1. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20031 Penetrating Probes: from SPS to RHIC Itzhak Tserruya Weizmann Institute QCD@Work, Conversano, June 14-18, 2003

2. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20032 Outline Introduction - Motivation: penetrating probes are directly sensitive to the two fundamental issues of RHI collisions: deconfinement and chiral symmetry restoration. RHIC and SPS Highlights - High p T phenomena - J/  suppression - Low-mass e + e - pairs - Direct photons Summary

3. 3 Penetrating probes  Relativistic Heavy Ion collisions aim at producing and studying high density matter.  “Penetrating probes” provide sensitive diagnostic tools.  Two types of penetrating probes: a) probes created at early stages which propagate through the medium and are modified by the medium. * QCD hard scattering probes:  jet quenching  suppression of high p T hadrons  J/  suppression b) e.m. probes (real or virtual photons) created inside the medium * Large mfp  no final state interaction carry information from place of creation to detectors.  low-mass e + e - pairs  real photons

4. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20034 The RHIC Experiments @ BNL STA R Begun operation June 2000 Outstanding start-up of machine and experiments

5. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20035 Jets: A New Probe For High Density Matter schematic view of jet production pp AA In the colored medium, quarks radiate energy (energy loss ~GeV/fm) modify jet shape. Jets from hard scattered quarks: - produced very early in the collision (τ <1fm/c) - expected to be significant at RHIC q q leading particle

6. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20036 PHENIXSTAR RHIC events Au-Au central collision at √s NN = 200 GeV p-p collision at √s = 200 GeV STAR

7. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20037 Jets: A New Probe For High Density Matter schematic view of jet production pp Not possible to observe jets directly in RHIC due to the large particle multiplicty. AA In the colored medium quarks radiate energy (energy loss ~GeV/fm) modify jet shape. Jets from hard scattered quarks: - produced very early in the collision (τ <1fm/c) - expected to be significant at RHIC Decrease their momentum  Suppression of high p T particles “Jet Quenching” n Identify jet and its possible modifications through leading particles q q leading particle

8. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20038 Nuclear modification factor Zero hypothesis: scale pp to AA with the number of NN collisions N coll : d 2 N AA /dp T d  (b) =  pp T AA (b) = N coll d 2 N pp /dp T d  ? Quantify “effect” with nuclear modification factor: If no “effect”: R AA < 1 at low p T in regime of soft physics R AA = 1 at high-p T where hard scattering dominates If “jet quenching”: R AA < 1 at high-p T AA

9. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 20039 High p T Suppression in Au-Au collisions !! Same behavior observed in the ratio of central to peripheral collisions Central/peripheral ratio AA / pp ratio Peripheral collisions look like pp. Central collisions are strongly suppressed

10. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200310 Discovery of High p T Hadron Suppression at RHIC… At CERN: all previous measurements see enhancement, not suppression: Low p t : soft processes  Npart R  Npart / Ncoll ~ 0.3 High p t : broadening due to rescattering (Cronin effect)  R > 1. PHENIX Preliminary CERN WA98: Understood enhancement from Cronin effect At RHIC: qualitatively new physics made accessible by RHIC’s higher energy and ability to produce (copious) perturbative probes

11. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200311 …Made the cover of PRL Jan. 2002 PHENIX

12. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200312 Suppression increases gradually with increasing collision centrality Nuclear modification factor R AA for charged particles in different centrality ranges in Au+Au collisions at 130GeV (result for most central collisions shown on all panels).

13. 13 protons  Suppression is particle dependent The proton puzzle: protons and antiprotons are not suppressed different production mechanism for protons and antiprotons? PHENIX Preliminary

14. 14 Unusual Particle Mix at p T > 1.5 GeV Peripheral collisions: p/  ~ 0.4 as in pp collisions. Central collisions: p/  ~1 higher than in pp or jets in e + e - collisions In-medium modification of fragmentation function?

15. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200315 Origin of the Suppression? Final state effect Energy loss of partons in dense matter Gyulassy, Wang, Vitev, Baier…. Hadronic absorption of fragments : (Absorption with comovers) Gallmeister, et al. PRC67,044905(2003) Parton recombination (coalescence) Fries, Muller, Nonaka, Bass nuclth/0301078 Lin & Ko, PRL89,202302(2002) Gluon Saturation  R dA ~ √AA ~ 0.5 (Color Glass Condensate) (McLerran, Kharzeev …) Multiple elastic scatterings (Cronin effect)  R dA > 1 Nuclear shadowing  R dA decreases Initial state effect No final state expected in d+Au collisions! d+Au is the “control” experiment

16. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200316 d – Au Results (I): Spectra Final spectra for charged hadron and identified pions. Data span 7 orders of magnitude.

17. 17 d - Au Results (II): Identified  0  Two independent measurements !  Suppression in AuAu is a final state effect !!  CGC ruled out as possible explanation of Au-Au results d-Au: Initial state effects only d-Au: Initial state effects only Au-Au: Initial + final states effects Au-Au: Initial + final states effects π0 π0

18. 18 Charged hadrons  See “Cronin” effect in d-Au?  Enhancement more pronounced in the charged hadron than in the  0 measurement ? d - Au Results (III): Charged Particles  Third independent measurement !!

19. 19 pTpT pTpT R R R R d-Au results (IV): Centrality Dependence Charged hadron spectra show centrality evolution with opposite trend to Au-Au collisions PHENIX preliminary

20. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200320 Suppression of high p T hadrons in central Au-Au collisions at RHIC energies Observed in the first RHIC run at √s NN = 130 GeV Confirmed in the second run at √s NN = 200 GeV No suppression observed in d-Au collisions at √s NN = 200 GeV, the third RHIC run, which ended a couple of months ago. The most significant RHIC result so far: The basis for the BNL press release issued on June 11: “Exciting first results from deuteron gold collisions at Brookhaven. Findings intensify search for new form of matter”

21. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200321 J/  Suppression Suppression Mechanism An “old” signature of QGP formation: (Matsui and Satz PL B178, (1986) 416). One of the first observations at CERN: * J/  suppression in 200 A GeV S-Au collisions explained by absorption in nuclear medium J/  + N  DD  abs ~ 6mb Anomalous suppression in Pb-Pb collisions at CERN  At high enough color density, the J  finds itself enveloped by the medium.  When screening radius < binding radius  J/  will dissolve (Debye screening)  The small cc production cross section makes it unlikely that they find each other at the hadronization stage

22. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200322 Anomalous J /  suppression in Pb-Pb collisions Normal nuclear absorption:  abs = 6.4 ± 0.8 mb NA50 Anomalous absorption in Pb-Pb for E T > 40 GeV or N part >100 or b < 8 fm

23. 23 J /  Suppression at SPS: Evidence of QGP? Conventional models ruled out Two-step pattern: successive melting of charmonium states  c (b.e.  250 MeV) and J/  (650 MeV) NA50 Also: Capella et al. Hadronic models: cold nuclear + “comover” dissociation QGP models: energy density thresholds + E T fluctuations  “thresholds” and high E T behavior favor QGP models ….

24. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200324 J/  at RHIC: Prospects  Suppression or enhancement? suppressed: because of Debye screening of the attractive potential between c and c in the deconfined medium. enhanced: charm cross section at RHIC is much larger than at SPS. The J/  melting mechanism could be compensated by recombination or coalescence of cc as the medium cools down.  Energy loss of charm quarks in the high density medium J/  is becoming a complex observable. Will require precise measurements of pp, pA and AA The PHENIX experiment was specifically designed to measure J/   e + e - at mid-rapidity and J/    +  - at forward rapidities

25. 25 J /  @ RHIC: Establishing pp baseline Clear J/  signals seen in both central and muon arms. Resolutions in agreement with expectations. Integrated cross-section : 3.98 ± 0.62 (stat) ± 0.56 (sys) ± 0.41(abs)  b In very good agreement with Color Evaporation Model calculations

26. 26 Poor statistics N=10.8  3.2 (stat)  3.8 (sys) J/   e + e - in Au-Au @ RHIC  Need much higher luminosity runs  Au-Au expected in run 2003-4 N coll scaling band Most probable value 90 % C.L. Incl. systematic errors Expectation with  abs =4.4 and 7.1 mb p-p

27. 27 Dileptons (e + e -,  +  - ): best probes to look for thermal radiation from QGP: q q   *  l + l - HG:  +  -   *  l + l - Photons * Same underlying physics but much higher background  less sensitivity Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero:  300 MeV 3  0 at high T and/or high  Constituent mass  current mass Chiral Symmetry (approximately) restored. Physics accessible through e.m. probes (I) Meson properties (m,  ) expected to be modified (?) * Best candidate:  -meson decay (   = 1.3fm/c) Low-mass dileptons: best probe of Chiral Symmetry Restoration

28. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200328 Physics accessible through e.m. probes (II)  meson * simultaneous measurement of   l + l - and   K + K - very powerful tool to evidence in-medium effects * strangeness enhancement Charm production * semileptonic decays of charmed mesons accessible at RHIC through high p T single electrons

29. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200329 Low-mass Dileptons: Main CERN Result Strong enhancement of low-mass e + e - pairs in A-A collisions (wrt to expected yield from known sources) Enhancement factor (m > 0.2 GeV/c 2 ): 2.6 ± 0.2 (stat) ± 0.6 (syst) No enhancement in pp nor in pA

30. 30 Multiplicity Dependence CERES Pb-Au 158 A GeV 95+96 data Enhancement factor rises linearly with dN/d   pair yield  (dN/d  ) 2 Data consistent with straight line passing through 1 at dN/d  =0 Largest enhancement at  500 MeV/c 2

31. 31 P t Dependence Enhancement much more pronounced at low pair p T : reaches a factor of 10 ! at masses of 0.4 – 0.6 GeV/c 2 CERES Pb-Au 158 A GeV 95+96 data at high pair p T, mass spectrum is much closer to cocktail

32. 32 Onset of Chiral Symmetry Restoration? What happens as CSR is approached? Dropping masses or line broadening? Quark-hadron duality down to m ~ 0.5 GeV/c 2 ? Dropping  -meson mass (Rapp, Wambach et al) In-medium  -meson broadening (G.E. Brown et al, using Brown-Rho scaling ) d.o.f. hadrons quarks Looking forward to high resolution CERES results

33. 33 Low-mass e + e - Pairs: Prospects at RHIC R. Rapp nucl-th/0204003 Strong enhancement of low- mass pairs persists at RHIC Contribution from open charm becomes significant Possibility to observe in-medium effects on the  ?

34. 34 Low and intermediate mass pairs at RHIC: first results e + e - pairs (real) e + e - pairs (mixed) Real and Mixed e + e - Distribution net e + e - e + e - from charm (PYTHIA) e + e - from light hadron decays Real - Mixed e + e - Distribution LMR (0.3 – 1.0 GeV): Predictions: = 9.2 x 10 -5 Measurements: Problem: combinatorial background too high S/B  1/300 Need an upgrade. R&D already started to develop an HBD

35. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200335  Meson * sensitive to strangeness production * simultaneous measurement of   e + e - and   K + K - very powerful tool to evidence in-medium effects unique capability of the PHENIX experiment mass [GeV/c 2 ]   e + e - mass [GeV/c 2 ]   K + K -

36. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200336 Intermediate Mass Region: CERN Data Enhancement of dimuons in the IMR (1 – 2.5 GeV/c 2 ) seen by: HELIOS – 3 NA38/50 – increasing with centrality Charm enhancement or thermal radiation from HG? HELIOS3- pW and SW 200 A GeV NA50 PbPb 158 A GeV Peripheral Central NA50 PbPb 158 A GeV Peripheral Central

37. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200337 Open Charm at RHIC  Measure inclusive single electrons  Subtract hadronic sources and gamma conversions.  Attribute difference to open charm.  p T distribution (in minimum bias and central collisions) and total cross section in very good agreement with Pythia. No charm enhancement? No high p T suppression of charm quarks? NOTE: Pythia comparison is on absolute scale, no free parameters.

38. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200338 Direct Photons at CERN Evidence for direct photons in central Pb-Pb collisions? 10-20% excess but 1-2  effect only WA98 No direct photons in peripheral Pb-Pb collisions Previous attempts with O,S beams by CERES, HELIOS2 and WA80 resulted only in upper limits

39. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200339 Direct photons at RHIC: first results No photon excess seen within errors Need better understanding of systematic errors

40. Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 200340 Summary Real photons - no convincing evidence of thermal radiation at the SPS. - expect RHIC results from the 2003-4 run. J/  suppression - most direct evidence of deconfinement at SPS? - Situation at RHIC more complex. Expect significant results from next run Jet quenching: - The most spectacular RHIC result so far. Enhancement of low-mass e + e - - thermal radiation from HG. - evidence of chiral symmetry restoration? - very difficult measurement: PHENIX upgrade underway.