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Surprises from RHIC

Surprises from RHIC. John G. Cramer Department of Physics University of Washington. Colloquium UW Physics Department March 4, 2002. Part 1. About RHIC (The R elativistic H eavy I on C ollider). Brookhaven/RHIC Overview. Systems: Au + Au CM Energies: 130 GeV/A

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Surprises from RHIC

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  1. Surprises from RHIC John G. Cramer Department of PhysicsUniversity of Washington Colloquium UW Physics Department March 4, 2002

  2. Part 1 About RHIC (The Relativistic Heavy Ion Collider) John G. Cramer

  3. Brookhaven/RHIC Overview Systems: Au + Au CM Energies: 130 GeV/A 200 GeV/A1st Collisions: 06/13/2000 Location: BrookhavenNationalLaboratory, Long Island,NY John G. Cramer

  4. The RHIC Accelerator System AGS Booster Ring Switchyard TandemVan de Graaff Yellow Ring RHIC Blue Ring John G. Cramer

  5. What does RHIC do? RHIC accelerates gold nuclei in two beams to about 100 Gev/nucleon each (i.e., to kinetic energies that are over 100 times their rest mass-energy) and brings these beams into a 200 GeV/nucleon collision. Four experiments, STAR, PHENIX, PHOBOS, and BRAHMS study these collisions. In the year 2000 run, RHIC operated at a collision energy of 130 Gev/nucleon. In 2001-2 it operated at 200 GeV/nucleon. John G. Cramer

  6. Magnet Time Projection Chamber Coils SiliconVertexTracker TPC Endcap & MWPC FTPCs ZCal ZCl VertexPositionDetectors Endcap Calorimeter Central Trigger Barrel or TOF BarrelEMCalorimeter RICH About the STAR Detector. STAR is a large solenoidaldetector based on a time-projection chamber. Ituses a 0.5 tesla magneticfield to momentum-analyzeabout 2,000 charged particlesper collision. John G. Cramer

  7. The STAR Collaboration John G. Cramer

  8. Central Au +Au Collision at sNN = 130 GeV Run: 1186017, Event: 32, central colors ~ momentum: low-- -high John G. Cramer

  9. Part 2 RHIC Surprises John G. Cramer

  10. In Search of the Quark-Gluon Plasma (QGP) A QGP should have more degrees of freedom than a pion gas. Entropy should be conserved during the fireball’s evolution. Hence, look in phase space for evidence of: Large size, Long lifetime, Extended expansion…… John G. Cramer

  11. Surprises from RHIC • Relativistic hydrodynamic calculations work surprisingly well, while cascade string-breaking models have problems. Near-threshold QGP behavior is not observed.The “Hydro Paradox”. • There is evidence for strong “quenching” of high momentum pions.QGP Absorption? • The ratio of the HBT radii Rout/Rside is ~1, while the closest model predicts 1.2, and most models predict 4 or more.In essence, all models on the market have been falsified.The “HBT Puzzle” • The pion phase space density is much larger than that observed at CERN or predicted by simple thermal models.A pion chemical potential ~ 50 MeV is needed to explain it.Stimulated emission of pions? John G. Cramer

  12. Surprise 1 Event-by-Event Elliptic Flowand Hydrodynamics John G. Cramer

  13. Sensitive to initial/final conditions and equation of state (EOS) ! coordinate-space-anisotropy  momentum-space-anisotropy y py px x Elliptic Flow and V2 John G. Cramer

  14. Elliptic Flow and Hydrodynamics John G. Cramer

  15. The Hydrodynamic Paradox The system behaves as if it hasreached thermodynamic equilibrium. How could there be enough time (in~10 fm/c) for the system to come to thermalequilibrium, as relativistic hydrodynamicsassumes? Quantum effects? Perhaps the multiparticlewave function collapses into a maximumentropy state => TD equilibrium. John G. Cramer

  16. Surprise 2 Pion Spectrum Measurements: Strong Absorption of 2 to 6 GeV/c Pions John G. Cramer

  17. Gedankenexperiments: p + QGP or HG Target High momentum pion beam High momentum pions Hadrongas (Transparent) Lower momentum pions High momentum pion beam QGP (Opaque) John G. Cramer

  18. High-Momentum p Absorption (1) Syst. errors from UA1 extrapolation (h+ + h-)/2 Au+Au Preliminary p+p MinBias/ UA1 Scales approximately A2 at high pT. John G. Cramer

  19. High-Momentum p Absorption (2) Suppression factor ~2 Systematic errors from UA1 extrapolation from 200 to 130 GeV Central/ UA1 Conclusion: Central RHIC Au+Au collisions show strongabsorption of high energy pions that is not observed in Pb+Pbcollisions at the CERN SPS or in less central collisions at RHIC.Smoking gun for QGP? John G. Cramer

  20. Surprise 3 Source Radii and Emission Duration fromBose-Einstein Interferometry John G. Cramer

  21. The Hanbury-Brown-Twiss Effect Coherent interference between incoherent sources! For non-interacting identical bosons: S(x,p)=S(x)S(p) Neglects • Momentum dependence of source • Quantum mechanics up to x and y • Final State Interactions after x and y Nonetheless • C2(q) contains shape information • True component-by-component in q John G. Cramer

  22. Bertsch-Pratt Momentum Coordinates John G. Cramer

  23. A Bose-Einstein Correlation “Bump” This 3D histogramhas been corrected forCoulomb repulsion ofidentical p-p- pairs andis a projection slice nearqlong=0 . The “bump” results fromBose-Einstein statistics ofidentical pions (Jp=0-). John G. Cramer

  24. “Naïve” picture (no space-momentum correlations): Rout2 = Rside2+(bpairt)2 One step further: Hydro calculation of Rischke & Gyulassy expects Rout/Rside ~ 2->4 @ kt = 350 MeV. Looking for a “soft spot” Small Rout/Rsideonly forTQGP=Tf (unphysical)). Expectations: Pre-RHIC HBT Predictions Rside Rout John G. Cramer

  25. Reality: STAR/RHIC HBT Measurements • ~10% Central AuAu(PbPb) events • y ~ 0 • kT0.17 GeV/c No significant increase in spatio-temporal size of the  emitting source at RHIC. Note the ~100 GeV gap from SPS to RHIC and the gapbetween AGS and SPS data. Ro/Rs ~ 1 John G. Cramer

  26. Rout and Rside are energy independent within error bars. Smooth energy dependence in Rlong No immediate indication of very different physics Fit Rlong to: AGS: A = 2.19 +/- .05 SPS: A = 2.90 +/- .10 RHIC: A = 3.32 +/- .03 Conclusion: Transverse Size ~ Constant vs. Energy M. Lisa et al., PRL 84, 2798 (2000) R. Soltz et al., to be sub PRC C. Adler et al., PRL 87, 082301 I.G. Bearden et al., EJP C18, 317 (2000) p- A = t0T in 1st order T/mT calculation t0 = average freeze-out timeT = freezeout temperature John G. Cramer

  27. RO/RS: STAR and PHENIX Agree, Models Fail. Compiled by S. Johnson STAR and PHENIX agree Best hydro model does not reproduce the data John G. Cramer

  28. Remedies for RHIC HBT Puzzle? • Problems: Ro/Rs (and implied emission duration) are too small, implying near-instantaneous emission. • Rl is also uncomfortably small, calling into question Bjorken “boost invariance”. • Solutions?:Allow single “avalanche” freezeout: tPT=tCF=tF? • Abandon outside-in freezeout scenario? Assume some mysterious energy-loss process at hottest part of collision fireball? • Abandon boost invariance? John G. Cramer

  29. Surprise 4 Particle Spectrum Measurements+Bose-Einstein Interferometry: Pion Phase Space Density John G. Cramer

  30. 2D Fit to Pion Spectrum (only) We can do a global fit of the uncorrectedpion spectrum vs. centrality by: • Assuming that the spectrumhas the form of a Bose-Einsteindistribution:d2N/mTdmTdy=A/[Exp(E/T) –1]and • Assuming that A and T have aquadratic dependence on thenumber of participants n:A(p) = A0+A1n+A2n2T(p) = T0+T1n+T2n2 STAR Preliminary John G. Cramer

  31. A 3D Correlation Histogram John G. Cramer

  32. Pion Phase Space Density at Midrapidity The Lorentz scalar phase space density áf(mT)ñ is the dimensionless average number of pions per 6-dimensional phase space cell Ñ3. At midrapidity áfñ is given by the expression: Average phasespace density HBT “volume” Jacobian Purity Momentum Spectrum John G. Cramer

  33. Momentum Volume The momentum volume can be determinedin two ways: • Fit the correlation function with a 3DGaussian and use the fit parameters toestimate the momentum volume vmom, • Direct summation of the 3D histogram channels. Method (1) is traditional, but Method (2) is less model-dependentand gives the best statistical accuracy. John G. Cramer

  34. <f> from Direct Histogram Sums STAR Preliminary John G. Cramer

  35. Tomasik & Heinz PSD Paper The longitudinal expansion hasreduced the phase space density andbroken the rule that the PSD goesto a Bose-Einstein distributionwhen ht=pt=0 (no flow). The reduction in the PSD leads toa need for a non-zero chemicalpotential m0 to reach high enoughPSD values to match RHIC/STARobservations. Notice that there is a “sweet spot”near pT=0.1 GeV/c at which <f>is independent of ht. John G. Cramer

  36. T&H Fit to Pion Spectra Because the longitudinal expan-sion reduces the phase space density,a non-zero chemical potential m0 isrequired to reproduce the mostcentral data. Pion phase space density dependson m0 and T in essentially the sameway, changing the PSD strength butnot its shape. However, the spectrumslope has very different dependenceson m0 and T, breaking this ambiguity. Therefore, fitting PSD and spectratogether constrains the parameters.However, the lowest curves wouldprefer a negative m0-value toreproduce the spectrum slope whilefitting the PSD. STAR Preliminary John G. Cramer

  37. T&H Fit to STAR Phase Space Density (HBT) Phase space density ~ 1Multiparticle and laser-likestimulated emission effects? STAR Preliminary John G. Cramer

  38. Summary What does it all mean? John G. Cramer

  39. Conclusion (1) The theoretical models ofRHIC physics now on themarket allow the source toexpand for too long, so thatthe theoretical predictions“outrun” the boundaries ofexperimental observation. Something is seriously wrongwith our understanding of thedynamics of RHIC collisions. John G. Cramer

  40. Conclusion (2) The useful theoretical models that has served us so well at the AGSand SPS for heavy ion studies have now been overloaded with a largevolume of puzzlingnew data from RHIC,and things are a bitup in the air. We need moretheoretical helpand more experi-mental data to meetthe challenge ofunderstanding whatis going on in theRHIC regime. It’s a very excitingtime for us STARexperimentalists! John G. Cramer

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