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A first look at Au+Au collisions at RHIC using the PHOBOS detector

A first look at Au+Au collisions at RHIC using the PHOBOS detector. Birger Back Argonne National Laboratory for the PHOBOS Collaboration. PHOBOS Collaboration (Nov 2001). ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOW

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A first look at Au+Au collisions at RHIC using the PHOBOS detector

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  1. A first look at Au+Au collisions at RHIC using the PHOBOS detector Birger Back Argonne National Laboratory for the PHOBOS Collaboration

  2. PHOBOS Collaboration (Nov 2001) 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, Alan Wuosmaa Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Nigel George, 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 Abigail Bickley, Richard Bindel, Alice Mignerey

  3. Outline • Apparatus • Triggering – centrality determination • Charged particle multiplicity • Energy dependence at h=0 , h = - ln{tan(q/2)} • Centrality dependence at h=0 • Pseudo-rapidity distributions – widths • Total charged particle multiplicity • Antiparticle / particle ratios • Summary

  4. The full PHOBOS Detector Trigger Paddles Mid-rapidity Spectrometer Cerenkov TOF ~4p Multiplicity Array 135,000 Silicon Pad channels: spectrometer + multiplicity

  5. PHOBOS in PHOTOS Octagon Detector ~ 25 cm Vertex Detector Ring Counter Sample Silicon pad sizes Octagon Detector: 2.7 x 8.8 mm2 Vertex Detector: 0.4 x 12 mm2 Ring Counter: 20 -106mm2 Spectrometer: 1 x 1 mm2 Spectrometer

  6. Phobos acceptance (zvtx=0) Spectrometer Vertex Spectrometer Vertex Spectrometer

  7. Triggering on Collisions x z Events Dt (ns) Positive Paddles Negative Paddles ZDC N ZDC P Au Au Paddle Counter PN PP ZDCCounter Valid Collision 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 GeV

  8. Determination of Npart b b Measurement Simulation ~ Nspectator=2A-Npart Npart Peripheral ~Npart Central

  9. Multiplicity of charged particles • Physics motivation • Time integral of particle production during the collision • Total entropy production • Lots of existing data for pp, pA, AB,AA • How do RHIC data fit into this picture? • Phobos multiplicity data • 56 GeV dN/dh(0) • 130 GeVdN/dh(0), dN/dh(0, cent), dN/dh(h) • 200 GeVdN/dh(0), dN/dh(0, cent), dN/dh(h)

  10. Multiplicity Measurements in 4p f h -5.4< h <5.4 Single-event display h ~0 Vertex “tracklets” – 3 point tracks

  11. Multiplicity at h=0 vs Energy (0-6%) RESULTS SsdN/dh 56 GeV 408612630 130 GeV 555612635 200 GeV 650 635 Ss2dN/dh/Npart 56 GeV 2.476.106.25 130 GeV 3.246.106.25 200 GeV 3.78 6.25 R200/130=1.1460.05 130 GeV PRL 85 (2000) 3100 200 GeV PRL (Accepted)

  12. Comparison to model predictions Experimental Observed increase smaller than predicted by HIJING with Jet Quenching

  13. dN/dh|h=0 comparison of data Grey band: Systematic uncertainty Reasonable agreement between experiments Up to 50% larger than pp

  14. dN/dh|h=0 comparison to models Grey band: Systematic uncertainty HIJING: PRL 86, 3496 (2001) EKRT: hep-ph/0106330 KN scaling PLB 507,121 (2001) dN/dh=(1-x)nppNpart/2+xnppNcoll “soft” “hard” Energy npp x 130 2.23 0.0960.04 200 2.42 0.1160.04

  15. dNch/dh shapes @ 130 GeV Phys. Rev. Lett. 87, 102303 (2001) 

  16. Energy dependence and comparison to pp Width increases with Ecm Increase Dh = Dybeam Scaling in fragmentation region HI part. Production is increased at mid-rapidity 7-10% syst error 7-10% syst error

  17. Scaling in the fragmentation region UA5: Alner et al., Z. Phys. C33,1 (1986) PHOBOS 2000/2001 7-10% syst error Fragmentation Fragmentation

  18. Centrality Dependence of Nchtot (|h|<5.4) Peripheral Central Ntot=(1-x)nppNpart/2+xnppNcoll “soft” “hard” 200 GeV x=0.037 Nchtot scales with Npart Variance decr. with Npart HIJING too low PRL 87, 102303 (2001) 

  19. Comparison to models 130 GeV PRL 87 (2001) 102303 AMPT – hadronic re-scattering

  20. Anti-particle / particle ratios + p K+ p+ p K- Tracking in the spectrometer Alternate 2T magnetic fields Energy loss and momentum p-

  21. Anti-particle/particle ratios Centrality =12 % <Npart> ~ 310 Central region not baryon-free K. Redlich, Quark Matter 2001 Stat.Syst. Phys.Rev.Lett. 87(2001) 102301

  22. Compare to Lower Energy Antiproton/proton ratio Increasing towards unity (baryon free) PHOBOS Phys. Rev. Lett. 87, 102301 (2001)

  23. Summary • dNch/dh at h=0 per participant • ~55% higher than pp at RHIC energies • Npart evolution between HIJING and EKRT • 9-11% contribution from hard scattering at h=0 • 3.7% for Ntot • dNch/dh in 4p • 130 GeV: <Nch> = 4200 +/- 470 (|h|< 5.4) for 3% central • Additional particle production near h=0 for central events • Wider than HIJING – hadronic re-scattering (AMPT) • Particle ratios • p/p ratio 0.60 +/- 0.04(stat) +/- 0.06(syst) • Central region not yet baryon free – maybe at 200 GeV??

  24. Some references • PHOBOS web-site: www.phobos.bnl.gov • Physics Results • Charged-particle pseudorapidity density distributions from Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102303 (2001)  • Ratios of charged particles to antiparticles near mid-rapidity in Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102301 (2001) • Charged particle multiplicity near mid-rapidity in central Au+Au collisions at 56 and 130 GeV Phys. Rev. Lett. 85, 3100 (2000) • Energy dependence of particle multiplicities near mid-rapidity in central Au+Au collisions Accepted Phys. Rev. Lett (Aug 2001) • Centrality Dependence of Charged Particle Multiplicity at h=0 in Au+Au Collisions at 130 GeV Submitted to Phys. Rev. C (August 2001) • Technical • Array of Scintillator Counters for PHOBOS at RHICNucl. Instr. Meth. A474, 38-45 (2001) • Silicon Pad Detectors for the PHOBOS Experiment at RHICNucl. Instr. Meth. A461, 143-149 (2001) • Development of a double metal, AC-coupled silicon pad detectorThe silicon detector for the PHOBOS experiment at RHICNucl. Instr. Meth. A389, 415 (1997)

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