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Results from the PHOBOS experiment at RHIC

Results from the PHOBOS experiment at RHIC. Steve Manly (Univ. of Rochester) 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 Steve Manly (Univ. of Rochester) 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 April 2001

  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

  4. The first look - 2000 run • Energy density and Entropy Production • Thermal Equilibration • Hadro-Chemistry Charged Particle Density Event Anisotropy - Flow Particle Ratios • Energy dependence (vs. AGS/SPS data) • System size (p+p, Npart dependence ) • Angular dependence ( and )

  5. Results to date - I Versus energy Central, =0, sNN = 56 and 130 GeV PRL 85 (2000) 3100 Versus centrality Varying centrality, =0,  sNN = 130 GeV QM2001, to be submitted soon dN/d Versus angle (and centrality) Varying centrality, ||<5.4,  sNN = 130 GeV QM2001, to be submitted soon

  6. Results to date - II p/p, K-/K+, -/+ ratios, central,  sNN = 130 GeV QM2001, Submitted to PRL hep-ex/0104032 Elliptic flow,  sNN = 130 GeV, as function of centrality and ||<5.3 QM2001, to be submitted soon

  7. 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 sNN = 130 GeV

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

  9. Si Si +z cm Vertex Determination with vertex detector counts Tracklets cm

  10. form 3D vertex z p - p- 100 MeV/c p+ Vertex determination with spectrometer arms tracklets

  11. dNch/dh measurements • What is the density of particles near h=0? • How does it compare to p+p and A+A @ SPS? • How does it vary with centrality? • How does the density of particles vary with angle? • Total multiplicity sNN = 56 GeV sNN = 130 GeV • Energy density • Entropy production • Relative importance of hard and soft production processes sNN = 130 GeV

  12. Tracklets and dNch/dh @ h=0 • Hundreds of tracklets per central event • Corrections • Background subtraction • Uncertainty due to model differences • Feed-down from strange decays See talk by Mike Reuter in Session J12 (Sunday 15:18)

  13. dNch/dh @ h=0 vs Energy PRL 85 (2000) 3100

  14. 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

  15. dNch/dh for ||<5.4 • |h| < 5.3 (Dh = 0.05-0.1), 0f2p (Df = 2p/32 -2p/64) 5.0m 1.1m 2.3m octagon Ring counter Interaction Point -1.1m -5.0m -2.3m

  16. RingsN Octagon RingsP “Unroll” the octagon and rings f h • Count hits • remove much background by demanding energy deposition consistency with angle • Occupancy per hit pad determined as fn of  via number of empty and hit pads • Corrections • residual background corrected via MC simulations See talk by Carla Vale in Session J12 (14:30, Sunday afternoon

  17. dNch/dh 25-35% 45-55% 35-45% dNch/dh 6-15% 15-25% 0-6% h h h dNch/dh vs Centrality Preliminary The width of the distribution changes with centrality Statistical errors only - 10% systematical uncertainty

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

  19. Determine ratio of p-/p+, K-/K+, p/p • Compare to AGS/SPS results Particle ratios: Hadro-chemistry • Baryo-Chemical Potential • Baryon Stopping See talk by Conor Henderson - session Q12 (11 am Monday)

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

  21. Results for ratios • Higher values of K-/K+ and p/p than at lower energies • Results consistent with B=45±5 MeV, which is much lower that that observed at SPS (~240-270 MeV) Assumes freezeout temp ~170 MeV in statistical model of Redlich (QM01)

  22. b (reaction plane) Elliptic flow Determine to what extent is the initial state spatial/momentum anisotropy preserved in the final state. dN/d(f -YR ) = N0 (1 + 2V1cos (f-YR) + 2V2cos (2(f-YR) + ... ) • Sensitive to the initial equation of state and the degree of thermalization. • Affects other variables, such as HBT and spectra.

  23. -2.0 < h < -0.1 0.1 < h < 2.0 Yna Ynb RingN RingP SubE (a) SubE (b) Elliptic Flow • Subevent technique: correlate reaction plane in one part of detector to  asymmetry in hit pattern in other part of detector • Correct for imperfect reaction plane resolution and hit saturation • (formalism given in A. M. Poskanzer,S. A. Voloshin Phys. Rev. C 58, 1671)

  24. 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, closer to hydrodynamic limit implying substantial thermalization

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

  26. Conclusions from year 1 • dNch/dh @h=0 per participant • Substantially higher than SPS (Pb-Pb) and p+p • Npart evolution between HIJING and EKRT • dNch/dh in 4-p • Additional particle production near h=0 for central events • Distribution wider than HIJING • Elliptic flow • V2– large, close to hydrodynamic limit • drops for |h| > 1.5 • Particle ratios • m B ~ 45 MeV vs 270 MeV at SPS

  27. Expectations for year 2 (starts mid-June!) • 100x statistics • Both arms completed • Physics: • low-pT physics • Spectra • HBT • Resonances (f at low pT) • Event-by-Event physics • Energy systematics • [Species systematics if enough running time]

  28. BACKUP SLIDES FOLLOW

  29. 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

  30. Spectrometer : Vertex Reconstruction form 3D vertex z

  31. Event Selection Rings N vertex available Rings P f -56cm -14cm z Octagon • To cover pseudo-rapidity -2.0 to 2.0, only events with vertex -38cm to -30 cm are used • Rings will cover 3.0 < |h| < 5.3 • 13K events are used finally for the analysis

  32. -2.0 < h < -0.1 0.1 < h < 2.0 Yna Ynb RingN RingP SubE (a) SubE (b) Flow Analysis* (Subevent correlation) • If we know the reaction plane perfectly: Vn = < cos (n(f-YR)) > • In real experiment, YR is unknown: use Yn • Vnobs = < cos (n(f-Yn)) > • <cos(n(Yna,b-YR))> = ( <cos(n(Yna- Ynb))> )1/2 • Finally, correct for event plane resolution • Vn= Vnobs / < cos (n(Yn -YR)) > * Phys. Rev. C 58, 1671 A. M. Poskanzer, S. A. Voloshin

  33. Subevent Plane Correlation Normalized Paddle Signal

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

  35. 12 12 DE (“MIP”) 8 8 4 4 0 0 6 6 0 0 -6 -6 -4 -4 2 2 4 4 -2 -2 Discriminating background with DE Monte Carlo Data DE (“MIP”) h h Not from vertex Si From vertex DE vs. h in the Octagon

  36. EM-Calorimeter Transition Radiation Detector Micro-Vertex Future plans: 2003 + beyond Discussing upgrade to focus on charm production at RHIC. Measure single electrons from displaced vertices • Existing Spectrometer • High rate (> 0.5 kHz) • High Resolution • Add • Micro-Vertex Detector • ALICE prototype TRD Electron-ID • EM-Calorimeter

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