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Highlights from RHIC

Highlights from RHIC. W.A. Zajc Columbia University. Conclusions. RHIC is a hadron collider of unprecedented versatility. The four RHIC experiments have a broad coverage ideally suited to exploit RHIC’s potential.

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Highlights from RHIC

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  1. Highlightsfrom RHIC W.A. ZajcColumbia University W.A. Zajc

  2. Conclusions • RHIC is a hadron collider of unprecedented versatility. • The four RHIC experiments have a broad coverage ideally suited to exploit RHIC’s potential. • RHIC and its experiments provide a superb environment for the study of QCD as a fundamental theory: • Phase transition(s) associated with • Confinement • Chiral symmetry breaking • Origin of proton spin • Initial results from RHIC show superb agreement between the experiments and indications of new behavior W.A. Zajc

  3. Why is RHIC? • To understand fundamental features of the strong interaction: • We have a theory of the strong interaction: • Where does the proton get its spin? • How does nuclear matter “melt”? (This works well except when the interaction is strong…) W.A. Zajc

  4. Making Something from Nothing • Explore non-perturbative “vacuum” that confines color flux by melting it • Experimental method: Energetic collisions of heavy nuclei • Experimental measurements:Use probes that are • Auto-generated • Sensitive to all time/length scales • Particle production • Our ‘perturbative’ region is filled with • gluons • quark-antiquark pairs • A Quark-Gluon Plasma (QGP) W.A. Zajc

  5. Energy density for “g” massless d.o.f 8 gluons, 2 spins;  2 quark flavors, anti-quarks, 2 spins, 3 colors 37 (!) “Reasonable” estimate Relevant Thermal Physics Q. How to liberate quarks and gluons from ~1 fm confinement scale? A. Create an energy density • Need better control of dimensional analysis: W.A. Zajc

  6. Pressure in plasma phase with “Bag constant” B ~ 0.2 GeV / fm3 Pressure of “pure” pion gas at temperature T Slightly More Refined Estimate • Compare • Select system with higher pressure: Phase transition at T ~ 140 MeV with latent heat ~0.8 GeV / fm3 Compare to best estimates (Karsch, QM01)from lattice calculations:T ~ 150-170 MeV latent heat ~ 0.70.3 GeV / fm3 W.A. Zajc

  7. Aside 1 • Previous approach (using a “pure” pion gas) works because QCD is a theory with a mass gap { • This gap is a manifestation of the approximate SU(2)R x SU(2)Lchiral symmetry of QCD with pions as the Nambu-Goldstone bosons W.A. Zajc

  8. requires T < TH Fit to this form with TH = 163 MeV Aside 2 The frightening density of hadronic levels led to concepts of • A “limiting temperature” TH (Hagedorn, 1965) • A phase transition(?) in hadronic matter before quarks were understood as underlying constituents W.A. Zajc

  9. g The Early Universe, Kolb and Turner Previous Attempts First attempt at QGP formation was successful (~1010 years ago) ( Effective number of degrees-of-freedom per relativistic particle ) W.A. Zajc

  10. Experimental Gauge Theory • QCD is the only fundamental gauge theory amenable to experimental study in both • Weak and strong coupling limits • Particle and bulk limits • RHIC • (Strong, bulk ) limit  heavy ion collisions • (Strong, particle) limit  spin physics • (Weak , particle) limit  W’s as helicity probes • (Weak , bulk ) limit  high pT probes of plasma state W.A. Zajc

  11. RHIC Specifications • 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Capable of colliding ~any nuclear species on ~any other species • Energy: • 500 GeV for p-p • 200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized) 6 3 5 1’ 4 1 2 W.A. Zajc

  12. How is RHIC Different? • It’s a collider • Detector systematics independent of ECM • (No thick targets!) • It’s dedicated • Heavy ions will run 20-30 weeks/year • It’s high energy • Access to perturbative phenomena • Jets • Non-linear dE/dx • Its detectors are comprehensive • ~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons W.A. Zajc

  13. Run-1 Results • RHIC worked (i.e, achieved its Year-1 goals): • Stable operation at 130 GeV • Delivery of 10% of design luminosity • All four experiments worked • All four experiments produced quality data within a few months of initial RHIC operation • Particle yields • Rapidity and pT spectra • Flow • Source sizes (Etc.) • This from a data set equivalent to 1-3 days running of RHIC at design luminosity W.A. Zajc

  14. STAR RHIC’s Experiments W.A. Zajc

  15. Kinematics Dynamics Kinematics 101 Fundamental single-particle observable: Momentum Spectrum W.A. Zajc

  16. BRAHMS Acceptance (PID) Acceptances PHOBOS Acceptance STAR Acceptance W.A. Zajc

  17. STAR Event Data Taken June 25, 2000. Pictures from Level 3 online display. W.A. Zajc

  18. STAR W.A. Zajc

  19. 2. Initial State Role of event geometry and gluon distributions • Final State • Yields of produced particles • Thermalization, Hadrochemistry 3. Plasma(?) Probes of dense matter Outline Will present sample of results from various points of the collision process: W.A. Zajc

  20. Results on Particle Composition Anti-particle/particle ratios from PHOBOS and BRAHMS mid-rapidity Anti-particle, particle spectra from PHENIX and STAR W.A. Zajc

  21. Agreement between Experiments • Excellent agreement in common observables provides confidence • Overlaps in detector capabilities a feature of RHIC program W.A. Zajc

  22. BRAHMS An experiment with an emphasis: • Quality PID spectra over a broad range of rapidity and pT • Special emphasis: • Where do the baryons go? • How is directed energy transferred to the reaction products? • Two magnetic dipole spectrometers in “classic” fixed-target configuration W.A. Zajc

  23. BRAHMS Results • Anti-proton to proton yields as a function of rapidity: (Two points are reflected about y=0) • Clear evidence for development of (nearly) baryon-free central region W.A. Zajc

  24. √s [GeV] Approaching the Early Universe • Early Universe: • Anti-proton/proton = 0.999999999 • We’ve created “pure” matterapproaching this value pbar/p • For the first timein heavy ion collisions, more baryons are pair-produced than brought in from initial state NA44 Pb+Pb E866 Au+Au W.A. Zajc

  25. Simple Chemistry • Assume • chemical description appropriate  (m,T) • Boltzmann approximation valid  n(m,T) ~ e m / T • Feed down negligible  use “raw” ratios • Then W.A. Zajc

  26. Locating RHIC on Phase Diagram • Baryon ratios determine m / T • K/p ratio, momentum spectra determine T • TCH = 190 ± 20 MeV, B = 45 ± 15 MeV (M. Kaneta and N. Xu, nucl-ex/0104021) • Final-state analysis suggests RHIC reaches the phase boundary • Difficult for hadrons to probe earlier than this “freeze-out” • <E>/N ~ 1 GeV(J. Cleymans and K. Redlich,Phys.Rev.C60:054908,1999 ) W.A. Zajc

  27. Dynamics 101 Q. How to estimate initial energy density? A. From rapidity density • “Highly relativistic nucleus-nucleus collisions: The central rapidity region”, J.D. Bjorken, Phys. Rev. D27, 140 (1983). • Assumes • ~ 1-d hydrodynamic expansion • Invariance in y along “central rapidity plateau”(I.e., flat rapidity distribution) • Then since boost-invariance of matter where t ~ 1 fm/c W.A. Zajc

  28. Determining Energy Density EMCAL • What is the energy density achieved? • How does it compare to the expected phase transition value ? PHENIX For the most central events: Bjorken formula for thermalized energy density time to thermalize the system (t0 ~ 1 fm/c) ~6.5 fm eBjorken~ 4.6 GeV/fm3 pR2 ~30 times normal nuclear density ~1.5 to 2 times higher than any previous experiments W.A. Zajc

  29. (scaled) spatial asymmetry STAR Centrality Dependence of Elliptic Flow Parameterize azimuthal asymmetry of charged particlesas 1 + 2 v2cos (2 f) Evidence that initial spatial asymmetry is efficiently translated to momentum space ( as per a hydrodynamic description) Evidence that initial spatial asymmetry is efficiently translated to momentum space ( as per a hydrodynamic description) W.A. Zajc

  30. PHOBOS An experiment with a philosophy: • Global phenomena • large spatial sizes • small momenta • Minimize the number of technologies: • All Si-strip tracking • Si multiplicity detection • PMT-based TOF • Unbiased global look at very large number of collisions (~109) W.A. Zajc

  31. PHOBOS Result on dNch/dh • Constrains (determines!) maximum multiplicities at RHIC energies • Does not constrain centrality dependence of same • Does not (quite) distinguish between • “Saturation” models, dominated by gg g • “Cascade” models, dominated by gg gg, gg ggq ( X.N. Wang and M. Gyulassy, nucl-th/0008014 ) W.A. Zajc

  32. PHENIX W.A. Zajc

  33. dN/dh / .5Npart Npart Physics, Consistency between Experiments • Trend • incompatible with final-state gluon saturation model • Good agreement with model based on initial-state saturation (Kharzeev and Nardi, nucl-th/0012025) • Excellent agreement between (non-trivial) PHENIX and PHOBOS analyses of this systematic variation with nuclear overlap. W.A. Zajc

  34. Gluon saturation at RHIC / xpz 2R dT =  /Q 2R m/pz Longitudinal Transverse When do the gluons overlap significantly? 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987) So for  /mx ~ 2R , ~ all constituents contributeParton density r ~ A xG(x,Q2) /R2 Parton cross section s ~ aS2/ Q2 Saturation condition rs ~ 1 QS2 ~ aS A xG(x,Q2) /R2 ~ A1/3 D. Kharzeev, nucl-th/0107033 W.A. Zajc

  35. Initial State Partons? • Procedure: • Determine scale QS2 as function of nuclear overlap • QS2 ~ A1/3 ~ Npart1/3 • Assume final-state multiplicities proportional to number of initial-state gluons in this saturated regime: • dN/dh = c Npart xG(x, QS2) • Note that DGLAP requires evolution of xG(x, QS2) : • xG(x, QS2) ~ ln (QS2 / LQCD2) • Results: • “Running” of the multiplicity yielddirectly from from xG(x, Q2) +DGLAP W.A. Zajc

  36. pp Extensions of Saturation Approach* • Use HERA data, counting rules • x G(x,Q2) ~ x-l (1-x)4 • Describe rapidity dependence: • y ~ ln(1/x)  QS2(s,y) = QS2(s,y=0)ely • Predict energy dependence: • x = QS / s QS2(s,y) = QS2(s0,y) (s/s0) l/2 • Predict10-14% increase betweens = 130 and 200 GeV • Versus 146% reported by PHOBOS * D. Kharzeev and E. Levin, nucl-th/0108006 W.A. Zajc

  37. Energy Loss of Fast Partons • Many approaches • 1983: Bjorken • 1991: Thoma and Gyulassy (1991) • 1993: Brodsky and Hoyer (1993) • 1997: BDMPS- depends on path length(!) • 1998: BDMS • Numerical values range from • ~ 0.1 GeV / fm (Bj, elastic scattering of partons) • ~several GeV / fm (BDMPS, non-linear interactions of gluons) W.A. Zajc

  38. Rare processes at RHIC Both PHENIX and STAR have measured charged particle spectra out to “small” cross sections W.A. Zajc

  39. Expectations • Particle production via rare processes should scale with Ncoll, the number of underlying binary nucleon-nucleon collisions • Assuming no “collective” effects • Test this on production at high transverse momentum W.A. Zajc

  40. A*B scaling at RHIC? • NO! • Both STAR and PHENIX data see deficit of particles at high transverse momenta,relative to expected A*B scaling: W.A. Zajc

  41. Additional Evidence • PHENIX sees an equal or larger effect in the p0 spectrum • Consistent with crude estimatesof additional energy loss in adeconfined medium W.A. Zajc

  42. Discovery of Jet Quenching? • Some would (have) claimed just that • To be done before this can be experimentally established: • Improve pTrange (Run-2) • Measure p-p spectrum (Run-2) • Study in “cold nuclear matter” with p-A collisions (Run-2?) • Vary system size (Run-2??) W.A. Zajc

  43. Summary of Results      (1  = 1 experiment ) • How many particles are produced? More than ever produced previously • Have we made contact with the early universe? Closer than all previous heavy ion experiments • Have we made sufficiently high energy densities for the phase transition? Yes • Is there evidence that the dense matter behaves collectively? Yes • Are there results consistent with the formation of a new state of matter? Yes (?!?) W.A. Zajc

  44. What’s left? The all-important spin program W.A. Zajc

  45. A polarized hadron collider is uniquely suited to some spin measurements: DG via Direct photons Hign pT pions J/Y production via W+/W- production Polarized Drell-Yan RHIC has been equipped To provide polarized beams of protons To make spin measurementsof same in (at least)PHENIX and STAR RHIC Spin Physics W.A. Zajc

  46. This is a world-class program RHIC Spin Potential • Wide range of measurements in many channels to address • DG • Sea quark polarization W.A. Zajc

  47. What’s Left? • Most of the program: • Energy scans • Species scans • All the systematic studies required before laying claim to new physics • Vital spin program • Example (A-A) program to do this: • Run-2: • Au+Au, crude p-p comparison run • First look at J/Y production, high pT • d-Au run? • Run-3: • High luminosity Au+Au (60%) of HI time • High luminosity light ions (40%) of HI time • Detailed examination of A*B scaling of J/Y yield • Run-4: • p-d/p-p comparisons • Baseline data for rare processes • Run-5: • “Complete” p-A program with p-Au • Energy scans • Systematic mapping of parameter space W.A. Zajc

  48. Run-2 Goals • Au-Au running • Achieve design values for • Energy (200 GeV) • Luminosity (2 x 1026 cm-2 s-1) • Interaction region (20 cm) • ~ 12 week physics run • ~ 100 x existing data sets from Run-1 • p-p running • Commission • proton collisions at 200 GeV(5 x 1030 cm-2 s-1) • Polarization for same (  50%) • ~ 5 weeks physics run • (Additional ‘heavy ion’ running to be determined) W.A. Zajc

  49. - Run-2 (A-A) Results • Significantly enhanced detectors  • Much greater integrated luminosity • Greatly extended reach in observables • pT to 20 GeV/c (currently 5 GeV/c) • Spectra of ’s and ’s (currently mass peaks only) • J/Y (no current data) • Extended understanding of RHIC physics • Access truly perturbative regime • Understand detailed hadro-chemistry • Understand (Debye?) screening in hot system W.A. Zajc

  50. An Interesting Technical Connection • RHIC experiments are coping with data volumes at the leading edge of current HEP experience: • Run-1 • ~1 Tb per day (recording rate per experiment) • Data volumes ~ 10 Tb (per experiment, from last year) • Run-2 and beyond • Roughly same recording rate (up to x2 higher?) • Greatly increased volumes (multi-100 Tb) • RHIC Computing Facility is a real-world existence proof for data analysis on this scale: • HPSS capacity of ~1.5 petabytes • HPSS optimized file mover • Port of Objectivity to Linux • “Grand Challenge” query processor • ROOT • Key members of development team (Brun, Rademakers, Buncic, Fine) have been supported by NP funds • NP supported contributions to ROOT • Multithreading • Port to SOLARIS • Objectivity encapsulation • StEvent W.A. Zajc

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