Anomalous Jy suppression in IndiumIndium collisions at 158 GeVnucleon

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Anomalous J/y suppression in Indium-Indium
collisions at 158 GeV/nucleon
Summary of the J/y results presented at Quark
Matter 2005
Study of the centrality dependence of
the J/y suppression, using different
normalization techniques Comparison with
theoretical models
Roberta Arnaldi INFN Torino (Italy) on behalf
of the NA60 Collaboration
Heavy Ion Forum CERN, October 6th 2005
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J/y suppression from p-A to Pb-Pb collisions
J/y suppression is generally considered one of
the most direct signatures of QGP formation
(Matsui-Satz 1986)
J/y production has been extensively studied in
p-A, S-U and Pb-Pb collisions by the NA38 and
NA50 experiments at the CERN SPS
Projectile
J/y
L
Target
J/y normal nuclear absorption curve
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Specific questions that remain open
Is the anomalous suppression also present in
lighter nuclear systems?
Study collisions between other systems, such as
Indium-Indium
Which is the variable driving the suppression?
Study the J/? suppression pattern as a function
of different centrality variables, including data
from different collision systems
What is the normal nuclear absorption
cross-section at the energy of the heavy ion data?
Study J/? production in p-A collisions at 158 GeV
What is the impact of the cc feed-down on the
observed J/y suppression pattern?
Study the nuclear dependence of cc production in
p-A collisions
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NA60s detector concept
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
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J/y production in Indium-Indium event selection
A clean sample of events is obtained with the
following requirements
Beam pile-up removed (in 12 ns window)
Matching between muon spectrometer and vertex
telescope tracks is not mandatory for J/y
studies. Requiring the matching
Interaction in one of the 7 targets (Z-vertex of
the collision determined with 200 µm accuracy)

• J/y mass resolution improves
  (from 105 MeV to 70 MeV)
• combinatorial background is reduced (from
  3 to lt1 in ? 1 s at the J/y peak)
• dimuon vertex is used to select only dimuons
  produced in Indium-Indium collisions
• statistics is reduced
  (dimuon matching e 70 at the J/y)

Phase space window -0.5 lt cos?CS lt 0.5 2.92
lt yLAB lt 3.92

• Available statistics for J/y studies vs.
  centrality (before matching)
• 43000 J/y
• 300 Drell-Yan (Mass gt4.2 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
the small contribution of secondary particles
emitted in the ZDC angular acceptance (? gt
6.3) the smearing due to the ZDC experimental
resolution (9 at the peak)
The link between NPart and other centrality
variables is established within the Glauber model
1 lt EZDC lt 2 TeV 2 lt EZDC lt 3 TeV 3 lt EZDC lt 4
TeV
? For each EZDC range we can estimate the
corresponding NPart, L, NColl
Number of participants
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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
1) DY normalization (J/? / DY standard
analysis) 2) analysis of the J/? sample
standalone
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The J/? / DY standard analysis
Background
without matching 6500 data set no centrality
selection

• Combinatorial background from ? and K decays
  estimated from the measured like-sign pairs (lt3
  contribution under the J/y)
• 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/y 12.4 (6500 A) 13.8 (4000 A)
• for DY 13.2 (6500 A) 14.1 (4000 A)
• (in mass window 2.94.5 GeV)

J/y
Charm
y
DY
A multi-step fit (max likelihood) is
performed a) M gt 4.2 GeV normalize the DY
b) 2.2 lt M lt 2.5 GeV normalize the charm (with
DY fixed)
c) 2.9 lt M lt 4.2 GeV get the J/y yield
(with DY charm fixed)
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J/y/DY advantages drawbacks
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
High mass dimuons are rare we are forced to
study the J/? with statistical errors imposed by
the 100 times smaller reference process
We cannot have more than 3 centrality bins
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Comparison with previous results
An anomalous suppression is present already
in Indium-Indium
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)
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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
dNJ/y/dEZDC
J/? sample matched sample of J/? (cleaner
spectrum). J/? events correspond to the signal
after the combinatorial background subtraction
Inefficiencies introduced by the cuts, used in
the events selection, affect in a negligible way
the J/y sample
EZDC (GeV)
Advantages
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
Drawbacks
there is no intrinsic absolute normalization.
? 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.
Nuclear absorption
The ratio Measured / Expected is normalized to
the standard analysis
The following pattern is observed Onset
of anomalous suppression in the range 80 lt
NPart lt 100 Saturation at large NPart
EZDC(TeV)
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Direct J/? sample stability of the result
J/? analysis shifted bins
J/? analysis
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
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Direct J/? sample comparison with previous
results
The J/y suppression patterns are in fair
agreement in the Npart variable
The S-U, In-In and Pb-Pb data points do not
overlap in the L variable
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Direct J/? sample comparison with previous
results (2)
very preliminary
Bjorken energy density, estimated using VENUS
A more significant comparison requires Pb-Pb
points with reduced error bars
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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
We consider models for which we have predictions
specifically made for In-In collisions
J/y absorption by produced hadrons (comovers)
Capella and Ferreiro, hep-ph/0505032 J/y
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 cc suppression by deconfined
partons when geometrical percolation sets in
Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004)
547.
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Suppression by produced hadrons (comovers)
The model takes into account nuclear absorption
and comovers interaction with sco 0.65 mb
(Capella-Ferreiro)
In-In _at_ 158 GeV
J/y / NColl
nuclear absorption
comover nuclear absorption
(E. Ferreiro, private communication)
NA60 In-In 158 GeVpreliminary
The smeared form (dashed line) is obtained taking
into account the resolution on NPart due to our
experimental resolution
Pb-Pb _at_ 158 GeV
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QGP hadrons regeneration in-medium effects
The model simultaneously takes into account
dissociation and regeneration processes in both
QGP and hadron gas (Grandchamp, Rapp, Brown)
In-In _at_ 158 GeV
fixed thermalization time
fixed thermalization time
centrality dependent thermalization time
BmmsJ/y/sDY
centrality dependent thermalization time
Nuclear Absorption
Suppression Regeneration
QGPhadronic suppression
Regeneration
Number of participants
NA60 In-In 158 GeVpreliminary
The smeared form (dashed line) is obtained taking
into account the resolution on NPart due to our
experimental resolution
Pb-Pb _at_ 158 GeV
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Suppression due to a percolation phase transition
Model based on percolation (Digal-Fortunato-Satz)
Sharp onset (due to the disappearance of the cc
meson) at Npart 125 for Pb-Pb and 140 for
In-In
The dashed line includes thesmearing due to the
ZDC resolution
Pb-Pb _at_ 158 GeV
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Summary of models comparison
Satz, Digal, Fortunato Rapp, Grandchamp,
Brown Capella, Ferreiro
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Summary

• The J/y 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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The NA60 experiment
http//cern.ch/na60
60 people 13 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. Damjanovic, A. David, A. de Falco,
N. de Marco,A. Devaux, A. Drees, L. Ducroux, H.
Enyo, 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