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

The NThe Npartpart dependence of the anomalous J/ dependence of the anomalous J/ suppression in Pb-Pb collisions at the suppression in Pb-Pb collisions at the

CERN SPSCERN SPSRoberta Arnaldi

(Universita’ di Torino, Italy)

NA50 collaboration

XXXVIth RENCONTRES DE MORIONDQCD High Energy Hadronic Interactions

Les Arcs, Savoie, France,March 17-24, 2001

Outline:

J//DY analysis vs EZDC

Link between EZDC and Number of Participants

J//DY analysis vs Npart

Discussion and conclusions

Study of J/ suppression versus centrality:

The NA50 collaborationThe NA50 collaborationM.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

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)

cc 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:


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


J/J/ suppression pattern suppression pattern

NA50 Pb-Pb collisions at 158 AGeV

The experimental apparatusThe experimental apparatus

3 centrality detectors

muon spectrometer

0<yCM<1 (2.8<ylab<4.0)|cosCS|<0.5

electromagnetic calorimeter (1.1<<2.3)

multiplicity detector (1.5<<3.5)

zero degree calorimeter (>6.3)

Nch ET EZDC

Pb targets1996 30% I

1998 7% I


J/J/ suppression vs. E suppression vs. ETT

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 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)

central collisions peripheral collisions

large b

EEZDCZDC and the centrality of the and the centrality of the collisioncollision

)()(158)( bNbNbE partspectZDC

the link between Npart , Nspect and b is deduced in the framework of the Glauber model

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


small b

J/J/ suppression as a function of suppression as a function of EEZDCZDC

J//MB vs. EZDC

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


Analysis techniqueAnalysis technique

*DYJ

th

DYZDC

th

MBZDC

MBZDC dEdN

dEdN

dEdN

exp

exp

JZDCdEdN

build a new estimator of J//DY to overcome the

statistical fluctuations in the DY sample

dbbbNb,0P1b,EP

dEdN

COLLZDCAB

th

DYZDC

dbbbPbEPdEdN

ZDCAB

th

MBZDC

,01,

• Ncoll(b), P(EZDC,b) are calculated within the Glauber model

• is obtained through event countingexp

JZDCdEdN

•use of “Minimum Bias Trigger”


free parameters are obtained by means of a fit to the experimental EZDC distribution

)())(()( bEbEb physicsZDCZDCEZDC

detector

)()(158)( bNbNbE partspectZDC

•at fixed b, P(EZDC,b) is gaussian distributed with

physics fluctuations


Pb peak

J//DY* vs. EZDC (J//DY*)/Abs vs. EZDC

two step-pattern is clearly visibleRoberta Arnaldi XXXVIth Rencontres de Moriond

J/J/ suppression vs. suppression vs. NNpartpartAlternative 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 unambiguously Npart

( useful for comparing results with other experiments)

the correlation between Npart and EZDC is obtained from:

Glauber model+

detector resolution


J//DY* vs. Npart (J//DY*)/Abs vs. Npart


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

DiscussionDiscussionIn a deconfinement scenario we expect a steep two-step pattern corresponding to the suppression of the direct J/ and J/ from c decay

x2

N2 N1

x1


b1 ~ 3 fm b2 ~ 8 fm

This simple assumption nicely reproduces the data the J/ EZDC suppression pattern is compatible with a sharp double step in Npart


N1 ~ 340, Npart(N1)~25N2 ~ 125, Npart(N2)~40

ConclusionsConclusions

From EZDC measurement Npart can be deduced unambiguously

J/ suppression can be studied vs. Npart

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:

J/ suppression as a function of EZDC

The double step pattern expected in case of deconfinement and observed as a function of ET is confirmed

N1 ~ 340N2 ~ 125


Documents

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