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Rachid Nouicer University of Illinois at Chicago for the Collaboration

The Latest Results from PHOBOS. Charged-Particle Multiplicity and Particle Ratios in Au + Au Collisions at RHIC Energies Using the PHOBOS Detector. Rachid Nouicer University of Illinois at Chicago for the Collaboration. The PHOBOS Collaboration. ~ 70 collaborators.

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Rachid Nouicer University of Illinois at Chicago for the Collaboration

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  1. The Latest Results from PHOBOS Charged-Particle Multiplicity and Particle Ratiosin Au + Au Collisions at RHIC EnergiesUsing the PHOBOS Detector Rachid Nouicer University of Illinois at Chicago for the Collaboration

  2. The PHOBOS Collaboration ~ 70 collaborators 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

  3. PHOBOS Apparatus • Completed Spring 2001 • 4p Multiplicity Array • - Octagon, Vertex & Ring Counters • Two Mid-rapidity Spectrometers • TOF wall for High-Momentum PID • Triggering • Scintillator Paddles + Cerenkov Counters • Zero Degree Calorimeter 135000 Silicon Pad channels

  4. Single Event in PHOBOS Collisions at 200 GeV!

  5. PHOBOS Silicon Detector Octagon Detector 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. Silicon Detector Performance p dE/dx resolution = 6.9% • Signal is well reproduced by MC • Signal/Noise ~ 14 to 20 • Particles are well identified by • spectrometer within 2T magnetic field K d p

  7. x z Triggering on Collisions & Centrality Positive Paddles Negative Paddles NegativeCerenkov PositiveCerenkov Negative ZDC Positive ZDC Au Au PN PP Peripheral Measurement 200 GeV Data • Coincidence between Paddle counters at Dt = 0 defines a valid collision • Paddle + ZDC timing reject background • Trigger sensitive to 97% of inelastic cross section ZDC Signal (a.u.) Nspectator=2A-Npart Central Paddle signal (a.u.) Npart

  8. f h Multiplicity Measurements in 4p -5.4< h <5.4 Single-event display |h| <1 Vertex “tracklets” – 3 point tracks

  9. Results : Charged Particle Multiplicity near Mid-rapidity vs Energy • no unexpected increase • 200 GeV AuAu 55% higher than pp • dN/dh = 650 +/- 35 • for AuAu at 200 GeV (6% central) Central PHOBOS : AuAu at RHIC 56 & 130 GeV : PRL 85, 3100(2000) 200 GeV : PRL 88, 22302(2002) Extrapolation to LHC :E(LHC)=30 E(RHIC) - A + A Fit : Model prediction : - KN Saturation Model : hep-ph/0111315

  10. Results Centrality Dependence & Comparison to Models PHOBOS : AuAu at RHIC PHOBOS : 130 GeV : PRC 65, 031901(2002) PHOBOS : 200 GeV submitted to PRC Saturation Model : nucl-th/0108006 |h| < 1 Two-component Fit : dN/dh= (1-x)nppNpart/2 + xnppNcoll “soft” “hard” Peripheral Central

  11. Results : Charged Particle Production vs h Au+Au at 200 GeV : latest results AuAu at 130 GeV : PHOBOS PRL 87, 102303 (2001)  Systematic Errors 200 GeV 130 GeV dN/dh dN/dh dN/dh Peripheral h h h dN/dh dN/dh dN/dh Central h h h

  12. Results : Energy Dependence & Comparison to pp A+A Au+Au & pp at 200 GeV HI Particle Production is Increased at Mid-Rapidity Width Increases with Ecm Most central RHIC : PHOBOS AuAu s = 200 GeV RHIC : PHOBOS AuAu s = 130 GeV SPS : EMU-13 PbPb s = 17 GeV Systematic errors not included

  13. Results : Limiting Fragmentation AuAu and pp show the same behavior, the extent of the fragmentation region grows with collision energy p + p A+A UA5, Z.Phys.C33, 1 (1986) Fragmentation Fragmentation Systematic errors not included

  14. Results : Limiting Fragmentation vs Centrality • The extent of the limiting fragmentation region is independent of centrality • but the shape in this region depends on centrality ! 200 GeV 200 GeV 130 GeV 130 GeV Fragmentation Fragmentation 200 GeV Data 130 GeV Data

  15. + Anti-particle / particle ratios Positive Charge p K+ p+ Negative Charge p • Tracking in the spectrometer • Alternate 2T magnetic fields • Energy loss and momentum K- p-

  16. Results : Anti-particle/particle ratios Centrality =12 % <Npart> ~ 310 Central region not Baryon-Free PHOBOS : Phys.Rev.Lett. 87, 102301(2001) K. Redlich, Quark Matter 2001 T= 170 MeV AuAu130 GeV Stat.Syst.

  17. Results : Compare to Lower Energy Antiproton/proton ratio Increasing towards unity (baryon free) PHOBOS PHOBOS : 130 GeV Phys. Rev. Lett. 87, 102301 (2001)

  18. Summary • dNch/dh @ |h|<1 per participant • AuAu 55% higher yield than pp at maximum RHIC energy • Npart >100 the R200/130 approximately constant at 1.14 +/- 0.01+/- 0.06 • AuAu data at 200 & 130 GeV are in good agreement with parton saturation model predictions and the two-component fits to the data (hard + soft) • Using the two-component fits the hard processes increase by 34 % and the soft by 7 % from 130 to 200 GeV in AuAu • dNch/dh in 4p • 200 GeV : <Nch> = 4960 +/- 250 (|h|< 5.4) for 6% central • Additional particle production near |h|< 1 for central events • Central AuAu has very similar shape to pp • Extent of fragmentation region grows with collision energy • Particle ratios • p/p ratio 0.60 +/- 0.04(stat) +/- 0.06(syst) • Central region not yet baryon free

  19. Some references • PHOBOS web-site: www.phobos.bnl.gov • Physics Results • Charged particle multiplicity near mid-rapidity in central Au+Au collisions at 56 and 130 GeV Phys. Rev. Lett. 85, 3100 (2000) • Ratios of charged antiparticles-to-particles near mid-rapidity in Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102301 (2001) • Charged-particle pseudorapidity density distributions from Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102303 (2001)  • Energy dependence of particle multiplicities near mid-rapidity in central Au+Au collisions Phys. Rev. Lett 88, 22302(2002) • Centrality Dependence of Charged Particle Multiplicity at |h|<1 in Au+Au Collisions at 130 GeV Phys. Rev. C65, 031901(2002) • Centrality Dependence of Charged Particle Multiplicity at |h|<1 in Au+Au Collisions at 130 and 200 GeV Submitted to Phys. Rev. C (2002) • Technical • Silicon Pad Detectors for the PHOBOS Experiment at RHICNucl. Instr. Meth. A461, 143-149 (2001) • Array of Scintillator Counters for PHOBOS at RHICNucl. Instr. Meth. A474, 38-45 (2001)

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