Dilepton production and the onset of deconfinement International Workshop “Critical Point and Onset of Deconfinement” Firenze, July 3-6 2006

• Introduction • J/  suppression studies at SPS energies • NA38/NA50: p-A, S-U, Pb-Pb • NA60: p-A, In-In • Other inputs: E866, HERA-B • Other J/  related topics: v 2 , p T , polarization • Conclusions E. Scomparin INFN-Torino (Italy)

The SPS energy range

SPS high energy ~ 200 GeV/nucleon SPS low energy ~ 20 GeV/nucleon SPS probably sitting in the region close to Deconfinement threshold Critical point • In the dilepton sector, SPS is well positioned to study • Onset of deconfinement  J/  suppression (this talk) • Approach to chiral symmetry restoration  in-medium modifications of vector mesons

Charmonium production at SPS

• Many relevant questions to be answered by studying charmonium production in heavy-ion collisions at the SPS • Is (at least part of the) suppression of charmonia that we observe in the data NOT due to usual hadronic processes ?

• Do we have evidence for a “threshold behaviour” of the suppression, that might be connected with the onset of deconfinement ?

• Can we observe the predicted “suppression hierarchy” for the various charmonia states ? • Study carried out by NA38/NA50/NA60 at the SPS from 1986 until today • Essentially the same experiment, although with very significant upgrades • Large set of results with very good statistics • (Lots of) systems studied, including: • p-p, p-d, p-Be, p-C, p-Al, p-Cu, p-Ag, p-W, p-Pb, p-U, O-Cu, O-U, S-U, In-In, Pb-Pb • Similar (but not identical) energy/kinematical domain between various data sets • Very significant contributions (in a slightly higher energy range) by E866 and HERA-B

The NA38/NA50/NA60 experiments

Based on the same muon spectrometer (inherited by NA10) no apparatus-dependent systematics Many updates in the target region, in parallel with the availability of radiation hard detectors NA60 NA50

2.5 T TARGET BOX BEAM BEAM TRACKER VERTEX TELESCOPE IC

not on scale

MUON FILTER

pA collisions: the reference

• Glauber fit to B µµ  J/  •  J/  abs = 4.48  at 400-450 GeV 0.42 mb Main problem: extrapolation to 158 GeV/c • S-U data (200 GeV) should not be used (absorption sources different wrt pA might be present) • Obtain normalization (  J/  pp ) at 200 GeV • using only pA data • assuming  J/  abs does not depend on  s • High statistics 400/450 data:  J/  /  DY ratios • Obtain  J/  abs = 4.18  0.35 mb

Expected (J/



)/DY at 158 GeV

• NA50 uses • Is (J/   )/DY Drell-Yan as a reference process equivalent to J/  to study J/ Yes, Drell-Yan A-dependence measured  DY = 0.995  0.016 (stat.)  0.019 (syst.)  suppression cross section per N-N collision ?

• Start from  J/  pp /  DY pp @ 450 GeV (1.4% error) • Rescale to 200 GeV • J/   see previous page ( 7.8% error , SU not used) • DY  LO calculation ( 2.5 % error ) • Rescale to 158 GeV • J/   fit a la Schuler to measured J/  • DY  LO calculation ( negligible error ) cross sections ( 1.5% error ) • Use Glauber (with neutron halo) to calculate centrality dependence of expected  J/  /  DY • Include experimental smearing on centrality determination ( E T , E ZDC , N ch ) Direct measurement of  J/  /  DY at 158 GeV would significantly decrease such errors (NA60)

Open questions on J/



production in p-A

1.0

0.9

0.8

E866 E789 E772 NA50 NA3 38.8 GeV 38.8 GeV 38.8 GeV 29.1 GeV 22.9 GeV Be/Fe/W Be/C/Cu/W H 2 /C/Ca/Fe/W H 2 /Pt - R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539 - K.G. Boreskov & A.B. Kaidalov, JETPL 77 (2003) 599

0.7

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

x F • Solid theoretical understanding is still missing • There are indications that  J/  may depend on  s inside the SPS energy range (becomes much smaller at RHIC!) • May have consequences for expected nuclear absorption at 158 GeV • There is feed-down from  ’ and  c   is an effective quantity (  eff ) • Use of  eff (or equivalently,   eff ) could introduce a bias (fraction of measured J/  coming from higher resonances can vary between p-A and A-A, due to different suppression mechanisms in the two systems)

 J/ 

/

 DY

in Pb-Pb collisions at 158 GeV

• Final NA50 set of data Old reference (include S-U in the determination) Small error, but assumes S-U is normal New reference (only p-A collisions are used) Larger error, but no assumption on S-U

Comparison between centrality estimators (E

T

, E

ZDC

, N

ch

)

Fair agreement between various centrality estimators

Suppression pattern (S-U vs Pb-Pb)

• Is J/  suppressed beyond nuclear absorption ?

 Yes, in central and semi-central PbPb collisions • Does the suppression exhibit a threshold behavior vs centrality?

 not easy to answer • Is there any sign of a “second drop” in the suppression pattern ?

 not evident, but no saturation of the suppression • Are we observing the suppression of  c occur at T~T c ?

in nuclear collisions, expected to Not obvious, recent HERA-B result R χ c  Σσ(χ c(i)  J/ψ  ) =0.21

 0.05

σ INCL (J/ψ )

Recent news from SPS: NA60

• Having observed an anomalous suppression in Pb-Pb collisions it is important to have a systematic study also with lighter ions Compare suppression pattern as a function of various centrality variables Try to single out a scaling variable for the anomalous suppression Study J/  suppression in Indium-Indium collisions

beam tracker

NA60: detector concept

targets 2.5 T dipole magnet vertex tracker Muon trigger and tracking NA50 spectrometer hadron absorber

Matching in coordinate and momentum space

or

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

Excellent vertex resolution ~ 200  m in the longitudinal coordinate ~ 20  m in the transverse coordinate Muon Other

!

J/



/ DY analysis

Set A ( lower ACM current) Set B ( higher ACM current)

• • • Combinatorial background (  , K decays) from event mixing method (negligible) Multi-step fit: a) DY (M>4.2 GeV), b) IMR (2.2

J/



/ DY vs. centrality

Anomalous suppression present in Indium-Indium • Qualitative agreement with NA50 results plotted as a function of N part • Data points have been normalized to the expected J/  normal nuclear absorption, calculated with  J/  abs = 4.18  0.35 mb as measured with p-A NA50 data 3 centrality bins, defined through E ZDC B. Alessandro et al., Eur. Phys. J. C39(2005) 335 bin1  bin2  bin3   N part   N part   N part  = 63 = 123 = 175

A different analysis technique

Measured J/  events are compared to the expected J/  centrality distribution calculated assuming nuclear absorption as the only suppression source , Very small statistical errors Many centrality bins More sensitive to systematics Nuclear absorption Normalization of the nuclear absorption curve we require the ratio measured/expected, integrated over centrality, to be equal to the same quantity for the J/  /DY analysis (0.87 ± 0.05)

Measured / Expected vs. N

part • Departure from the expected normal nuclear absorption in peripheral events • Saturation in more central events ?

Comparison with NA38/NA50

The J/  suppression patterns are in fair agreement when plotted against N part

Comparison with the extreme case of a step-like function

1 A1 Step position A2 N part Step position: N part = 82 ± A1= 0.98 ± 0.03

A2= 0.85 ±  2 /dof = 2.0

0.01

9 • Resolution on N part estimate (due to the measured E ZDC resolution) taken into account • A certain amount of physics smearing can be accommodated by the data

Comparison with a recent model

Nucl. abs. only Nucl. abs. + hadron gas Maximum hadronic absorption (Hagedorn gas) not enough to reproduce In-In and Pb-Pb Mechanisms at the parton level must be invoked Becattini, Maiani et al., Phys. Lett. B632(2006) 233

Summary on systematic errors

Various sources of systematic errors have been investigated and their effect on the measured suppression pattern is the following: • • Event selection  1-2% Input to Glauber model (In density distributions) >10% for E ZDC < 3 TeV negligible elsewhere • Link E ZDC – N part 5 -10 % for E ZDC < 3 TeV negligible elsewhere • • • Error on  J/  pp (450 GeV)  Error on  abs  8% centrality independent 3-4 % (almost) centrality independent Error due to the J/  /DY normalization  ~ 6% centrality independent The most central bin is affected by a sizeable systematic error relatively to the others. There is also a ~10% systematic error, independent on centrality The shape of the suppression pattern can be accurately evaluated, but its absolute normalization is more uncertain

J/



suppression studies: where are we ?

• Results for various p-A and A-A systems indicate that • J/  is suppressed beyond normal nuclear absorption • in Pb-Pb collisions (NA50) • in In-In collisions (NA60) • J/  is not suppressed beyond normal nuclear absorption • in S-U collisions (NA38) • Is there a threshold effect ?

• Results are not conclusive • Anomalous suppressions sets in at N part ~100 at SPS energy • Coherent interpretation of RHIC and SPS results is challenging • • J/  regeneration at RHIC ?

Sequential suppression , with only  c melting observed at SPS/RHIC ?

Azimuthal distribution of J/

 • Possible sources of J/  v 2 • Charm elliptic flow  For J/  formed by cc recombination, if c quarks thermalize early Not likely to occur at SPS energies Greco, Ko, Rapp, PLB595(2004) 202 • cc break-up on co-moving hadrons  More pions in-plane than out-of-plane (pion elliptic flow) J/  exiting in-plane more absorbed Gives of v 2 negative values with smooth centrality dependence Heiselberg, Mattiello, PRC60(1999)44902 • cc break-up by QGP hard gluons  Parton density azimuthally anisotropic J/  exiting out-of-plane more absorbed Give of v 2 positive onset when values with sudden critical conditions for QGP are reached Wang, Yuan, PLB540(2002) 62 Zhu, Zhuang, Xu, PLB607 (2005) 107

Preliminary NA50 results (Pb-Pb)

• Reaction plane estimated using e.m. calorimeter (6 azimuthal sectors)  determine event plane  2 Correction for event plane resolution still under investigation • Calculate v’ • v’ 2 n =  cos[n(   always smaller than v 2  n )]  • Small positive J/   More J/  v 2 on average exiting in plane • Negative J/   v 2 more unlikely no major role for breakup by comovers

Preliminary NA60 results (In-In)

• Event plane method has been used • Correction for reaction plane resolution applied central peripheral • More peripheral data  hint for a non isotropic emission pattern with positive v 2 ?

• Only 50% of the statistics analyzed

J/



transverse momentum distributions

Study, for A-A collisions, the dependence of  p T 2  on L Both Pb-Pb and In-In points are well reproduced assuming that p T distributions are broadened by initial-state parton multiple scattering  p T 2  =  p T 2  pp + a gN L L (fm) Pb-Pb + In-In  p T 2  pp [(GeV/c) 2 ] a gN [(GeV/c) 2 /fm] 1.10  0.02 0.081  0.003

J/



central to peripheral ratio “R

CP

”

Define R i CP (p T )  N ψ, i (p T ) N DY, i N ψ,1 (p T ) N DY,1 i=1 most peripheral bin i=5 most central bin • J/  is suppressed mainly at low transverse momentum • For p J/  T > 3.5 GeV/c, the centrality dependence of suppression is weak

Polarization of the J/

 The polarization of the J/  provides a detailed test of quarkonium production models Quarkonium polarization: • CSM : predicts transverse polarization • CEM : predicts no polarization • NRQCD : predicts transverse polarization at large p T Results up to now (E866, CDF…) do not show an increase of the polarization for high p T In nucleus-nucleus collisions B.L. Ioffe and D.E. Kharzeev, PRC68 (2003) 061902 “Quarkonium Polarization in Heavy-ion collisions as a possible signature of the QGP” “…polarization exhibits strong non-perturbative effects. The QGP is expected to screen away the non perturbative physics: the J/ perturbative QCD…”  which escape from the plasma should possess polarization as predicted by

Polarization of the J/



in Indium-Indium (NA60)

• Study performed in the kinematical region 0 < p T < 5 GeV/c 3.2 < y LAB - 0.7 < cos < 3.8

 H < 0.7

• Acceptance correction performed using a 3-D method • cos  H distribution is fitted with

dσ dcosθ

H 

1



λcos

2

θ

H • Polarization angle is computed in the helicity frame (z-axis coincident with the J/ frame)  direction in the center of mass bin corresponding to 1 < p T < 2 GeV/c cos  H

Preliminary NA60 results

According to theory, in case of QGP formation the expected value for the polarization is remains l = 0.6

(for p T ~ 0), and even taking into account the initial transverse momentum of gluons, significantly higher than zero l = 0.35 – 0.4

p T (GeV/c) … values closer to zero x F

Conclusions

• Study of quarkonium production very important for HI collisions • Onset of deconfinement • Thermometer of the medium (sequential suppression) • SPS results indicate anomalous suppression (signal is there !) • Quantitative and detailed comparison of J/  various systems is still not conclusive suppression between • Pb-Pb vs In-In  qualitative agreement when plotted against N part • S-U (asymmetric system) seems to be in disagreement  Role of different energy density profile ? • Understanding the SPS+RHIC set of results is crucial • Next steps • NA50  • NA60  analysis without Drell-Yan essential to compare Pb-Pb and In-In pA results @ 158 GeV , study of A-dependence of  c production