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Dilepton production and the onset of deconfinement

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

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Dilepton production and the onset of deconfinement

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  1. 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: v2, pT, polarization • Conclusions E. Scomparin INFN-Torino (Italy)

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

  3. 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

  4. NA50 Dipole field2.5 T NA60 TARGET BOX MUON FILTER BEAM BEAMTRACKER VERTEX TELESCOPE IC not on scale 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

  5. pA collisions: the reference • Glauber fit to Bµµ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/DYpp@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 (ET, EZDC, Nch) Direct measurement of J/ /DY at 158 GeV would significantly decrease such errors (NA60)

  7. - R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539 • K.G. Boreskov & A.B. Kaidalov, JETPL 77 (2003) 599 B&K 1.0 Vogt: final state absorption 0.9 0.8 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 Open questions on J/ production in p-A  E866 38.8 GeV Be/Fe/W E789 38.8 GeV Be/C/Cu/W E772 38.8 GeV H2/C/Ca/Fe/W NA50 29.1 GeV Be/Al/Cu/Ag/W NA3 22.9 GeV H2/Pt xF • 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)

  8. J/ /DY in Pb-Pb collisions at 158 GeV • Final NA50 set of data New reference (only p-A collisions are used) Larger error, but no assumption on S-U Old reference (include S-U in the determination) Small error, but assumes S-U is normal

  9. Comparison between centrality estimators (ET, EZDC, Nch) 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~Tc ? 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. Muon trigger and tracking Iron wall magnetic field hadron absorber Muon Other or ! NA60: detectorconcept 2.5 T dipole magnet NA50 spectrometer beam tracker vertex tracker targets Matching in coordinate and momentum space • Improved dimuonmass resolution • Origin of muonscan 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) 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<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%

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

  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 Very small statistical errors Many centrality bins More sensitive to systematics Nuclear absorption 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) Normalization of the nuclear absorption curve

  16. Measured / Expected vs. Npart • 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 Npart

  18. Meas/Exp 1 A1 A2 Npart Step position Comparison with the extreme case of a step-like function Step position: Npart = 82 ± 9 A1= 0.98 ± 0.03 A2= 0.85 ± 0.01 2/dof = 2.0 • Resolution on Npart estimate (due to the measured EZDC resolution) taken into account • A certain amount of physics smearing can be accommodated by the data

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

  20. 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) • Link EZDC – Npart • 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 EZDC < 3 TeV negligible elsewhere 5 -10 % for EZDC < 3 TeV negligible elsewhere 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 normalizationis more uncertain

  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 Npart~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/ v2 • 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 v2 with smooth centrality dependence Heiselberg, Mattiello, PRC60(1999)44902 Give positive values of v2 with sudden onset when critical conditions for QGP are reached • cc break-up by QGP hard gluons •  Parton density azimuthally anisotropic • J/ exiting out-of-plane more absorbed Wang, Yuan, PLB540(2002) 62 Zhu, Zhuang, Xu, PLB607 (2005) 107

  23. 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’n =  cos[n( - n)]  • v’2 always smaller than v2 • Small positive J/ v2 on average • More J/ exiting in plane • Negative J/ v2 more unlikely • no major role for breakup by comovers

  24. central peripheral Preliminary NA60 results (In-In) • Event plane method has been used • Correction for reaction plane resolution applied • More peripheral data  hint for a non isotropic emission pattern • with positive v2 ? • Only 50% of the statistics analyzed

  25. pT2J/ (GeV/c)2 L (fm) J/ transverse momentum distributions Study, for A-A collisions, the dependence of  pT2 on L Both Pb-Pb and In-In points are well reproduced assuming that pT distributions are broadened by initial-state parton multiple scattering preliminary pT2 = pT2pp + agN L

  26. J/ central to peripheral ratio “RCP” Define i=1 most peripheral bin i=5 most central bin • J/ is suppressed mainly • at low transverse momentum • For pT > 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 pT Results up to now (E866, CDF…) do not show an increase of the polarization for high pT 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/ which escape from the plasma should possess polarization as predicted by perturbative QCD…”

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

  29. l l pT (GeV/c) l l xF Preliminary NA60 results According to theory, in case of QGP formation the expected value for the polarization isl = 0.6(for pT ~ 0), and even taking into account the initial transverse momentum of gluons, remains significantly higher than zero l = 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 Npart • 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

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