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Multiplicity Measurements with the PHOBOS Detector for the PHOBOS Collaboration Winter Workshop

This talk provides an overview of the multiplicity measurements conducted using the PHOBOS detector at the 18th Winter Workshop on Nuclear Dynamics. It covers the techniques employed for multiplicity measurements, centrality determination, and the results obtained. The talk also offers a glimpse into future research prospects.

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Multiplicity Measurements with the PHOBOS Detector for the PHOBOS Collaboration Winter Workshop

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  1. Multiplicity Measurements withThe PHOBOS Detector Russell Betts (UIC) for the PHOBOS Collaboration 18th Winter Workshop on Nuclear Dynamics Nassau, Jan 20th-27th,2002

  2. The PHOBOS Collaboration Birger Back, Nigel George, Alan Wuosmaa Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Stephen Gushue, George Heintzelman, 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 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. Completed Spring 2001 • 4p Multiplicity Array • - Octagon, Vertex & Ring Counters • Two Mid-rapidity Spectrometers • TOF wall for High-Momentum PID • Triggering • Scintillator Paddles • Zero Degree Calorimeter 137000 Silicon Pad channels

  4. Outline of Talk • Centrality Determination Nparticipant and Ncollision • Techniques for Multiplicity Measurements • Tracklets • Hit Counting • Energy Deposition • Results • Energy Dependence for  1 • Centrality Dependence • dN/d Shapes • Summary and Taster of Future Delights

  5. x z Events Dt (ns) Triggering on Collisions Positive Paddles Negative Paddles ZDC N ZDC P Au Au PN PP Paddle Counter ZDC Counter Valid Collision • Coincidence between Paddle counters at Dt = 0 defines a valid collision. • Paddle + ZDC timing reject background. • Sensitive to 97±3 % of inelastic cross section for Au+Au.

  6. Trigger Selection - ZDC vs Paddles b Central b Peripheral

  7. Determining Centrality Counts • HIJING + GEANT • Glauber Calculation • Model of Paddle Response Paddle signal (a.u.) Counts Npart

  8. Uncertainty on Npart • Measurement sensitive to trigger bias • “Minimum-bias” still has bias • Affects most peripheral events Counts • Estimating 97% when really 94% overestimates Npart Paddle signal (a.u.)

  9. Energy Spectrum (DE) in Si pads 1 hit Data MC 2 hits Multiplicity Distributions Hits in One Layer of Silicon Rings Vertex Octagon

  10. Au+Au Collision Event Display

  11. Event Vertex Finding +z • Vertex Resolution: • sx ~ 450 mm • sy ~ sz ~ 200 mm

  12. Vertex Tracklet Reconstruction Tracklets are two point tracks that are constrained by the event vertex. dh = h1 – h2 df = f1 – f2 |dh| < 0.04 |df| < 0.3

  13. Combinatorial Background All Pairs of Hits “Background Flip” Outer Hit Bin 10 (Data)

  14. Backgrounds Weak Decays d Electrons

  15. Vertex Tracklet Systematic Error • Reconstruction: Vertex selection, Tracklet algorithm etc. • 1.8% • Weak Decays: Mostly Ks and L - 2% • Background: Combinatorial, d-electrons - 1.5% • MC Generators: Different particle production, background etc. • - 5% • Total:7.5%

  16. -3 0 +3 -5.5 +5.5 Analog and Digital Hit-Counting f h Octagon, Ring and Vertex Detectors (unrolled) Count Hits or Deposited Energy

  17. 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 DE vs. h in the Octagon From vertex

  18. dNch O(h,b) ×fB(h,b) Shits = dh A(ZVTX) DE deposition in multiplicity detectors for 1 event. f h • Count hits binned in h, centrality (b) • Calculate acceptance A(ZVTX)for that event • Find the occupancy per hit pad O(h,b) • Fold in a background correction factor fB(h,b)

  19. “Measuring” the Occupancy Method: Assume Poisson statistics 0-3% Octagon (central) Rings Ntracks/hit pad N=number of tracks/pad m =mean number of tracks/pad 50-55% (peripheral) h The numbers of empty, and occupied, pads determine the occupancy as a function of h,b

  20. Estimating remaining backgrounds 1.0 0.8 0.6 0.4 0.2 -6 -6 -4 -4 -2 -2 0 0 2 2 4 4 6 6 MC, Occupancy Corrected 600 dNch/dh 400 MC “truth” fB=MCTruth/MCOcc 200 h Compare PHOBOS Monte Carlo “data” analyzed using occupancy corrections to “truth” - the difference gives corrections for remaining background. fB(h,b) h

  21. Energy Loss  Multiplicity 300 mm Si Energy deposited in ith pad (truncated) corrected for angle of incidence Mean energy loss for one particle traversing pad RATIO OF TOTAL TRACKS TO PRIMARY TRACKS 0.30 - 0.40 • Measured S/N = 10 - 20 << Landau Width • Use Non-Hit pads - for • Common-Mode Noise Suppression • M = 240 ± 15 ± 5 ± CMN for one sensor (120 channels) at h = 0

  22. Uncertainty in Theoretical Predictions

  23. Constraining the Models

  24. Ratio 200/130 GeV Ratio 200/130 averaged for four PHOBOS methods Phobos Measurement R200/130 = 1.14 +/- 0.05 Moderate Increase in Energy Density? Systematic Uncertainty

  25. Hard and Soft Processes • Soft processes (pT < 1 GeV) • Color exchange excites baryons • Baryons decay to soft particles • Varies with number of struck nucleons • “Wounded Nucleon Model” • Hard processes (pT > 1 GeV) • Gluon exchange in a binary collision creates jets • Jets fragment into hadrons, dominantly at mid-rapidity (mini)jet (mini)jet

  26. Multiple Collisions with Nuclei pp collisions pA collisions Spectators Participants Spectators • Nuclei are extended • RAu ~ 6.4 fm (10-15 m) • cf. Rp ~ .8 fm • Geometrical model • Binary collisions (Ncoll) • Participants (Npart) • Nucleons that interact inelastically • Spectators (2A – Npart) • p+A: Npart = Ncoll + 1 • (Npart ~ 6 for Au) • A+A: Ncoll Npart4/3 b 1200 Ncoll Npart 400 b(fm) 9 0 18

  27. Hard & Soft What about non-central events? We already expect that charged particle production can have two components: Fraction from hard processes proton-proton multiplicity We can tune the relative contribution by varying the collision centrality Is this Description unique ?

  28. Parton Saturation • Gluons below x~1/(2mR) overlap in transverse plane with size 1/Q • Gluons recombine at a critical density characterized by “saturation” scale Qs2 • Below this scale, the nucleus looks “black” to a probe t Scale depends on volume(controlled by centrality!) “Colored Glass Condensate” McLerran, Venugopalan, Kharzeev, Dumitru, Schaffner-Bielich…

  29. Data and Models for 130 GeV Yellow band: Systematic Error

  30. Data and Models for 200 GeV Yellow band: Systematic Error

  31. Shapes of dN/dh Distributions at 130 GeV - Hit Counting • Shapes only weakly dependent on centrality • Differ in details

  32. 130 GeV (0-6%) AMPT (35-45%) (p-p) HIJING Most of “new” behavior is at mid-rapidity – detailed comparison with pp and pA required.

  33. Energy Dependence and Comparison to pp • Width increases with Ecm • Increase D = Dybeam • Scaling in fragmentation region HI part. Production is increased at mid-rapidity 7-10% syst error 7-10% syst error

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

  35. Summary Energy and Centrality Dependence of Mid-Rapidity Multiplicity has Constrained Models and given Insight into Interplay of Different Processes Shapes of Multiplicity Distributions show Scaling in Fragmentation Region illustrating Common Mechanism for Particle Production which Evolves to Features Unique to HI Situation at Mid-Rapidity To Come: Shapes versus Centrality at 200 GeV Multiplicity at 20 GeV pp Data with PHOBOS at 200 GeV

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