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Results Summary of PHOBOS Experiment at RHIC

This overview presents the results from the PHOBOS experiment at RHIC in Krakow, Poland. The experiment includes the PHOBOS Collaboration, Argonne National Laboratory, Brookhaven National Laboratory, and more. The results cover charged particle density, particle ratios, elliptic flow, and centrality determination.

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Results Summary of PHOBOS Experiment at RHIC

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  1. Overview of Resultsfrom PHOBOS experiment at RHIC Andrzej Olszewski Institute of Nuclear Physics, Kraków, Poland for the PHOBOS Collaboration

  2. PHOBOS at RHIC

  3. PHOBOS Collaboration ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ROCHESTER UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND Birger Back, Nigel George, Alan Wuosmaa Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Stephen Gushue, Dale Hicks, Burt Holzman,Robert Pak, Marc Rafelski, Louis Remsberg, Peter Steinberg, Andrei Sukhanov Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Michel Rbeiz, 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 Chia Ming Kuo, Willis Lin, Jaw-Luen Tang Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly,Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer Richard Bindel, Alice Mignerey

  4. The PHOBOS Detector (2001) ZDC Paddle Trigger Counter Time of Flight Spectrometer Vertex Octagon Ring Counters Cerenkov y f x q z 1m • 4p Multiplicity Array - Octagon, Vertex & Ring Counters • Mid-rapidity Spectrometer • TOF wall for high-momentum PID • Triggering • Scintillator Paddles Counters • Zero Degree Calorimeter (ZDC) 137000 silicon pad readout channels

  5. Central Part of the Detector (not to scale) 0.5m

  6. PHOBOS in PHOTOS Octagon Detector ~ 25 cm Vertex Detector Ring Counter Silicon pad sizes Octagon Detector: 2.7 x 8.8 mm2 Vertex Detector: 0.5 x (12-24) mm2 Ring Counter: (5x5) - (10x10) mm2 Spectrometer: (1x1) - (0.5x19) mm2 Spectrometer

  7. PHOBOS Running Summary Year 2001 running Year 2000 running • Commissioning: (mid-July) • Add 2nd spectrometer arm • Au+Au collisions sNN = 130 GeV and 200 GeV First published result on dNch/d|||<1,sNN= 200 GeV • Physics run: (mid-August) • 2 spectrometer arm setup • Au+Au collisions at sNN = 200 GeV: ~3.5 M collisions by end of August • Commissioning:(May-July) • Part of silicon installed • Au+Au collisions at sNN = 56 GeV and 130 GeV First published results on dNch/d|||<1, sNN=56 and 130 GeV • Physics run: (July-August) • 1 spectrometer arm setup • Au+Au collisions at sNN = 130 GeV: ~3.5 M collisions total Essentially flawless performance of PHOBOS detector

  8. Results to Date _ Charged particle density • Versus energy Central collisions, 0: • sNN = 56 and 130 GeV PRL 85 (2000) 3100 • sNN = 200 GeV, submitted to PRL • Versus centrality: • sNN=130 GeV,0 submitted to PRC • Versus angle and centrality • sNN= 130 GeV, ||<5.4 PRL 97 (2001) 102303 Particle ratios p/p, K-/K+, -/+ Central collisions: •  sNN = 130 GeV PRL 97 (2001) 102301 Elliptic flow • Versus angle andcentrality • sNN= 130 GeV, ||<5.3 QM2001, to be submitted soon

  9. Triggering on Collisions Events Coincidence between Paddle counters at Dt = 0 defines a valid collision Paddle + ZDC timing reject background Sensitive to 97% of inelastic cross section for Au+Au at sNN = 130/200 GeV Dt (ns) Positive Paddles Negative Paddles ZDC N ZDC P Au Au Paddle Counter PN PP ZDCCounter Valid Collision

  10. Selecting Collision Centrality Data Counts Paddle signal (a.u.) 3<|h|<4.5 Larger signal = more central collision. PN PP Peripheral Collision:  Small number of participating nucleons Central Collision:  Large Npart b “side” view of colliding nuclei “side” view of colliding nuclei

  11. Centrality Determination Npart Multiplicity in Paddles HIJING + GEANT % Error on Npart Npart (3<||<4.5) Analysis is limited to events with Npart > 70 % (Paddle mult.) % (Npart)

  12. Charged Particle Density Study • Energy/entropy density production • Response to properties of nuclear/partonic medium • Saturation • Jet quenching • Importance of hard and soft processes • Re-scattering effects • Long-range particle correlations • Memory of the initial geometry in the final state Measurement Charged Particle Density Event Anisotropy - Flow Context Energy Dependence System Size Angular Dependence

  13. Tracklets • Tracklet: Two-hit combination + vertex position • >300 tracklets/central event in Vertex, >100 in Spectrometer Vertex detector

  14. Analog and Digital Hit-Counting f h -5.5 +5.5 -3 +3 0 Hits in Octagon, Ring and Vertex for single event Digital Count hits above energy threshold, assume Poisson- statistics in the distribution of hits among the pads Analog Use deposited energy (dE/dx) in each pad to estimate number of particles that crossed the pad

  15. Charged Particle Density Four counting methods • Tracking detectors - h 0 measurements • Tracklets in Spectrometer • Tracklets in Vertex detector • Single layer detectors - 4 measurements • Use deposited energy (dE/dx) in each Si-pad • Count hits above threshold, assume Poisson-statistics All four measurements corrected for: secondary particles, feed-down from weak decay, stopping particles Systematic uncertainty: from 4.5% (Tracklets in Spectrometer) to 10% (Hit counting) All four methods deliver consistent results - final results averaged

  16. Charged Particle Density at h0 PHOBOS first measurements - charged particle density - in mid-rapidity - for 6% of the most central events dNch/d|||<1(56 GeV) = 408  12(stat)  30(syst) dNch/d|||<1(130 GeV) = 555  12(stat)  35(syst) dNch/d|||<1(200 GeV) = 650  35(syst)

  17. dNch/d|||<1 vs Energy PHOBOS 200 _ RHIC 130 p+p PHOBOS 56 SPS nucl-ex/0108009 Submitted to PRL Preliminary AGS

  18. dNch/d|||<1: Ratio 200/130 GeV 90% confidence PHOBOS Measurement nucl-ex/0108009 New results for 200 GeV dNch/d|||<1 = 650  35 dNch/d|||<1/0.5Npart = 3.78  0.25 R200/130 = 1.14 +/- 0.05(sys)

  19. Multiplicity ath=0 vs Centrality Evolution with Npart • Scaled multiplicity increases with Npart • Similar to Kharzeev/Nardi dNch/d = a·Npart+b·Ncoll • Stronger than in EKRT • Less steep than in HIJING nucl-ex/0105011 sNN =130GeV

  20. Multiplicity in 4 - Centrality Dependence sNN = 130GeV PRL 97 (2001) 102303 The width of the distribution changes with centrality

  21. Multiplicity in 4 - Centrality Dependence PRL 97 (2001) 102303 (dNch/d)/(0.5Npart) Total Nch(||  5.4) central(0-6%) central(0-6%) peripheral(35-45%) • 3% most central collisions <Nch> = 4200  470 • Additional particle production near =0

  22. Change in dN/dh with Energy 200 GeV - 6% • First attempt to compare dN/d shape for Au+Au at 130 and 200 GeV • ‘Limiting fragmentation’ check by plotting dN/d with  - Ybeam • Agreement for AA in the fragmentation region • Different slope when compared to pp 130 GeV - 6% UA5 200 GeV (NSD) Systematic errors not shown

  23. Azimuthal Angular Distributions “head on” view of colliding nuclei YRreaction plane b V2 determines to what extent the initial state spatial/momentum anisotropy is preserved in the final state. dN/d(f -YR ) = N0 (1 + 2V1cos (f-YR) + 2V2cos (2(f-YR)) + ... )

  24. Centrality Dependence of V2 b Peripheral Collisions Central Collisions b Preliminary |h| < 1.0 sNN=130GeV V2 SPS 17 GeV PHOBOS Systematic error ~ 0.007 Normalized Paddle Signal (STAR : Normalized Nch ) Anisotropy increases for peripheral collisions Large V2 signal compared to lower energy

  25. V2 (elliptical flow) vs h sNN= 130 GeV PHOBOS Preliminary V2 STAR (PRL) All Charged PHOBOS Systematic error ~ 0.007 h Averaged over centrality V2 drops for |h| > 1.5

  26. Microscopic viewpoint: Antiproton/proton ratio determined by: Baryon stopping; Pair production; Absorption in nuclear medium Why Measure Antiparticle/Particle Ratios? Net Baryon Number y • Thermodynamic viewpoint: Particle ratios can be used to estimate hadro-chemical potentials

  27. Anti-particle / particle Ratios p 70 cm K+ p+ p K- Tracking in the spectrometer Alternate 2T magnetic fields Energy loss and momentum p-

  28. Results for Ratios PRL 97 (2001) 102301 K-/K+ vs Energy p/p vs Energy p-/p+ = 1.00 ±0.01 (stat) ± 0.02 (syst) K-/K+ = 0.91 ± 0.07 (stat) ± 0.06 (syst) p/p = 0.60 ± 0.04 (stat) ± 0.06 (syst) • Higher values of K-/K+ and p/p than at lower energies

  29. Results for Ratios PRL 97 (2001) 102301 • Results consistent with B=45±5 MeV, which is much lower than that observed at SPS (~240-270 MeV) Assumes freezeout temp ~170 MeV in statistical model of Redlich (QM01)

  30. Summary 1 Charged Particle Densities (Entropy) • dNch/dh at h0 per participant • First look at Au+Au at 200 GeV - increase in density by 14% compared to 130GeV • Logarithmic increase with energy from AGS to RHIC • Npart evolution stronger then linear, indicates increasing contributions from hard processes • dNch/dh in 4p • Additional particle production concentrated near h0 for central events • Decreasing width with increasing centrality • On average 4200 particles in central collisions at 130GeV

  31. Summary 2 • Elliptic flow • Increase of elliptic flow (V2) for more peripherial events • Increase of flow effect with increasing energy • V2 at mid-rapidity up to 0.06 • V2 drops for |h| > 1.5 • Particle ratios • K-/K+ and p/p significantly higher than at AGS or SPS m B ~ 45 MeV vs 270 MeV at SPS • p/p between HIJING and RQMD predictions • Central region closer to baryon free state

  32. Outlook: Year 2001 • 100x statistics • Physics: • low-pT physics • Spectra • HBT • Resonances (f at low pT) • Event-by-Event physics • Energy systematics • Species systematics

  33. The End

  34. Silicon Signals 200 vs 130 GeV Signal shapes are (almost) identical: Methods developed for 130 GeV good also at 200 GeV

  35. dNch/d at mid-rapidity: ||<1 Tracklet counting method (2 Si layers) Tracklets  three-point tracks two-hit combinations+measured event vertex • x ~ 450 m • y ~ 200 m • z ~ 200 m The measurements in the two different Si pad detectors (different location, granularity, acceptance, systematics) Spectrometer Vertex Detector D = (2 + 2) 1/2 < 0.015 || < 0.04 , || < 0.3

  36. Tracklet analysis (Zvtx, Nhits) = TOTAL systematic errors: 4.5% (Spectrometer) 7.5% (Vertex) FINAL RESULTS Combined Vertex and Spectrometer measurements (weighted by the inverse of their total systematic error)

  37. Hit Counting method • Count hits in (,Npart) bins • Evaluate number of particles per hit pad * Assume Poisson statistics P(N)=Ne-/N!  - determined by the measured ratio (p) of occupied to empty pads:  = ln(1+p) * Perform a multi-Landau fits to E (,Npart) spectra (convoluted with gaussian) • Calculate acceptance • Fold in a final background correction (MC) FBkg(,Npart) =(dNch/d)MCTruth/(dNch/d)MCReconstructed Nhits(,Npart) Ntr/hit(,Npart) A(,Zvtx) FBkg(,Npart)

  38. Analog method • Get energy deposited in each Si pad E(,Npart) • Divide this energy by the average energy per track<E()> • Correct for the fraction of primaries fprim() <E> and fprim are obtained from HIJING+GEANT simulations Hit Counting and Analog methods agree to within 5% (systematic errors in each method are 10%)

  39. Secondary particles (+2%) Little material between interaction point and sensitive volume Antiproton absorption in detector (+8%) From GEANT simulations Feed-down from weak decays (-2%) Reduced by tracking within 10cm of vertex Further limit by distance-of-closest-approach cut on tracks Corrections to the Raw Numbers

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