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The N part dependence of the anomalous J/  suppression in Pb-Pb collisions at the CERN SPS

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. Outline:. Study of J/  suppression versus centrality:. J/  /DY analysis vs E ZDC

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The N part dependence of the anomalous J/  suppression in Pb-Pb collisions at the CERN SPS

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

  2. XXXVIth Rencontres de Moriond Roberta Arnaldi 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

  3. pairs are produced very early in the collision by gluon fusion XXXVIth Rencontres de Moriond Roberta Arnaldi The search for QGP: the J/ 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)  probe the medium they cross  strongly bound states are not easy to break in the (relatively) soft interactions with comoving hadrons Binding energy: • J/  650 MeV • ’  50 MeV • c  250 MeV

  4. XXXVIth Rencontres de Moriond Roberta Arnaldi J/ suppression pattern NA50  Pb-Pb collisions at 158 AGeV • 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 What is expected in this case? double step pattern due to:  suppression of directly produced J/  melting of c state

  5. XXXVIth Rencontres de Moriond Roberta Arnaldi The experimental apparatus muon spectrometer Pb targets 1996  30% I 1998  7% I 0<yCM<1 (2.8<ylab<4.0) |cosCS|<0.5 zero degree calorimeter(>6.3) multiplicity detector(1.5<<3.5) electromagnetic calorimeter(1.1<<2.3) 3 centrality detectors EZDC Nch ET

  6. J/ suppression vs. ET The reference: J/ suppression as expected from nuclear absorption (estimated from pp, pA, SU data) with abs=6.4mb The J/ suppression pattern in Pb-Pb collisions shows: • sharp onset of the J/ suppression around ET ~ 40 GeV • steady decrease for high ET 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)

  7. central collisions XXXVIth Rencontres de Moriond Roberta Arnaldi EZDC and the centrality of the collision peripheral collisions small b large b From EZDC measurement the centrality of the collisions can be estimated  is obtained through a fit to the measured EZDC. Energy carried by participants is < 10% for EZDC >12 TeV the link betweenNpart , Nspectandb is deduced in the framework of the Glauber model

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

  9. is obtained through event counting XXXVIth Rencontres de Moriond Roberta Arnaldi Analysis technique build a new estimator of J//DY to overcome the statistical fluctuations in the DY sample • use of “Minimum Bias Trigger” • Ncoll(b), P(EZDC,b) are calculated within the Glauber model

  10. XXXVIth Rencontres de Moriond Roberta Arnaldi • at fixed b, P(EZDC,b) is gaussian distributed with     sdetector physics fluctuations free parameters are obtained by means of a fit to the experimental EZDC distribution Pb peak

  11. XXXVIth Rencontres de Moriond Roberta Arnaldi J//DY* vs. EZDC (J//DY*)/Abs vs. EZDC  two step-pattern is clearly visible

  12. XXXVIth Rencontres de Moriond Roberta Arnaldi J/ suppression vs. Npart  Alternative way of presenting the results: use of EZDC to estimate a centrality variable directly related to the geometry of the collision  EZDC measures Nspect hence from EZDC measurements we can estimate unambiguouslyNpart ( useful for comparing results with other experiments) the correlation between NpartandEZDC is obtained from: Glauber model + detector resolution

  13. XXXVIth Rencontres de Moriond Roberta Arnaldi J//DY* vs. Npart (J//DY*)/Abs vs. Npart

  14. x2 x1 N2 N1 XXXVIth Rencontres de Moriond Roberta Arnaldi 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 We check if the J//DY* behaviour vs EZDC is compatible with a sharp double step in Npart, taking into account the experimental resolution We assume to have two breaks in the suppression pattern at Npart= N1, N2. We suppose that at N1, N2 a fraction x1, x2 of the detected J/ are suppressed

  15. XXXVIth Rencontres de Moriond Roberta Arnaldi This simple assumption nicely reproduces the data the J/ EZDC suppression pattern is compatible with a sharp double step in Npart b1 ~ 3 fm b2 ~ 8 fm N1 ~ 340, Npart(N1)~25 N2 ~ 125, Npart(N2)~40

  16. XXXVIth Rencontres de Moriond Roberta Arnaldi Conclusions • J/ suppression as a function of EZDC • From EZDC measurement Npart can be deduced unambiguously J/ suppression can be studied vs. Npart The double step pattern expected in case of deconfinement and observed as a function of ET is confirmed • The J/  suppression vs EZDC is compatible with a sharp double step in Npart, once the experimental resolution is taken into account. The critical values of Npart are: N1 ~ 340 N2 ~ 125

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