear 2000 with the target in vacuum [14] with the data from previous runs [1–4] indicatesthat interactions with air inﬂuence both ET and EZDC data and that the inﬂuence is relatedmostly to the peripheral domain Np < ∼ 100.The contaminations at moderate centrality, 100 < ∼ Np < ∼ 200, may come from interactionsof reactions fragments with nuclei in zero degree calorimeter. If such an interaction takesplace, a fraction of the produced particles (those having large transversal momenta) mayleave the calorimeter without deposing their energy in it. This could cause sizable lossesof zero-degree energy, which result in distortion of the shape of the minimum bias sample.Therefore, the mentioned problem will likely persist even in the new vacuum data, if theyare analyzed within the ‘minimum bias’ procedure, unless losses of the zero degree energyare taken into account.We conclude, that the new data on the EZDC dependence of the J/ψ suppression patternin Pb+Pb at the CERN SPS [1] are at variance with the other data published by the samecollaboration [2–4]. This problem is related only to the data obtained within the ‘minimumbias’ analysis. A possible source of the discrepancy might be distortion of the minimumbias sample most likely by losses of zero degree energy because of interactions of reactionfragments with nuclei inside the zero degree calorimeter.ACKNOWLEDGMENTSWe are grateful to R. Arnaldi, P. Bordalo, M. Gonin, L. Kluberg, M. Mac Cormick,E. Scomparin, N. Topilskaya for their interest to the paper and for interesting discussions.We acknowledge the ﬁnancial support of the Alexander von Humboldt Foundation, DFG,GSI and BMBF, Germany.5051015202530354045500 20 40 60 80 100 12050 100 150 200 250 300 350RET (GeV)NpPb-Pb 1996Pb-Pb 1996 min. biasPb-Pb 1998 min. biasmodel Amodel Bmodel Cmodel C with ET lossFIG. 1. The dependence of the J/ψ to Drell-Yan ratio R on the transverse energy ET. Thecorresponding average number of participant nucleons Np are shown on the upper axis. The pointswith error bars are the NA50 data. The lines correspond to diﬀerent parametrizations. ModelsA an B are ﬁtted to the ET data set. Model C being ﬁtted to the EZDC data set (see Fig. 2)demonstrates disagreement with the ET data.6051015202530350 5 10 15 20 25 30350 300 250 200 150 100 50REZDC (TeV)NpPb-Pb 1996 min. biasPb-Pb 1998 min. biasmodel Amodel Bmodel CFIG. 2. The dependence of the J/ψ to Drell-Yan ratio R on the energy of zero degree calorimeterEZDC. The corresponding average number of participant nucleons Np are shown on the upper axis.The points with error bars are the NA50 ‘minimum bias’ analysis data. The lines correspond todiﬀerent parameterizations. Model C is ﬁtted to the EZDC data set. Models A and B being ﬁttedto the ET-data (see Fig. 1) demonstrate disagreement with the EZDC data.705101520253035400 5 10 15 20 25 30350 300 250 200 150 100 50REZDC (TeV)NpPb-Pb 1996Pb-Pb 1998model Amodel Bmodel CFIG. 3. The dependence of the J/ψ to Drell-Yan ratio R on the energy of zero degree calorimeterEZDC. The corresponding average number of participant nucleons Np are shown on the upper axis.The points with error bars are the NA50 standard analysis data. The lines correspond to diﬀerentparametrizations. Models A and B are ﬁtted to the ET-data (see Fig. 1) Model C is ﬁtted to the‘minimum bias’ EZDC data (see Fig. 2). 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