Surprises from RHIC STAR John G Cramer Department
- Slides: 40
Surprises from RHIC STAR John G. Cramer Department of Physics University of Washington Colloquium UW Physics Department March 4, 2002
Part 1 About RHIC (The Relativistic Heavy Ion Collider) STAR March 4, 2002 2 John G. Cramer
Brookhaven/RHIC Overview Systems: Au + Au CM Energies: 130 Ge. V/A 200 Ge. V/A 1 st Collisions: 06/13/2000 Location: Brookhaven National Laboratory, Long Island, NY STAR March 4, 2002 3 John G. Cramer
The RHIC Accelerator System AGS Booster Ring Switchyard Tandem Van de Graaff Yellow Ring RHIC STAR March 4, 2002 4 Blue Ring John G. Cramer
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 Ge. V/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 Ge. V/nucleon. STAR March 4, 2002 5 John G. Cramer
About the STAR Detector. STAR is a large solenoidal detector based on a timeprojection chamber. It uses a 0. 5 tesla magnetic field to momentum-analyze about 2, 000 charged particles per collision. Magn et Coils TPC Endcap & MWPC ZC al Endcap Calorime ter Barrel EM Calorim eter STAR March 4, 2002 6 Time Projecti on Silicon Chamb Vertex er Tracker FTPCs ZCl Vertex Positio n Central Detect Trigger ors Barrel or RI TOF CH John G. Cramer
The STAR Collaboration STAR March 4, 2002 7 John G. Cramer
Central Au +Au Collision at s. NN = 130 Ge. V Run: 1186017, Event: 32, central colors ~ momentum: low - - - high STAR March 4, 2002 8 John G. Cramer
Part 2 RHIC Surprises STAR March 4, 2002 9 John G. Cramer
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…… STAR March 4, 2002 10 John G. Cramer
Surprises from RHIC 1. Relativistic hydrodynamic calculations work surprisingly well, while cascade string-breaking models have problems. Near-threshold QGP behavior is not observed. The “Hydro Paradox”. 2. There is evidence for strong “quenching” of high momentum pions. QGP Absorption? 3. 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” 4. The pion phase space density is much larger than that observed at CERN or predicted by simple thermal models. A pion chemical potential ~ 50 Me. V is needed to explain it. STAR Stimulated emission 11 John G. Cramer March 4, 2002 of pions?
Surprise 1 Event-by-Event Elliptic Flow and Hydrodynamics STAR March 4, 2002 12 John G. Cramer
Elliptic Flow and V 2 Sensitive to initial/final conditions and equation of state (EOS) ! coordinate-space-anisotropy momentum-space-anisotropy y py px x STAR March 4, 2002 13 John G. Cramer
Elliptic Flow and Hydrodynamics STAR March 4, 2002 14 John G. Cramer
The Hydrodynamic Paradox The system behaves as if it has reached thermodynamic equilibrium. How could there be enough time (in ~10 fm/c) for the system to come to thermal equilibrium, as relativistic hydrodynamics assumes? Quantum effects? Perhaps the multiparticle wave function collapses into a maximum entropy state => TD equilibrium. STAR March 4, 2002 15 John G. Cramer
Surprise 2 Pion Spectrum Measurements: Strong Absorption of 2 to 6 Ge. V/c Pions STAR March 4, 2002 16 John G. Cramer
Gedankenexperiments: p + QGP or HG Target High momentum pion beam High momentum pions Hadron gas (Transparent) High momentum pion beam Lower momentum pions QGP (Opaque) STAR March 4, 2002 17 John G. Cramer
High-Momentum p Absorption (1) Syst. errors from UA 1 extrapolation Au+Au Preliminary p+p (h+ + h-)/2 Min. Bias/ UA 1 Scales approximately A 2 at high p. T. STAR March 4, 2002 18 John G. Cramer
High-Momentum p Absorption (2) • Suppression factor ~2 • Systematic errors from UA 1 extrapolation from 200 to 130 Ge. V Central/ UA 1 Conclusion: Central RHIC Au+Au collisions show strong absorption of high energy pions that is not observed in Pb+Pb collisions at the CERN SPS or in less central collisions at RHIC. Smoking gun for QGP? STAR March 4, 2002 19 John G. Cramer
Surprise 3 Source Radii and Emission Duration from Bose-Einstein Interferometry STAR March 4, 2002 20 John G. Cramer
The Hanbury-Brown-Twiss Effect Coherent interference between incoherent sources! For non-interacting identical bosons: S(x, p)=S(x)S(p) Neglects q Momentum dependence of source • Quantum mechanics up to x and y q Final State Interactions after x and y Nonetheless q C 2(q) contains shape information q True component-by-component in q STAR March 4, 2002 21 John G. Cramer
Bertsch-Pratt Momentum Coordinates STAR March 4, 2002 22 John G. Cramer
A Bose-Einstein Correlation “Bump” This 3 D histogram has been corrected for Coulomb repulsion of identical - - pairs and is a projection slice near qlong=0. The “bump” results from Bose-Einstein statistics of identical pions (J =0 ). STAR March 4, 2002 23 John G. Cramer
Expectations: Pre-RHIC HBT Predictions “Naïve” picture (no space-momentum correlations): u Rout 2 = Rside 2+(bpairt)2 Rside Rout One step further: u Hydro calculation of Rischke & Gyulassy expects Rout/Rside ~ 2>4 @ kt = 350 Me. V. u Looking for a “soft spot” u Small Rout/Rside only for TQGP=Tf (unphysical)). STAR March 4, 2002 24 John G. Cramer
Reality: STAR/RHIC HBT Measurements • ~10% Central Au. Au(Pb. Pb) events • y~0 • k. T 0. 17 Ge. V/c No significant increase in spatio-temporal size of the emitting source at RHIC. Note the ~100 Ge. V gap from SPS to RHIC and the gap between AGS and SPS data. Ro/Rs ~ 1 STAR March 4, 2002 25 John G. Cramer
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 C 18, 317 (2000) - 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 A = t 0 T in 1 st order T/m. T calculation t 0 = average freeze-out time T = freezeout temperature STAR March 4, 2002 26 John G. Cramer
RO/RS: STAR and PHENIX Agree, Models Fail. Compiled by S. Johnson STAR and PHENIX agree Best hydro model does not reproduce the data STAR March 4, 2002 27 John G. Cramer
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: t. PT=t. CF=t. F? Abandon outside-in freezeout scenario? Assume some mysterious energy-loss process at hottest part of collision fireball? Abandon boost invariance? STAR March 4, 2002 28 John G. Cramer
Surprise 4 Particle Spectrum Measurements + Bose-Einstein Interferometry: Pion Phase Space Density STAR March 4, 2002 29 John G. Cramer
2 D Fit to Pion Spectrum (only) We can do a global fit of the uncorrected pion spectrum vs. centrality by: (1) Assuming that the spectrum has the form of a Bose-Einstein distribution: d 2 N/m. Tdy=A/[Exp(E/T) – 1] and (2) Assuming that A and T have a quadratic dependence on the number of participants n: Pr STA elim R ina ry A(p) = A 0+A 1 n+A 2 n 2 T(p) = T 0+T 1 n+T 2 n 2 STAR March 4, 2002 30 John G. Cramer
A 3 D Correlation Histogram STAR March 4, 2002 31 John G. Cramer
Pion Phase Space Density at Midrapidity The Lorentz scalar phase space density áf(m. T)ñ is the dimensionless average number of pions per 6 -dimensional phase space cell Ñ 3. At midrapidity áfñ is given by the expression: Average phase space density Jacobian Purity Momentum Spectrum STAR March 4, 2002 32 HBT “volume” John G. Cramer
Momentum Volume The momentum volume can be determined in two ways: (1) Fit the correlation function with a 3 D Gaussian and use the fit parameters to estimate the momentum volume vmom, (2) Direct summation of the 3 D histogram channels. (3) Method (1) is traditional, but Method (2) is less model-dependent and gives the best statistical accuracy. STAR March 4, 2002 33 John G. Cramer
<f> from Direct Histogram Sums Pr STA elim R ina ry STAR March 4, 2002 34 John G. Cramer
Tomasik & Heinz PSD Paper The longitudinal expansion has reduced the phase space density and broken the rule that the PSD goes to a Bose-Einstein distribution when ht=pt=0 (no flow). The reduction in the PSD leads to a need for a non-zero chemical potential m 0 to reach high enough PSD values to match RHIC/STAR observations. Notice that there is a “sweet spot” near p. T=0. 1 Ge. V/c at which <f> is independent of ht. STAR March 4, 2002 35 John G. Cramer
T&H Fit to Pion Spectra Because the longitudinal expansion reduces the phase space density, a non-zero chemical potential m 0 is required to reproduce the most central data. Pion phase space density depends on m 0 and T in essentially the same way, changing the PSD strength but not its shape. However, the spectrum slope has very different dependences on m 0 and T, breaking this ambiguity. Therefore, fitting PSD and spectra together constrains the parameters. However, the lowest curves would prefer a negative m 0 -value to reproduce the spectrum slope while fitting the PSD. STAR March 4, 2002 Pr STA elim R ina ry 36 John G. Cramer
T&H Fit to STAR Phase Space Density (HBT) Phase space density ~ 1 Multiparticle and laser-like stimulated emission effects? Pr STA elim R ina ry STAR March 4, 2002 37 John G. Cramer
Summary What does it all mean? STAR March 4, 2002 38 John G. Cramer
Conclusion (1) The theoretical models of RHIC physics now on the market allow the source to expand for too long, so that theoretical predictions “outrun” the boundaries of experimental observation. Something is seriously wrong with our understanding of the dynamics of RHIC collisions. STAR March 4, 2002 39 John G. Cramer
Conclusion (2) The useful theoretical models that has served us so well at the AGS and SPS for heavy ion studies have now been overloaded with a large volume of puzzling new data from RHIC, and things are a bit up in the air. We need more theoretical help and more experimental data to meet the challenge of understanding what is going on in the RHIC regime. It’s a very exciting time for us STAR experimentalists! STAR March 4, 2002 40 John G. Cramer
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