1. Dilepton production and the onset of deconfinement International Workshop “Critical Point and Onset of Deconfinement” Firenze, July 3-6 2006 E. Scomparin INFN-Torino (Italy) 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

2. The SPS energy range SPS low energy ~ 20 GeV/nucleon SPS high energy ~ 200 GeV/nucleon 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 SPS probably sitting in the region close to Deconfinement threshold Critical point

3. Charmonium production at SPS 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 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 ?

4. 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 NA50 MUON FILTER BEAM TRACKER TARGET BOX VERTEX TELESCOPE Dipole field 2.5 T BEAM IC not on scale NA60

5. pA collisions: the reference Glauber fit to B µµ  J/ at 400-450 GeV  J/ abs = 4.48  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

6. Expected (J/)/DY at 158 GeV NA50 uses Drell-Yan as a reference process to study J/ suppression Is (J/)/DY equivalent to J/ cross section per N-N collision ?  Yes, Drell-Yan A-dependence measured  DY = 0.995  0.016 (stat.)  0.019 (syst.) 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/ cross sections (1.5% error) DY  LO calculation (negligible 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)

7. Open questions on J/ production in p-A 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) -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 1.0 0.9 0.8 0.7 B&KB&K Vogt: final state absorption R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539 - R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539 - K.G. Boreskov & A.B. Kaidalov, JETPL 77 (2003) 599 E86638.8 GeVBe/Fe/W E78938.8 GeVBe/C/Cu/W E77238.8 GeVH 2 /C/Ca/Fe/W NA5029.1 GeVBe/Al/Cu/Ag/W NA322.9 GeVH 2 /Pt E86638.8 GeVBe/Fe/W E78938.8 GeVBe/C/Cu/W E77238.8 GeVH 2 /C/Ca/Fe/W NA5029.1 GeVBe/Al/Cu/Ag/W NA322.9 GeVH 2 /Pt xFxF 

8.  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

9. Comparison between centrality estimators (E T, E ZDC, N ch ) Fair agreement between various centrality estimators

10. 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 in nuclear collisions, expected to occur at T~T c ? Not obvious, recent HERA-B result =0.210.05

11. 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

12. hadron absorber Muon Other and trackingMuon trigger magnetic field Iron wall NA50 spectrometer 2.5 T dipole magnet Matching in coordinate and momentum space targets beam tracker vertex tracker or ! NA60: detector concept Improved dimuon mass resolution Origin of muons can be accurately determined ~ 200 m in the longitudinal coordinate ~ 20 m in the transverse coordinate Excellent vertex resolution

13. J/ / DY analysis Set A ( lower ACM current) Combinatorial background (, K decays) from event mixing method (negligible) Multi-step fit: a) DY (M>4.2 GeV), b) IMR (2.2<M<2.5 GeV), c) charmonia (2.9<M<4.2 GeV) Mass shape of signal processes from MC (PYTHIA+GRV94LO pdf) Results from set A and B statistically compatible  use their average in the following Stability of the J/  / DY ratio: change of input distributions in MC calculation  0.3% (cos), 1% (rapidity) level of muon spectrometer target cut  < 3% Set B ( higher ACM current)

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

15. A different analysis technique Measured J/ events are compared to the expected J/ centrality distribution, calculated assuming nuclear absorption as the only suppression source 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) Very small statistical errors Many centrality bins More sensitive to systematics

16. Measured / Expected vs. N part Departure from the expected normal nuclear absorption in peripheral events Saturation in more central events ?

17. Comparison with NA38/NA50 The J/ suppression patterns are in fair agreement when plotted against N part

18. Comparison with the extreme case of a step-like function N part Meas/Exp 1 Step position A1 A2 Step position: N part = 82 ± 9 A1= 0.98 ± 0.03 A2= 0.85 ± 0.01  2 /dof = 2.0 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

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

20. Summary on systematic errors Various sources of systematic errors have been investigated and their effect on the measured suppression pattern is the following: 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 Event selection  1-2% Input to Glauber model (In density distributions) Link E ZDC – N part Error on  J/  pp (450 GeV)  8% centrality independent Error on  abs  3-4 % (almost) centrality independent Error due to the J/  /DY normalization  ~ 6% centrality independent >10% for E ZDC < 3 TeV negligible elsewhere 5 -10 % for E ZDC < 3 TeV negligible elsewhere

21. 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 ?

22. 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 negative values of v 2 with smooth centrality dependence cc break-up by QGP hard gluons  Parton density azimuthally anisotropic J/ exiting out-of-plane more absorbed Give positive values of v 2 with sudden onset when critical conditions for QGP are reached Heiselberg, Mattiello, PRC60(1999)44902 Wang, Yuan, PLB540(2002) 62 Zhu, Zhuang, Xu, PLB607 (2005) 107

23. Preliminary NA50 results (Pb-Pb) Correction for event plane resolution still under investigation Calculate v’ n =  cos[n(  -  n )]  v’ 2 always smaller than v 2 Reaction plane estimated using e.m. calorimeter (6 azimuthal sectors)  determine event plane  2 Small positive J/ v 2 on average  More J/ exiting in plane Negative J/ v 2 more unlikely  no major role for breakup by comovers

24. Preliminary NA60 results (In-In) More peripheral data  hint for a non isotropic emission pattern with positive v 2 ? Only 50% of the statistics analyzed centralperipheral Event plane method has been used Correction for reaction plane resolution applied

25. J/ transverse momentum distributions p T 2  = p T 2  pp + a gN L p T 2  J/ (GeV/c) 2 L (fm) preliminary 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

26. J/ central to peripheral ratio “R CP ” Define i=1 most peripheral bin i=5 most central bin J/ is suppressed mainly at low transverse momentum For p T > 3.5 GeV/c, the centrality dependence of J/ suppression is weak

27. 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 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” Results up to now (E866, CDF…) do not show an increase of the polarization for high p T “…polarization exhibits strong non-perturbative effects. The QGP is expected to screen away the non perturbative physics: the J/ which escape from the plasma should possess polarization as predicted by perturbative QCD…”

28. 0 < p T < 5 GeV/c 3.2 < y LAB < 3.8 - 0.7 < cos H < 0.7 cos H distribution is fitted with Polarization of the J/ in Indium-Indium (NA60) cos H bin corresponding to 1 < p T < 2 GeV/c Study performed in the kinematical region Acceptance correction performed using a 3-D method Polarization angle is computed in the helicity frame (z-axis coincident with the J/ direction in the center of mass frame)

29. According to theory, in case of QGP formation the expected value for the polarization is = 0.6 (for p T ~ 0), and even taking into account the initial transverse momentum of gluons, remains significantly higher than zero Preliminary NA60 results xFxF p T (GeV/c) = 0.35 – 0.4 … values closer to zero

30. 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/ suppression between various systems is still not conclusive 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  analysis without Drell-Yan essential to compare Pb-Pb and In-In NA60  pA results @ 158 GeV, study of A-dependence of  c production