1 / 40

Penetrating Probes: from SPS to RHIC

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

alessa
Download Presentation

Penetrating Probes: from SPS to RHIC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. 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 pT phenomena - J/ suppression - Low-mass e+e- pairs - Direct photons • Summary

  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 pT 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. STAR The RHIC Experiments @ BNL • Begun operation June 2000 • Outstanding start-up of machine and experiments

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

  6. p-p collision at √s = 200 GeV STAR PHENIX STAR RHIC events Au-Au central collision at √sNN = 200 GeV

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

  8. Quantify “effect” with nuclear modification factor: AA AA AA Nuclear modification factor • Zero hypothesis: scale pp to AA with the number of NN collisions Ncoll: • d2NAA/dpTd (b) = pp TAA(b) = Ncoll d2Npp /dpTd ? • If no “effect”: • RAA < 1 at low pT in regime • of soft physics • RAA = 1 at high-pT where • hard scattering dominates • If “jet quenching”: • RAA < 1 at high-pT

  9. Central/peripheral ratio Same behavior observed in the ratio of central to peripheral collisions High pT Suppression in Au-Au collisions !! AA / pp ratio Peripheral collisions look like pp. Central collisions are strongly suppressed

  10. Discovery of High pT Hadron Suppression at RHIC… • At CERN: all previous measurements see enhancement, not suppression: Low pt : soft processes  Npart R  Npart / Ncoll ~ 0.3 High pt : broadening due to rescattering (Cronin effect)  R > 1. CERN WA98: Understood enhancement from Cronin effect PHENIX Preliminary • At RHIC: qualitatively new physics made accessible by RHIC’s higher energy and ability to produce (copious) perturbative probes

  11. PHENIX …Made the cover of PRL Jan. 2002

  12. Suppression increases gradually with increasing collision centrality Nuclear modification factor RAA for charged particles in different centrality ranges in Au+Au collisions at 130GeV (result for most central collisions shown on all panels).

  13. Suppression is particle dependent PHENIX Preliminary protons p0 • The proton puzzle: • protons and antiprotons are not suppressed • different production mechanism for protons and antiprotons?

  14. Unusual Particle Mix at pT > 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. Gluon Saturation  RdA~ √AA ~ 0.5 (Color Glass Condensate) • (McLerran, Kharzeev …) • Multiple elastic scatterings (Cronin effect) RdA > 1 • Nuclear shadowing  RdA decreases Initial state effect • 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) Origin of the Suppression? No final state expected in d+Au collisions! d+Au is the “control” experiment

  16. d – Au Results (I): Spectra Final spectra for charged hadron and identified pions. Data span 7 orders of magnitude.

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

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

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

  20. 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” Suppression of high pT hadrons in central Au-Au collisions at RHIC energies • The most significant RHIC result so far: • Observed in the first RHIC run at √sNN = 130 GeV • Confirmed in the second run at √sNN = 200 GeV • No suppression observed in d-Au collisions • at √sNN = 200 GeV, the third RHIC run, which • ended a couple of months ago.

  21. J/ Suppression • An “old” signature of QGP formation: (Matsui and Satz PL B178, (1986) 416). Suppression Mechanism • 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 • 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

  22. Anomalous J/ suppression in Pb-Pb collisions NA50 Normal nuclear absorption: abs = 6.4 ± 0.8 mb Anomalous absorption in Pb-Pb for ET > 40 GeV or Npart >100 or b < 8fm

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

  24. 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 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

  25. J/Y @ RHIC: Establishing pp baseline Clear J/Y 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) mb In very good agreement with Color Evaporation Model calculations

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

  27. Physics accessible through e.m. probes (I) • Low-mass dileptons: best probe of Chiral Symmetry Restoration Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero: < qbarq >  300 MeV3  0 at high T and/or high  Constituent mass  current mass Chiral Symmetry (approximately) restored. Meson properties (m,) expected to be modified (?) * Best candidate: -meson decay ( = 1.3fm/c) • 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

  28. 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 pT single electrons

  29. No enhancement in pp nor in pA 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/c2 ): 2.6 ± 0.2 (stat) ± 0.6 (syst)

  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/c2

  31. Pt Dependence CERES Pb-Au 158 A GeV 95+96 data • Enhancement much more pronounced at low pair pT : • reaches a factor of 10 ! at masses of 0.4 – 0.6 GeV/c2 • at high pair pT, mass spectrum is much closer to cocktail

  32. In-medium -meson broadening hadrons d.o.f. (Rapp, Wambach et al) Dropping -meson mass quarks (G.E. Brown et al, using Brown-Rho scaling ) Quark-hadron duality down to m ~ 0.5 GeV/c2 ? Onset of Chiral Symmetry Restoration? What happens as CSR is approached? Dropping masses or line broadening? Looking forward to high resolution CERES results

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

  35. * 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  Meson   K+K-   e+e- mass [GeV/c2] mass [GeV/c2]

  36. Intermediate Mass Region: CERN Data Enhancement of dimuons in the IMR (1 – 2.5 GeV/c2) 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

  37. Open Charm at RHIC • Measure inclusive single electrons • Subtract hadronic sources and gamma conversions. • Attribute difference to open charm. • pT distribution (in minimum bias and central collisions) and total cross section in very good agreement with Pythia. No charm enhancement? No high pT suppression of charm quarks? NOTE: Pythia comparison is on absolute scale, no free parameters.

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

  39. Direct photons at RHIC: first results No photon excess seen within errors Need better understanding of systematic errors

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

More Related