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The N part dependence of the anomalous J/ suppression in Pb-Pb collisions at the CERN SPS Roberta Arnaldi (Universita di Torino, Italy) NA50 collaboration XXXVIth RENCONTRES DE MORIOND QCD High Energy Hadronic Interactions Les Arcs, Savoie, France, March 17-24, 2001 Outline: J/ /DY analysis vs E ZDC Link between E ZDC and Number of Participants J/ /DY analysis vs N part Discussion and conclusions Study of J/ suppression versus centrality:

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The NA50 collaboration M.C. Abreu 6,a, B. Alessandro 10, C. Alexa 3, R. Arnaldi 10, M. Atayan 12, C. Baglin 1, A. Baldit 2, M. Bedjidian 11, S. Beole 10, V. Boldea 3, P. Bordalo 6,b, A. Bussiere 1, L. Capelli 11, L. Casagrande 6,c, J. Castor 2, C. Castanier 2, B. Chaurand 9, B. Cheynis 11, E. Chiavassa 10, C. Cicalo 4, T. Claudino 6, M.P. Comets 8, N. Constans 9, S. Constantinescu 3, P. Cortese 10,d, N. De Marco 10, A. De Falco 4, G. Dellacasa 10,d, A. Devaux 2, S. Dita 3, O. Drapier 11, B. Espagnon 2, J. Fargeix 2, P. Force 2, M. Gallio 10, Y.K. Gavrilov 7, C. Gerschel 8, P. Giubellino 10, M.B. Golubeva 7, M. Gonin 9, A.A. Grigorian 12, S. Grigorian 12, J.Y. Grossiord 11, F.F. Guber 7, A. Guichard 11, H. Gulkanyan 12, R. Hakobyan 12, R. Haroutunian 11, M. Idzik 10,e, D. Jouan 8, T.L. Karavitcheva 7, L. Kluberg 9, A.B. Kurepin 7, Y. Le Bornec 8, C. Lourenco 5, P. Macciotta 4, M. Mac Cormick 8, A. Marzari-Chiesa 10, Masera 10, A. Masoni 4, M. Monteno 10, A. Musso 10, P. Petiau 9, A. Piccotti 10, J.R. Pizzi 11, W. Prado da Silva 10,f, F. Prino 10, G. Puddu 4, C. Quintans 6, S. Ramos 6,b, L. Ramello 10,d, P. Rato Mendes 6, L. Riccati 10, A. Romana 9, H. Santos 6, P. Saturnini 2, E. Scalas 10,d, E. Scomparin 10, S. Serci 4, R. Shahoyan 6,f, F. Sigaudo 10, S. Silva 6, M. Sitta 10,d, P. Sonderegger 5,b, X. Tarrago 8, N.S. Topilskaya 7, G.L. Usai 4, E. Vercellin 10, L. Villatte 8, N. Willis 8. 1 LAPP, CNRS-IN2P3, Annecy-le-Vieux, France. 2 LPC, Univ. Blaise Pascal and CNRS-IN2P3, Aubiere, France. 3 IFA, Bucharest, Romania. 4 Universita di Cagliari/INFN, Cagliari, Italy. 5 CERN, Geneva, Switzerland. 6 LIP, Lisbon, Portugal. 7 INR, Moscow, Russia. 8 IPN, Univ. de Paris-Sud and CNRS-IN2P3, Orsay, France. 9 LPNHE, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France. 10 Universita di Torino/INFN, Torino, Italy. 11 IPN, Univ. Claude Bernard Lyon-I and CNRS-IN2P3, Villeurbanne, France. 12 YerPhI, Yerevan, Armenia. a also at UCEH, Universidade de Algarve, Faro, Portugal. b also at IST, Universidade Tecnica de Lisboa, Lisbon, Portugal. c now at CERN. d Universita del Piemonte Orientale, Alessandria and INFN-Torino, Italy. e now at Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, Cracow, Poland. f now at UERJ, Rio de Janeiro, Brazil, g on leave of absence ofYerPhI,Yerevan,Armenia Roberta Arnaldi XXXVIth Rencontres de Moriond

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The search for QGP: the J/ The search for QGP: the J/ J/ 650 MeV 50 MeV c 250 MeV Physics motivation: look for QGP formation studying charmonium suppression in heavy-ion collisions J/ suppression by colour screening is predicted to be an unambiguous signature of QGP formation: (Matsui,Satz Phys. Lett. B178(1986) 416) pairs are produced very early in the collision by gluon fusion probe the medium they cross strongly bound states are not easy to break in the (relatively) soft interactions with comoving hadrons Binding energy: Roberta Arnaldi XXXVIth Rencontres de Moriond

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What is expected in this case? double step pattern due to: suppression of directly produced J/ melting of c state At SPS energies, sufficiently high T or for the phase transition could be reached. We could determine such threshold looking at the onset of the anomalous J/ suppression : important to measure the centrality of the collisions Roberta Arnaldi XXXVIth Rencontres de Moriond J/ suppression pattern NA50 Pb-Pb collisions at 158 AGeV

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The experimental apparatus 3 centrality detectors muon spectrometer 0<y CM <1 (2.8<y lab <4.0) |cos CS |<0.5 electromagnetic calorimeter (1.1< <2.3) multiplicity detector (1.5< <3.5) zero degree calorimeter ( >6.3) N ch E T E ZDC Pb targets 1996 30% I 1998 7% I Roberta Arnaldi XXXVIth Rencontres de Moriond

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J/ suppression vs. E T The reference: J/ suppression as expected from nuclear absorption (estimated from pp, pA, SU data) with abs =6.4 mb The J/ suppression pattern in Pb-Pb collisions shows: sharp onset of the J/ suppression around E T ~ 40 GeV steady decrease for high E T values the observed J/ suppression pattern provides significant evidence for deconfinement of quarks and gluons in Pb-Pb collisions (NA50 Coll., Phys. Lett. B 477 2000 28-36)

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central collisionsperipheral collisions large b E ZDC and the centrality of the collision the link between N part, N spect and b is deduced in the framework of the Glauber model From E ZDC measurement the centrality of the collisions can be estimated is obtained through a fit to the measured E ZDC. Energy carried by participants is 12 TeV Roberta Arnaldi XXXVIth Rencontres de Moriond small b

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J/ suppression as a function of E ZDC J/ /MB vs. E ZDC the solid line represents the absorption curve with abs = 6.4 ± 0.8 mb as obtained from p-p, p-A, S-U data ratio between two directly measured variables Roberta Arnaldi XXXVIth Rencontres de Moriond

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Analysis technique build a new estimator of J/ / DY to overcome the statistical fluctuations in the DY sample N coll (b), P(E ZDC,b) are calculated within the Glauber model is obtained through event counting use of Minimum Bias Trigger Roberta Arnaldi XXXVIth Rencontres de Moriond

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free parameters are obtained by means of a fit to the experimental E ZDC distribution detector at fixed b, P(E ZDC,b) is gaussian distributed with physics fluctuations Roberta Arnaldi XXXVIth Rencontres de Moriond Pb peak

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J/ /DY* vs. E ZDC (J/ /DY*)/Abs vs. E ZDC two step-pattern is clearly visible Roberta Arnaldi XXXVIth Rencontres de Moriond

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J/ suppression vs. N part Alternative way of presenting the results: use of E ZDC to estimate a centrality variable directly related to the geometry of the collision E ZDC measures N spect hence from E ZDC measurements we can estimate unambiguously N part ( useful for comparing results with other experiments) the correlation between N part and E ZDC is obtained from: Glauber model + detector resolution Roberta Arnaldi XXXVIth Rencontres de Moriond

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J/ /DY* vs. N part (J/ /DY*)/Abs vs. N part Roberta Arnaldi XXXVIth Rencontres de Moriond

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We check if the J/ /DY* behaviour vs E ZDC is compatible with a sharp double step in N part, taking into account the experimental resolution We assume to have two breaks in the suppression pattern at N part = N 1, N 2. We suppose that at N 1, N 2 a fraction x 1, x 2 of the detected J/ are suppressed Discussion In a deconfinement scenario we expect a steep two-step pattern corresponding to the suppression of the direct J/ and J/ from c decay x2x2 N2N2 N1N1 x1x1 Roberta Arnaldi XXXVIth Rencontres de Moriond

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b 1 ~ 3 fm b 2 ~ 8 fm This simple assumption nicely reproduces the data the J/ E ZDC suppression pattern is compatible with a sharp double step in N part Roberta Arnaldi XXXVIth Rencontres de Moriond N 1 ~ 340, Npart (N 1 )~25 N 2 ~ 125, Npart (N 2 )~40

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Conclusions From E ZDC measurement N part can be deduced unambiguously J/ suppression can be studied vs. N part The J/ suppression vs E ZDC is compatible with a sharp double step in N part, once the experimental resolution is taken into account. The critical values of N part are: J/ suppression as a function of E ZDC The double step pattern expected in case of deconfinement and observed as a function of E T is confirmed N 1 ~ 340 N 2 ~ 125 Roberta Arnaldi XXXVIth Rencontres de Moriond

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