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Results from the Relativistic Heavy Ion Collider (Part I)

Results from the Relativistic Heavy Ion Collider (Part I). James Nagle. HAWAII 2001: DNP/ JPS Meeting October 17-20, 2001 Maui, Hawaii. Outline. Physics goals of Relativistic Heavy Ions RHIC experiments and Run I results. Initial Conditions Parton Density Energy Density.

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Results from the Relativistic Heavy Ion Collider (Part I)

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  1. Results from the Relativistic Heavy Ion Collider (Part I) James Nagle HAWAII 2001: DNP/ JPS Meeting October 17-20, 2001 Maui, Hawaii

  2. Outline • Physics goals of Relativistic Heavy Ions • RHIC experiments and Run I results Initial Conditions Parton Density Energy Density Collective Expansion Pressure and Flow Hard Processes Quark / Gluon jets Charm Production Direct Photons Hadroniziation Particle Ratios Correlations

  3. Structure of the Proton See the whole proton Momentum transfer Q2 = 0.1 GeV2 Wavelength l = h/p See the quark substructure Q2 = 1.0 GeV2 See many partons (quarks and gluons) Q2 = 20.0 GeV2

  4. Heavy Ion Collisions Nuclei collide at near the speed of light and a parton cascade results…. 104 gluons, q, q’s 200 hadrons 200 hadrons

  5. Lattice QCD The nature of this bath of quarks and gluons cannot be calculated directly with Quantum Chromodynamics (QCD). Thus we rely on lattice QCD calculations. Lattice QCD predicts a phase transition to a Quark-Gluon Plasma where the long range confining force is screened. Phase Transition: T = 150-200 MeV e ~ 0.6-1.8 GeV/fm3 Assumes thermal system. e/T4 T/Tc (F. Karsch, hep-lat/9909006)

  6. Early Universe QGP, quasi-free quarks, gluons T (MeV) RHIC 200 SPS AGS Color superconductor? nuclei packed hadrons m (MeV) Baryon density Phase Diagram of Nuclear Matter

  7. Relativistic Heavy Ion Collider • Au + Au collisions at 200 GeV/u • p + p collisions at 500 GeV • spin polarized protons • lots of combinations in between Central Collisions/sec Luminosity (cm-2 sec1) Equivalent Collider Energy (GeV/u)

  8. STAR Four Major Experiments

  9. Two Large Experiments STAR Hadronic Observables over a Large Acceptance Event-by-Event Capabilities Solenoidal magnetic field Large coverage Time-Projection Chamber Silicon Tracking, RICH, EMC, TOF PHENIX Electrons, Muons, Photons and Hadrons Measurement Capabilities Focus on Rare Probes: J/y, high-pT Two central spectrometers with tracking and electron/photon PID Two forward muon spectrometers

  10. Two Small Experiments PHOBOS Charged Hadrons in Central Spectrometer Nearly 4p coverage multiplicity counters Silicon Multiplicity Rings Magnetic field, Silicon Strips, TOF BRAHMS Hadron PID over broad rapidity acceptance Two conventional beam line spectrometers Magnets, Tracking Chambers, TOF, RICH Paddle Trigger Counter TOF Spectrometer Octagon+Vertex Ring Counters

  11. Run I at RHIC June 2000 Running in August PerformanceAu + Au RHIC Design snn 130 GeV 200 GeV L [cm-2 s -1 ] ~ 2 x 1025 2 x 1026 Interaction rates ~ 100 Hz 1400 Hz

  12. (1) Initial Conditions • What is the energy density achieved? • How does it compare to the expected phase transition value from lattice QCD? • What is the initial density of created partons?

  13. eBj~ 23.0 GeV/fm3 eBj~ 4.6 GeV/fm3 Lattice ec pR2 2ct0 Energy Density Bjorken formula for thermalized energy density in terms of measured transverse energy ET PHENIX: Central Au Au yields Thermalization time ?

  14. Charged Particle Multiplicity Agreement between all four RHIC experiments at 130 GeV New Result from PHOBOS from Run II at 200 GeV Particle production rising faster than in pp (p pbar) Over 5000 charged particles produced in central collisions at 200 GeV Do we see gluon saturation? PHENIX STAR PHOBOS BRAHMS

  15. probe rest frame r/ ggg Gluon Saturation the saturation scale pT2 ~ A1/32 /2mG(x,pT2) Test this by varying the A of the colliding nuclei One can look for saturation effects at higher x than at HERA in Deep Inelastic Scattering (e+p) due to nuclear size. Wavefunction of low x partons overlap and the self-coupling gluons fuse, thus saturating the density of gluons in the initial state 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987).

  16. Spectators Spectators Collision Characterization In Run I, we only collided one nuclear species (Gold). However, we can vary the collision size by selecting different impact parameter events Different number of participating nucleons Binary collisions Participants Spectators Participants = 2 x 197 -Spectators Impact Parameter (fm)

  17. Experimental Tests Number of charged particles increases per participating nucleon as expected from hard processes. EKRT model of gluon saturation is ruled out by data, but more systematic studies with different nuclei and lower energy data are needed. nucl-ex/0105011 dNch/dh/(0.5Np) Phys. Rev. Lett. 86, 3500 (2001) Npart

  18. Other Tests Saturation models can predict the scaling with centrality and energy of the momentum space distributions. Kharzeev & Levin, nucl-th/0108006 Schaffner-Bielich et al, nucl-th/0108048

  19. Hard Probes of the Plasma Quarks and quarkonium states are excellent probes of the plasma Beams of hard probes: colored quarks, J/y,…. Colorless Hadrons Colored QGP

  20. hadrons leading particle q q hadrons leading particle Hard Scattering Hard scattering of partons (large Q2) have calculable rates in perturbative QCD. The partons manifest themselves as jets, fragmented hadrons in a collimated angular cone. Schematic View of Jet Production OPAL Event Display

  21. Ingredients In order to calculate the yield of high pT hadrons Flux of incoming partons (structure functions) from Deep Inelastic Scattering Fragmentation functions D(z) in order to relate jets to observed hadrons Perturbative QCD

  22. Heavy Ion Reactions Partons are expected to lose energy via gluon radiation in traversing a color deconfined quark-gluon plasma. q q Experimentally look for a suppression of high pT hadrons from jet fragmentation. Baier, Dokshitzer, Mueller, Schiff, hep-ph/9907267 Gyulassy, Levai, Vitev, hep-pl/9907461 Wang, nucl-th/9812021 and many more…..

  23. Neutral Pions PHENIX measures p0 with two types of calorimeters Significant suppression relative to p-p scaling observed in central collisions

  24. Unidentified Charged Hadrons Preliminary

  25. Suppression Observed Suppression relative to independent binary collisions scaling Preliminary Ratio Ratio h- h+/- p0 PT (GeV/c) PT (GeV/c) • Unidentified spectra from STAR and PHENIX agree within systematics, but different trend at high pt • p0 shows a larger suppression, surprising since most expect unidentified to be dominated by p+/- over kaons and (anti)protons

  26. Jet Fragmentation? Why are the charged unidentified and the p0 different? Unidentified charged hadrons have a dominant proton/antiproton contribution above pT > 2 GeV ! This is not the expected jet fragmentation function D(z).

  27. Hydro + Jets Hydrodynamic expansion may boost “soft” physics into what was previously thought to be only “hard” physics pT region. Teaney et al. Vitev et al. predict that “hard” jet fragmentation eventually dominates over hydrodynamics for antiprotons above pT~5 GeV

  28. Energy Loss from p0 Thus, just focus on p0 Calculation of X.N. Wang includes a particular shadowing parameterization for the structure functions and kT broadening (Cronin). Agreement with data implies: dE/dx = 0.25 GeV/fm p0 Central data Scaled pp Shadowing + Cronin Energy Loss X.N. Wang, Phys. Rev. C 61, 064910 (2000) (nucl-th/9812021)

  29. e+ e+ g* Dx Parton Energy Loss In Cold Nuclear Matter E866 at Fermilab HERMES at DESY Drell-Yan production in proton-A collisions is sensitive to parton energy loss. Deep-Inelastic Scattering on Nuclei is sensitive to parton energy loss and hadron formation. m+ g* m- Dx Need to separate shadowing from energy loss, and then -dE/dx = 2.32  0.52  0.5 GeV/fm Modified fragmentation functions measured. hep-ex/0109014 hep-ex/0012049

  30. Dx Dx Disappearing Density Stage 1: Initial hard scattering occurs Parton sees flux of other partons from the remainder of the two nuclei If parton starts at 900, then no net energy loss, just kT scatter Stage 2: Parton has coherent energy loss as it traverses the energy dense medium created in the collision However, the density of the medium is dropping rapidly (eg. e ~ time-4/3) Thus this parton does not see RAU of the highest density deconfined medium

  31. Other Observables Quark jets dominate over gluon jets at high pT X.N.Wang, Phys.Rev.C 58 (1998) 2321 Gluons have larger energy loss in deconfined quark-gluon plasma.

  32. q q q q Back-to-Back Correlations At pT<1.5 GeV, hydrodynamic description is good. Above this, hard scattering may dominate which is uncorrelated with impact parameter plane, unless…. Parton energy loss could be correlated to reaction plane due to different average path lengths

  33. Be Careful • This is a very exciting result because it is an observation in AA of a suppression relative to point-like scaling and is qualitatively in agreement with parton energy loss expectations. • However, many caveats remain. • Cronin Effect and Pt broadening • Nuclear shadowing of structure functions • Hadron ratios in the fragmentation • Does the parton hadronize inside? • Too early for conclusions. q q

  34. Heavy Flavor From screening of cc pairs in a deconfined plasma we may expect a suppression of J/y and other quarkonium states. Open charm measurements are a necessary normalization for the J/y result. Also, charm production and kinematics give useful information on energy loss, shadowing, and saturation physics. D0 K-p+ D0D0m+m- K+ K-nmnm D0 K- e+ ne D0D0 e+e- K+ K-nene D0 K-m+ nm D0D0m+e- K+ K-nenm

  35. Single Electron Spectra One must account for contributions: p0, h Dalitz g conversions Remaining signal is then from charm and bottom thermal production new physics Simulation p0,h Dalitz Now Real Data ! D0 K-p+ g conversions Charm Beauty Drell-Yan

  36. Run II and Beyond RHIC is at 200 GeV and expects to achieve full luminosity. AuAu running until Nov. 26, and then start the first polarized proton run to begin the spin program. Many good reasons to be excited: (1) Hadron spectra beyond Pt > 10 GeV ! * this helps resolve many open questions (2) Back-to-Back correlation measurements (3) Charm via single electrons, e+e- pairs, and muons (4) Second excellent probe of QGP (J/y, y‘,U) (5) Much more than can be listed here Critical measurements needed from proton-nucleus (or dA) and energy loss in cold nuclear matter.

  37. Conclusions • First RHIC run is a success ! • All four experiments have physics results with many papers already accepted/published • (1) Initial Conditions look favorable for QGP formation • (2) First measures of high pT hadrons very exciting • (3) Charm physics starting with single electrons • RHIC Run II is underway and all four experiments are taking data. With an increase in RHIC luminosity, expectations for physics conclusions are high.

  38. AA Other Effects

  39. AA A CERN-SPS Results

  40. Nuclear Modifications Zheng Huang, Hung Jung Lu, Ina Sarcevic: Nucl.Phys.A637:79-106,1998 (hep-ph/9705250 )

  41. Central to Peripheral Central/peripheral PT

  42. x distribution Does not include kt broadening For pt = 4 GeV/c hadron from jet fragmentation, what is x distribution of parent parton?

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