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Results from the PHOBOS experiment at RHIC

Results from the PHOBOS experiment at RHIC. Gunther Roland (MIT) for the PHOBOS Collaboration. PHOBOS Collaboration. ARGONNE NATIONAL LABORATORY Birger Back, Nigel George, Alan Wuosmaa BROOKHAVEN NATIONAL LABORATORY

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Results from the PHOBOS experiment at RHIC

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  1. Results from the PHOBOS experiment at RHIC Gunther Roland (MIT) for the PHOBOS Collaboration

  2. PHOBOS Collaboration ARGONNE NATIONAL LABORATORY Birger Back, Nigel George, Alan Wuosmaa BROOKHAVEN NATIONAL LABORATORY Mark Baker, Donald Barton, Alan Carroll, Stephen Gushue, George Heintzelman, Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov INSTITUTE OF NUCLEAR PHYSICS, KRAKOW Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak MASSACHUSETTS INSTITUTE OF TECHNOLOGY Wit Busza* , Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch NATIONAL CENTRAL UNIVERSITY, TAIWAN Willis Lin, JawLuen Tang UNIVERSITY OF ROCHESTER Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs UNIVERSITY OF ILLINOIS AT CHICAGO Russell Betts, Clive Halliwell, David Hofman, Burt Holzman, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter UNIVERSITY OF MARYLAND Richard Bindel, Edmundo Garcia-Solis, Alice Mignerey * Spokesperson

  3. Paddle Trigger Counter TOF Spectrometer Octagon+Vertex Ring Counters PHOBOS Detector 96000 Silicon Pad channels 4-p Multiplicity Array Mid-rapidity Spectrometer Scintillator Paddles + Zero Degree Calorimeter for triggering TOF wall for high-momentum PID see talk by R. Pak Tue 3:20 PM

  4. Energy Dependence (vs. AGS/SPS Data) System Size (p+p, Npart Dependence ) Year 1: The Big Picture Q: • Entropy Production • Thermal Equilibration • Hadro-Chemistry Charged Particle Density Event Anisotropy - Flow Particle Ratios Context:

  5. Event selection and Centrality Determination • Minimum bias trigger on Au+Au interactions • Determine number of participating nucleons See talk by Judith Katzy Wed, 4:40 PM

  6. Positive Paddles Negative Paddles ZDC N ZDC P Au Au PN PP x z Selecting Collisions • Coincidence between Paddle counters • Paddle + ZDC timing reject background • Sensitive to 97% of inelastic cross-section for Au+Au at sqrt(sNN) = 130 GeV

  7. Determination of Npart Data • HIJING +GEANT • Glauber Calculation • Model of Paddle trigger Paddle signal (a.u.) MC Npart

  8. dNch/dh @ h=0 vs Energy For method , see poster by P. Decowski • Two values of sqrt(sNN): • 56 GeV • 130 GeV • Select 6% most central events • Count Tracklets near h = 0 Q: • What is the density of particles near h=0? • How does it compare to p+p and A+A @ SPS

  9. Hundreds of tracklets per central event Corrections Background subtraction Uncertainty due to model differences Feed-down from strange decays Stopping particles Total uncertainty on dNch/dh is ±8% Tracklets and dNch/dh

  10. dNch/dh @ h=0 vs Energy PRL 89 (2000) 3100

  11. Model comparisons HIJING EKRT (saturation model) 40% increase from p+p to central Au+Au Data constrain model parameter space Further insight from centrality dependence

  12. dNch/dh @ h=0 vs Centrality See talk by Judith Katzy, Wed 4:40 PM • Count tracklets • 12 centrality bins for upper 55% of cross-section Q: How does dNch/dh@h=0 evolve from 2.3 (p+p) to 3.25 (Au+Au) per participant pair?

  13. dNch/dh @ h=0 vs Npart Preliminary Yellow band: Systematic uncertainty dNch/dh/(0.5*Npart) Npart Good agreement with previous PHOBOS point Good agreement with recent PHENIX data Neither HIJING nor EKRT describe data well

  14. Comparison to SPS PHOBOS Au+Au` WA98 Pb+Pb dNch/dh/(0.5*Npart) dNch/dh/(0.5*Npart) p+p p+p Npart Npart General features (rapid rise/flat top) similar Note that WA98 dNch/dh measured in lab frame

  15. dNch/dh in 4-p See talk by Alan Wuosmaa Mon 5:35 PM • Determine dNch/dh for –5.4 < h < 5.4 • Shape • Total multiplicity • Evolution vs Npart • Hit counting Q: Increased particle production everywhere or only near h=0?

  16. dNch/dh vs Centrality Preliminary dNch/dh 45-55% 35-45% 25-35% dNch/dh 15-25% 6-15% 0-6% h h h Statistical errors only - 10-20% systematical uncertainty

  17. Evolution of dNch/dh vs Npart Preliminary Statistical errors only <Nch> = 4100 +/- 410 for 3% most central Additional particle production near h=0 Wider + more particles relative to HIJING Data Npart=356 Npart=215 (dNch/dh)/(½Npart) Npart=103 h HIJING (dNch/dh)/(½Npart) h

  18. Event Anisotropy See talk by Inkyu Park Wed 5:55 PM • Elliptic flow of charged particles • V2 for –5.0 < h < 5.3 • Evolution of V2 vs Npart Q: 4100 independent particles or thermally equilibrated strongly interacting matter?

  19. Q: Does the initial space anisotropy translate into final state momentum space anisotropy? Elliptic Flow b (reaction plane) dN/d(f -YR ) = N0 (1 + 2V1cos (f-YR) + 2V2cos (2(f-YR) + ... )

  20. Centrality Dependence Preliminary |h| < 1.0 V2 Hydrodynamic model Preliminary SPS AGS Systematic error ~ 0.007 Normalized Paddle Signal Large V2 Signal compared to lower energy

  21. PHOBOS V2 STAR (PRL) h V2 vs h Preliminary Systematic error ~ 0.007 Averaged over centrality V2 drops for |h| > 1.5

  22. Hadro-Chemistry See talk by Nigel George Fri 4:25 PM • Determine ratio of p-/p+, K-/K+, p/p • Compare to AGS/SPS results Q: • Baryo-Chemical Potential • Baryon Stopping

  23. Particle ID dE/dx in silicon Two B-field polarities Many systematic effects cancel in the ratio Tracking and Particle ID

  24. Kinematic Coverage P Acceptance near y=0.5 Identical for positive particles in BPLUS/negative particles in BMINUS K P

  25. Particle Ratios Preliminary Centrality =12 % <Npart> ~ 310 Central region not baryon-free

  26. Energy Dependence Preliminary Ratios @ ycm closer to unity than at lower energy Estimate mB = 50 MeV vs 200 MeV at SPS

  27. Summary • dNch/dh @h=0 per participant • 70% higher than SPS for central Au+Au • 40% higher than p+p at RHIC energy • Npart evolution between HIJING and EKRT • dNch/dh in 4-p • <Nch> = 4100 +/- 410 (|h|< 5.4) for 3% central • Additional particle production near h=0 for central events • Wider than HIJING • Cross-over in dNch/dh per Npart near h = 4 • Elliptic flow • V2 up to 0.06 – close to hydrodynamic limit • larger than SPS • V2 drops for |h| > 1.5 • Particle ratios • p/p ratio 0.55 +/- 0.1 • between HIJING and RQMD • m B ~ 50 MeV vs 200 MeV at SPS

  28. Outlook I: 2001 • 100x statistics • Both arms completed • Physics: • low-pT physics • Spectra • HBT • Resonances (f at low pT) • Event-by-Event physics • Energy systematics • Species systematics

  29. Outlook II: 2003 + beyond Q: Charm Production at RHIC? A: Measure single electrons from displaced vertices EM-Calorimeter Transition Radiation Detector • Existing Spectrometer • High rate (> 0.5 kHz) • High Resolution • Add • Micro-Vertex Detector • ALICE prototype TRD Electron-ID • EM-Calorimeter Micro-Vertex

  30. pT Dependence Consistent with flat pT dependence Same as SPS data, HIJING prediction

  31. Tracklets are 3-point tracks Vertex + 2 planes in SPEC, VTX Quality controlled by cuts in Pseudorapidity residual: dh Azimuthal angle residual: df Good efficiency Reduced sensitivity Non-vertex backgrounds Strangeness / gamma conversion Tracklets to measure dN/dh

  32. Outlook II: 2003 + beyond

  33. Estimate of Npart • Combining all sources of statistical and systematic error, we measure: • At lower energy, larger relative fluctuations in paddles depletes most central bin

  34. Results Energy sNN = 56 GeV sNN = 130 GeV Measurable dN/dh | |h|<1 408 ±12(stat)±30(syst) 555 ±12(stat) ±35(syst) dN/dh | |h|<1 per participant pair 2.47 ±0.10±0.25 3.24 ±0.10±0.25 Ratio (density per participant pair) 1.31 ±0.04±0.05

  35. Centrality: Participants vs. Spectators The collision geometry (i.e. the impact parameter) determines the number of nucleons that participate in the collision “Spectators” Only ZDCs measure Npart Zero-degreeCalorimeter “Spectators” Many things scale with Npart: • Transverse Energy • Particle Multiplicity • Particle Spectra “Participants” Detectors at 90o

  36. Data-taking in RHIC Year-1 “Comissioning” Run • sNN = 56 GeV • sNN = 130 GeV CR00 “Physics” Run • sNN = 130 GeV PR00

  37. Measurement sensitive to trigger bias “Minimum-bias” still has bias Affects most peripheral events Uncertainty on Npart % Error on Npart This measurement Npart • Estimating 96% when really 90% overestimates Npart • Affects measurement of Npart more in peripheral events than in central

  38. Physics Goal of RHIC & Flow b (reaction plane) Initial state anisotropy Flow Equation of state Flow strength Directed flow Elliptic flow dN/d(f -YR ) = N0 (1 + 2V1cos (f-YR) + 2V2cos (2(f-YR) + ... )

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