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Jet Quenching at RHIC

Jet Quenching at RHIC. Saskia Mioduszewski Brookhaven National Laboratory 28 June 2004. Outline. Introduction to high p T (hard scattering) RHIC (Relativistic Heavy Ion Collider) Physics goals of heavy ion collisions Hard processes in heavy ion collisions

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Jet Quenching at RHIC

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  1. Jet Quenching at RHIC Saskia Mioduszewski Brookhaven National Laboratory 28 June 2004

  2. Outline • Introduction to high pT (hard scattering) • RHIC (Relativistic Heavy Ion Collider) • Physics goals of heavy ion collisions • Hard processes in heavy ion collisions • Expected behavior if A+A is an incoherent sum of individual p+p collisions • In-medium effects • Summary

  3. cone of hadrons “jet” p p Spectrum becomes “harder” at high pT – deviates from exponential (Note the log scale on the y-axis!) Discovery of high pT production in p+p (CERN-ISR) “hard scattering”  high pT

  4. International conference on high-energy physics, Paris, 1982 Results from CERN experiment UA2 really convinced everyone that jets in hadron-hadron collisions had been seen Jets & proton-antiproton collisions

  5. 1984 BNL note about RHIC physics Jets in nuclear collisions

  6. Subsequent hadron measurements at high pT show same effect

  7. Thermally-shaped Soft Production Hard Scattering Production cross section of p0 measured by PHENIX • Good agreement with NLO perturbative QCD calculations • High pT particle yields serve as a calibrated probe of the nuclear medium in nucleus+nucleus (A+A) and deuteron+nucleus (d+A) collisions

  8. STAR The RHIC Experiments

  9. On the Scale of Downtown Boston….

  10. nuclear matter p, n Fundamental Puzzles of Hadrons • Confinement • Quarks do not exist as free particles • Large hadron masses • Free quark mass ~ 5-7 MeV • Quarks become “fat” in hadrons, constituent mass ~ 400 MeV • Complex structure of hadrons • Sea anti-/quarks • Gluons These phenomena must have occurred with formation of hadrons

  11. Energy Density of Nuclear Matter Quark-Gluon Plasma q, g nuclear matter p, n distance of two nucleons: 2 r0 ~ 2 fm size of nucleon rn ~ 0.8 fm density or temperature • normal nuclear matter e0 : • e0 ~ 0.15 GeV/fm3 • critical density ec: • ec ~ 0.7 GeV/fm3

  12. Lattice QCD at Finite Temperature • Coincident transitions: deconfinement and chiral symmetry restoration Ideal gas(Stefan-Boltzmann limit) F. Karsch, hep-ph/010314 (mB=0) Critical energy density: Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero: At high temperature and/or baryon density Constituent mass  current mass Chiral Symmetry (approximately) restored. TC ~ 175 MeV  eC ~ 0.7 GeV/fm3

  13. early universe P. Braun-Munzinger, nucl-ex/0007021 250 RHIC quark-gluon plasma SPS Temperature T [MeV] 200 Lattice QCD AGS deconfinement chiral restoration thermal freeze-out 150 SIS hadron gas neutron stars atomic nuclei 100 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] 50 0 Schematic Phase Diagram of Strongly Interacting Matter Test QCD under extreme conditions and in large scale systems Search for deconfined QGP phase SISAGS  SPS RHICLHC From high baryon density regime to high temperature regime

  14. RHIC Physics Program • RHIC was proposed in 1983 • One of the main emphases is study of properties of matter under extreme conditions • large energy densities • high temperatures • To achieve these conditions we collide heavy nuclei at very high energies • Extremely useful to have probes with known properties

  15. vacuum QGP Detecting the QGP “matter box” • Rutherford experiment a atom discovery of nucleus SLAC electron scattering e  proton discovery of quarks • “ideal” experiment • Experiments with QGP not quite that simple • QGP created in nucleus-nucleus collisions can not be put in “box” • Thousands of particles produced during collision penetrating beam absorption or scattering pattern

  16. cone of hadrons “jet” increased gluon-radiation within plasma? Hard scattering p p Jets in heavy ion collisions hard-scattered parton in p+p hard-scattered parton during Au+Au hadron distribution softened, jets broadened?

  17. SppS Collisions UA1, 900 GeV proton anti-proton s = 200, 546, 900 GeV 10’s of particles

  18. RHIC Collisions sNN = 130, 200 GeV Gold Gold (center-of-mass energy per nucleon-nucleon collision) 1000’s of particles

  19. Au+Au peripheral Phys Rev Lett 90, 082302 Jets in Heavy Ion Collisions

  20. Not all A+A collisions are the same -- “Centrality” Spectators Participants For a given b, Glauber model predicts Npart (No. participants) and Nbinary (No. binary collisions) 15 fm b 0 fm 0 N_part 394

  21. Yield of p0 measured by PHENIX Au+Au collisions p+p collisions

  22. +A DIS (1973) AGS  Point-like Scaling DIS scales with A E. Gabathuler, Proc. 6th Int. Symposium on Electron and Photon Interactions at High Energies (1973), Bonn.

  23. Scaling from p+p to A+A • For hard-scattering processes, expect point-like scaling. For inclusive cross sections : • For semi-inclusive yields, expect :

  24. “Binary-Scaling” and RAA • The probabilityfor a“hard” collision for any two nucleons is small • The total probability in A+A collision ismultipliedby the number of times we try, i.e. – the cross-section scales with the number of binary collisions -Nbinary • Define Nuclear Modification Factor RAA  Effect of nuclear medium on yields

  25. “no effect” Systematizing Our Expectations • Describe in terms of scaled ratio RAA= 1 for “baseline expectations” > 1 “Cronin effect” • Will present most of Au+Au and d+Au data in terms of this ratio

  26. Motivation Effect of collision medium on hadron pT spectra: • Parton scattering with large momentum transfer  Hard-scattered partons (jets) present in early stages of collisions • Hot and dense medium  Hard-scattered partons sensitive to hot/dense medium Theory predicts radiative energy loss of parton in QGP • Emission of hadrons  High pT hadrons (jet fragments) Dense medium (QGP) would cause depletion in spectrum of leading hadron at high pT - “jet quenching”

  27. X-N. Wang, Phys. Rev. C58 (1998) 2321 High pT in Au+Au collisions Theoretical prediction Investigate hadron pT spectra for evidence of parton energy loss (“jet quenching”) induced by dense medium

  28. Central Au+Au • * p+p scaled by Nbinary(central) • Peripheral Au+Au • *p+p scaled by Nbinary(peripheral) Yield of p0 in Au+Au compared to p+p collisions

  29. Nuclear Modification Factor RHIC 200 GeV central - Suppression peripheral – Nbinary scaling  Comparison of peripheral to central binary scaling

  30. RAA is well below 1 for both charged hadrons and neutral pions. • The neutral pions fall below the charged hadrons since they do not contain contributions from protons and kaons. Strong Suppression! RAA for p0 and charged hadrons PHENIX AuAu 200 GeV p0 data: nucl-ex/0304022, submitted to PRL. charged hadron (preliminary) : NPA715, 769c (2003).

  31. RAA as a Function of Collision Energy • Previous measurement from CERN-SPS observed no suppression (pT reach limited to 4 GeV/c) * • RHIC measurement shows suppression up to 10 GeV/c (how far in pT will it extend?) • Latest RHIC measurement at s=62 GeV shows suppression at high pT * Re-analysis of WA98: d’Enterria nucl-ex/0403055

  32. Au+Au peripheral Au+Au central pedestal and flow subtracted ? Phys Rev Lett 90, 082302 Azimuthal distributions in Au+Au Near-side: peripheral and central Au+Au similar to p+p Strong suppression of back-to-back correlations in central Au+Au collisions

  33. Proton/deuteron nucleus collision Nucleus- nucleus collision d+Au Control Experiment • Collisions of small with large nuclei were always foreseen as necessary to quantify cold nuclear matter effects. • Recent theoretical work on the “Color Glass Condensate” model provides alternative explanation of data: • Jets are not quenched, but are a priori made in fewer numbers. • Color Glass Condensatehep-ph/0212316; Kharzeev, Levin, Nardi, Gribov, Ryshkin, Mueller, Qiu, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu • Small + Large distinguishes all initial and final state effects.

  34. d+Au results from presented at a press conference at BNL on June, 18th, 2003 Is The Suppression Always Seen at RHIC? • NO! • Run-3: a crucial control measurement via d+Au collisions

  35. Conclusion The combined data from Runs 1-3 at RHIC on p+p, Au+Au, and d+Au collisions establish that a new effect (a new state of matter?) is produced in central Au-Au collisions Au + Au Experiment d + Au Control Experiment Final Data Preliminary Data

  36. Theoretical Understanding? Both • Au-Au suppression (I. Vitev and M. Gyulassy, hep-ph/0208108) • d-Au enhancement(I. Vitev, nucl-th/0302002) understood in an approach that combines multiple scattering with absorption in a dense partonic medium (15 GeV/fm3 ~100 x normal nuclear matter) • Our high pT probeshave been calibratedand are now being used to explore the precise propertiesof the medium d-Au Au-Au

  37. Direct Photons in AuAu Many sources, different pT regions • Thermal Sources (pT < 3-4 GeV) -Partonic (QGP!) , Hadronic Gas (new resonance diagrams  theoretical uncertainties) -Largest Backgrounds, PHENIX systematics still under investigation in this momentum region • Hard Scattering (pT > 3-4GeV) -In central AuAu, p0/meson background suppressed -”Cleanest” region (pQCD dominates) -PHENIX has good sensitivity here  High pT photons provide alternative to high pT hadrons, but Photons do not interact strongly in medium

  38. PHENIX Run2 200 GeV p-p Calculated g from p0 Phys. Rev. Lett. 91, 241803 (2003) p+p->p0 + X PHENIX Direct g’s: Step 0) Measure Background • We are looking for the signal over a large background • Requires precise knowledge of the p0’s Vogelsang calculation reference: JHEP 9903 (1999) 025/ Private Comm.

  39. Ratio g/p0measured g/p0expected bkg Direct Photon Result in p+p Collisions g/p0 PHENIX Preliminary PHENIX Preliminary • Excess Above Background Double Ratio: • [g/p]measured / [g/p]background  gmeasured/gbackground • The excess above 1 is the direct photon signal • Small direct g signal found in 200 GeV p+p

  40. PHENIX Preliminary PbGl / PbSc Combined 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 0-10% Central 200 GeV AuAu • [g/p0]measured / [g/p0]background= gmeasured/gbackground Central Au+Au Direct Photon Result

  41. Summary of High pT Physics at RHIC PHENIX Preliminary PHENIX Preliminary

  42. Summary • Goal of colliding heavy ions at high energies is to detect and study the properties of QCD phase transition (QGP) • One possible signature of the QGP is energy loss of “hard-scattered” partons in the dense medium • Have measured charged particle and neutral pion yields up to pT ~10 GeV/c • Spectra exhibit significant suppression in yield at high pT in central collisions relative to binary-scaled p+p collisions, which requires a very dense medium • Confirmed that it is a final-state effect with d+Au data • Consistent with parton energy loss in dense, strongly interacting medium • Suppression of hadrons at high pT allows for “easier” measurement of pQCD photons

  43. RHIC Performance Run Year Species s1/2 [GeV ]  Ldt Ntot tot. data 01 2000 Au - Au 130 1 μb-1 10M 3 TB 02 2001/2002 Au - Au 200 24 μb-1 170M ~20 TB p- p 200 0.15 pb-1 3.7G ~10 TB 03 2002/2003 d - Au 200 2.74 nb-1 5.5G 46 TB p - p 200 0.35 pb-1 4.0G 35 TB

  44. 30-40% Central AuAu 200 GeV 20-30% Central AuAu 200 GeV 50-60% Central AuAu 200 GeV 0-10% Central 200 GeV AuAu 10-20% Central 200 GeV AuAu 60-70% Central AuAu 200 GeV 70-80% Central AuAu 200 GeV 40-50% Central AuAu 200 GeV Centrality Dependence of Direct Photon Signal PHENIX Preliminary

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