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