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Anomalous J/ suppression in Indium-Indium collisions at 158 GeV/nucleon

Roberta ArnaldiINFN Torino (Italy) on behalf of the NA60 Collaboration

Quark Matter 2005 XVIII International Conference on Nucleus-Nucleus Collisions

August 4-9, Budapest, Hungary

Study of the centrality dependence of the J/ suppression, using different normalization techniques

Comparison with theoretical models

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J/ production has been extensively studied in p-A, S-U and Pb-Pb collisions by the NA38 and NA50 experiments at the CERN SPS

J/ suppression from p-A to Pb-Pb collisions

( ) absLS J e

4.18 0.35mbJabs

J/ normal nuclear

absorption curve

J/

L

Projectile

Target

J/suppression is generally considered one of the most direct signatures of QGP formation (Matsui-Satz 1986)

• Light systems and peripheral Pb-Pb collisions: J/ is subject to nuclear absorption and the scaling variable for this process is L (length of nuclear matter crossed by the J/,

• Central Pb-Pb collisions: the L scaling is broken and an anomalous suppression sets in

~

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Study the J/ suppression pattern as a function of different centrality variables, including data from different collision systems

Study collisions between other systems, such as Indium-Indium

Which is the variable driving the suppression?

Is the anomalous suppression also present in lighter nuclear systems?

Study the nuclear dependence of c production in p-A collisions

What is the impact of the c feed-down on the observed J/ suppression pattern?

Specific questions that remain open

Study J/ production in p-A collisions at 158 GeV

What is the normal nuclear absorption cross-section at the energy of the heavy ion data?

New and accurate measurements are needed to answer these questions

S-U

In-In

Pb-Pb

Npart

L (f

m)

pure Glauber calculation

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NA60’s detector concept

MUON FILTER

BEAMTRACKER

TARGETBOX

VERTEX TELESCOPE

Dipole field2.5 T

BEAM

IC

not to scale

• Origin of muons can be accurately determined• Improved dimuon mass resolution

Matching in coordinate and in momentum space

ZDC allows studies vs. collision centrality

Idea: place a high granularity and radiation-hard silicon tracking telescope in the vertex regionto measure the muons before they suffer multiple scattering and energy loss in the absorber

beam

~ 1m Muon Spectrometer

MWPC’s

Trigger Hodoscopes

Toroidal Magnet

IronwallHadron absorber

ZDC

Target area

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Interaction in one of the 7 targets (Z-vertex of the collision determined with ~ 200 µm accuracy)

A clean sample of events is obtained with the following requirements:

Phase space window:-0.5 < cosCS < 0.5 & 2.92 < yLAB < 3.92

J/ production in Indium-Indium: event selection

Beam pile-up removed (in ±12 ns window) Matching between muon spectrometer and vertex telescope tracks is not mandatory for J/ studies. Requiring the matching:

• J/ mass resolution improves (from ~105 MeV to ~70 MeV)

• combinatorial background is reduced (from ~ 3% to <1% in 1 σ at the J/ peak)

• dimuon vertex is used to select only dimuons

produced in Indium-Indium collisions

• statistics is reduced (dimuon matching : ~ 70% at the J/) Available statistics for J/ studies

vs. centrality (before matching):

• ~ 43000 J/• ~ 300 Drell-Yan (Mass >4.2 GeV)

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Centrality estimate

NA60 can estimate the centrality of the collisions from the energy released in the Zero Degree Calorimeter (ZDC)

taking into account:

ZDC Spect PartE =N ×158GeV + αN

the small contribution of secondary particles emitted in the ZDC angular acceptance (η > 6.3)

the smearing due to the ZDC experimental resolution (~9% at the peak)

Number of participants

1 < EZDC < 2 TeV2 < EZDC < 3 TeV3 < EZDC < 4 TeV…

For each EZDC range we can estimate the corresponding NPart, L, NColl …

Target

Projectile

Target

Projectile

EZDC (GeV)

The link between NPart and other centrality variables is established within the Glauber model

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1) DY normalization (J/ / DY standard analysis)

2) analysis of the J/ sample standalone

J/ production analyses methods

Two different analyses have been performed to investigate the centrality dependence of the J/ production.

They correspond to two different ways of normalizing the J/ yield:

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J/

’

DY

Background

Charm

A multi-step fit (max likelihood) is performed:

a) M > 4.2 GeV : normalize the DY

b) 2.2 < M < 2.5 GeV: normalize the charm (with DY fixed)

c) 2.9 < M < 4.2 GeV: get the J/ yield (with DY & charm fixed)

Combinatorial background from and K decays estimated from the measured like-sign pairs (<3% contribution under the J/)

Signal mass shapes from Monte Carlo:• PYTHIA and GRV 94 LO parton densities• GEANT 3.21 for detector simulation • reconstructed as the measured data

Acceptances from Monte Carlo simulation:• for J/ : 12.4 % (6500 A); 13.8 % (4000 A)

• for DY : 13.2 % (6500 A); 14.1 % (4000 A) (in mass window 2.9–4.5 GeV)

without matching6500 data setno centrality selection

The J/ / DY standard analysis

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J//DY: advantages & drawbacks

We cannot have more than 3 centrality bins

High mass dimuons are rare: we are forced to study the J/ with statistical errors imposed by the 100 times smaller reference process

Advantages:

DY is a hard process. Its production cross section scales as the number of binary collisions and does not suffer sizeable final state effects

J/ / DY is a ratio of events collected with the same trigger (i.e. dimuon trigger). event selection is the same for both event samples inefficiencies or experimental biases (affecting both J/ and DY) cancel out

Easy comparison with the results previously obtained by the NA50 experiment

Drawbacks:

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The normal absorption curve is based on the NA50 results. Its uncertainty (~ 8%) at 158 GeV is dominated by the (model dependent) extrapolation from the 400 and 450 GeV data

need p-A measurements at 158 GeV: data collected in 2004 (analysis under way)

Comparison with previous results

An “anomalous suppression” is present already in Indium-Indium

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Direct J/ sample

The idea is to directly compare the measured J/ sample (as a function of centrality) with the theoretical distribution expected in case of pure nuclear absorption

Advantages:

Drawbacks:there is no intrinsic absolute normalization.

small statistical errors possibility to obtain a detailed pattern as a function of centrality

only one trigger involved (i.e. dimuon trigger)

inefficiencies are negligible

J/ sample: matched sample of J/ (cleaner spectrum). J/ events correspond to the signal after the combinatorial background subtraction

EZDC (GeV)

dNJ/

/d

EZ

DC

εvertex dimuon > 99.5 %

ε ver

tex

Inefficiencies introduced by the cuts, used in the events selection, affect in a negligible way the J/ sample

does not show a centrality dependence

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Direct J/ sample: the result

Data are compared with a theoretical J/ distribution, obtained within the Glauber model, taking into account the survival probability to the nuclear absorption.

The following pattern is observed:

Onset of anomalous suppression in the range 80 < NPart < 100

Saturation at large NPart

Nuclearabsorption

The ratio Measured / Expected is normalized to the standard analysis

EZDC(TeV)

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Direct J/ sample: stability of the result

The observed pattern is confirmed by a similar analysis with a reduced number of bins

Shifting the 16 EZDC bins by half the bin

width does not change the observed suppression pattern

J/ analysis

J/ analysis shifted binsJ/ analysis (2 TeV EZDC bins)

J/ analysis (1 TeV EZDC bins)

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Direct J/ sample: comparison with previous results

The S-U, In-In and Pb-Pb data points do not overlap in the L variable

The J/ suppression patterns are in fair agreement in the Npart variable

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Direct J/ sample: comparison with previous results (2)

NA50 Pb-PbNA60 In-In

NA50 Pb-PbNA60 In-In

A more significant comparison requires Pb-Pb points with reduced error bars

very

preliminary

Bjorken energy density, estimated using VENUS

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Direct J/ sample: comparison with theoretical models

Good accuracy of NA60 data allows a quantitive comparison with the available theoretical predictions

It is important to emphasize that these models were previously tuned on the p-A, S-U and Pb-Pb suppression patterns obtained by NA38 and NA50

J/ absorption by produced hadrons (comovers) Capella and Ferreiro, hep-ph/0505032;

J/ suppression in the QGP and hadronic phases including thermal regeneration and in-medium properties of open charm and charmonium states

Grandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301;

hep-ph/0403204

c suppression by deconfined partons when geometrical percolation sets in

Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.

We consider models for which we have predictions specifically made for In-In collisions:

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Suppression by produced hadrons (“comovers”)

In-In @ 158 GeV

The model takes into account nuclear absorption and comovers interaction with co = 0.65 mb (Capella-Ferreiro)

J/

NC

oll

nuclear absorption

comover + nuclear absorption

Pb-Pb @ 158 GeV

(E. Ferreiro, private communication)

The smeared form (dashed line) is obtained taking into account the resolution on NPart due to our experimental resolution

NA60 In-In 158 GeVpreliminary

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QGP + hadrons + regeneration + in-medium effects

The smeared form (dashed line) is obtained taking into account the resolution on NPart due to our experimental resolution

Pb-Pb @ 158 GeV

NA60 In-In 158 GeVpreliminary

B

J/

/D

Y

Nuclear Absorption

Regeneration

QGP+hadronic suppression

Suppression + Regeneration

In-In @ 158 GeV

Number of participants

fixed thermalization timecentrality dependent thermalization time

fixed thermalization timecentrality dependent thermalization time

The model simultaneously takes into account dissociation and regeneration processes in both QGP and hadron gas (Grandchamp, Rapp, Brown)

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The dashed line includes thesmearing due to the ZDC resolution

Suppression due to a percolation phase transition

Sharp onset (due to the disappearance of the c

meson) at Npart ~ 125 for Pb-Pb and ~ 140 for In-In

Model based on percolation (Digal-Fortunato-Satz)

Pb-Pb @ 158 GeV

NA60 In-In 158 GeVpreliminary

The measured data show a similar pattern but the anomalous suppression sets in at Npart ~ 90

NA60 In-In 158 GeVpreliminary

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Summary of models comparison

Satz, Digal, FortunatoRapp, Grandchamp, BrownCapella, Ferreiro

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Summary

• The J/ shows an anomalous suppression already in Indium-Indium

• The suppression is centrality dependent and sets in at a number of participants ~ 90

• None of the available models properly describes the observed suppression pattern

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http://cern.ch/na60

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Stony Brook

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~ 60 people13 institutes8 countries

R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen,B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanović, A. David, A. de Falco, N. de Marco,

A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A. Grigorian, J.Y. Grossiord,N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço,

J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri

The NA60 experiment