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Pradeep Sarin For the PHOBOS Collaboration May 25, 2002

Charged particle multiplicity distributions in Au+Au collisions up to 200 GeV from the PHOBOS detector at RHIC. Charged particle multiplicity distributions in Au+Au collisions up to 200 GeV from the PHOBOS detector at RHIC. Pradeep Sarin For the PHOBOS Collaboration May 25, 2002

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Pradeep Sarin For the PHOBOS Collaboration May 25, 2002

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  1. Charged particle multiplicity distributions in Au+Au collisions up to 200 GeV from the PHOBOS detector at RHIC Charged particle multiplicity distributions in Au+Au collisions up to 200 GeV from the PHOBOS detector at RHIC Pradeep Sarin For the PHOBOS CollaborationMay 25, 2002 Meeting of the Division of Particles and Fields, American Physical Society

  2. The PHOBOS Collaboration ARGONNE NATIONAL LABORATORYBirger Back, Alan Wuosmaa BROOKHAVEN NATIONAL LABORATORY Mark Baker, Donald Barton, Alan Carroll, Nigel George, Stephen Gushue, George Heintzelman, Burt Holzman, Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov INSTITUTE OF NUCLEAR PHYSICS, KRAKOWAndrzej Budzanowski, Roman Hołyński, Jerzy Michałowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Woźniak MASSACHUSETTS INSTITUTE OF TECHNOLOGYMaartin Ballintijn, Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane, Judith Katzy, Piotr Kulinich, Jang Woo Lee, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Carla Vale, Gerrit van Nieuwenhuizen, Gábor Veres, Robin Verdier, Bernard Wadsworth, Bolek Wysłouch NATIONAL CENTRAL UNIVERSITY, TAIWANChia Ming Kuo, Willis Lin, Jaw-Luen Tang UNIVERSITY OF ILLINOIS AT CHICAGORussell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer UNIVERSITY OF MARYLANDAbigail Bickley, Richard Bindel, Alice Mignerey, Marguerite Belt Jones UNIVERSITY OF ROCHESTERJoshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs 70 Collaborators from 3 countries

  3. Probing the phase diagram of QCD..or..Why collide heavy ions? VAPOR QGP A question of scale! ~1012 K WATER ICE NUCLEI Melt IceBoil WaterVapor Do this at many pressures.. … Thomson, Kelvin, Carnot (1840-1860)Understand phase diagram of water… Theories of Condensed Matter Collide nucleiMake QGP Do this at many energies.. … AGS, SPS, RHIC (1987-present)Understand QCD phase diagram … Theories of Matter

  4. Relativistic Heavy Ion Collider (RHIC) • Luminosities: ~1026 cm-2 s-1 for Au+Au ~1032 cm-2 s-1 for p+p • 4 Experiments to observe heavy ion collisions • 2 rings, 3.83 km circumference • Can collide any two nuclear species • .. Up to a CMS energy = 200 GeV for Au+Au = 500 GeV for p+p

  5. Melting the QCD Vacuum VNI Simulations: Geiger, Longacre, Srivastava, nucl-th/9806102 Animation: J. Mitchell (UW, Seattle) Parton Cascade HardCollisions Freezeout Colliding nuclei

  6. Why count charged particles? • Charged-particle production in heavy-ion collisions: • Geometry of collision: impact parameter, participants/spectators • Entropy production: Au+Au1000’s of particles Where does it come from? Initial, partonic or hadronization stage? What are the mechanisms? Binary collisions, gluon saturation, fragmentation? • Lots of existing data for pp, pA, AB, AA collisions at different energies • How do RHIC data fit into this picture?

  7. PHOBOS Experimental Setup Nominal Interaction Point • 137,000 Silicon Readout Channels • 1,300 Scintillator Readout Channels

  8. Triggering on Collisions & Classifying them tn tP x ZDC N ZDC P Au Au z PN PP Paddle Counter - Coincidence between Paddle counters near |tp-tn| = 0 defines a collision - Paddle + ZDC timing reject background - Sensitive to 97% of inelastic cross section for Au+Au at sNN = 200 GeV • Impact parameter b not accessible • Classify events by mean number of participants <Npart> • Compare Paddle detector signal response to Monte Carlo simulations to estimate <Npart> ….. Geometry! (Glauber model)

  9. 4 Silicon Multiplicity Detector f z or h Single Event Display 12m of Be beam-pipe Octagon detector module4 x 32 pads each Ring detector module8 x 8 pads each

  10. Choosing phase space variables.. A+BC+X All CMS Energies Fragmentation Region Fragmentation Region -1 0 1 xF~0.1 ~(ybeam-2) -ybeam ybeam ~4.8 (sNN = 130GeV) 0 -ybeam ybeam ~5.4 (sNN = 200GeV) 0 Experimentally, easier to measure! C X  B A

  11. 5 4 3 2 1 0 1 2 3 4 5 h Coverage of Detectors 5m 3.2m 2m 1m Octagon Rings Paddle Counters For vertex at Z=0. h

  12. Measuring dNch/dh - Overview dNch = dh Single Event DisplayEnergy deposition in multiplicity detectors for 1 event. f z or h Count hit pads N(h,b) binned in h, centrality (b) Correct for detector Acceptance A(h,ZVTX) Correct for the occupancy per hit pad O(h,b) Fold in a background correction factor fB(h,b) N(h,b)×O(h,b)× fB(h,b) A(h,ZVTX)

  13. 0-3% (central) Octagon Rings O(h,b) O(h,b) 50-55% (peripheral) m h f Occupancy Correction O(h,b) N = number of tracks/pad m= mean number of tracks/pad f z or h z

  14. 1.0 0.8 0.6 fB(h,b) 0.4 0.2 h -6 -6 -4 -4 -2 -2 0 0 2 2 4 4 6 6 MC, reconstructed Output Background Correction 600 dNch/dh 400 fB=MCTruth/MCOccCorr MC Input(primaries) 200 h Compare PHOBOS Monte Carlo input to reconstructed output the ratio gives corrections for background hits fromsecondaries generated in thebeampipe and decay particles.

  15. 200 GeV 130 GeV Putting it all together… Au+Au at 200 GeV : Preliminary AuAu at 130 GeV : PHOBOS PRL 87, 102303 (2001)  Typical systematic errors 130 GeV: <Nch>=4100 ± 210 200 GeV: <Nch>= 4960 ± 250

  16. T P T Results & Conclusions - I pp Central AA 900 GeV 545 GeV 200 GeV 53 GeV Fragmentation Fragmentation UA5, Z.Phys.C33, 1 (1986) Systematic errors not shown SPS data: PRC 62, 014903 (2000) • Limiting Fragmentation in AA collisions!“At high energies the number of particles produced by the fragmenting target is independent of the target, projectile and beam energy”-Benecke et al, PRC 188 (1969) 2159 • Chou-Yang geometrical model of hadronic collisions The extent of the limiting curve grows with increasing energy

  17. Central AA Peripheral AA 200 GeV 200 GeV 130 GeV 130 GeV Fragmentation Fragmentation xF Fragmentation Region Fragmentation Region -1 +1 0 ybeam 200 GeV Data 130 GeV Data -ybeam 0 ~ (ybeam-2) Results & Conclusions - II Expect Feynman scalingand limiting fragmentation to work here.. .. It does! • The extent of the limiting curve is independent of centrality of collision • The shape in this region depends on centrality: in the far-forward  region, the yield is larger for peripheral collisions!

  18. Results & Conclusions - III Charged particle multiplicities at mid-rapidity, Centrality Dependence • PHOBOS measurements at 2 energies • Pictures of initial state: Initial state gluon saturation or 2-component model: “hard” binary collisions of nucleons+“soft” color exchange among wounded nucleonsHard to discriminate… • In any case: very little left for final state entropy production! PHOBOS PRC, In Press (nucl-ex/0201005) X~0.09-0.11npp~2.2-2.4 Model curves: Kharzeev and Levin, Phys. Lett. B523, 79 (2001)

  19. Results & Conclusions – IV Charged particle multiplicities at mid-rapidity, Energy Dependence pp PHOBOS PRL 88, 22302 (2002) • Smooth ln(sNN ) evolution of mid-rapidity charged particle multiplicities • Yield at mid-rapidity is 55% higher than pp collisions at similar center-of-mass energies

  20. Summary • dNch/d in 4 • 200 GeV : <Nch> = 4960 +/- 250 (||< 5.4) for 6% central • 130 GeV : <Nch> = 4100 +/- 210 (||< 5.4) for 6% central • Evidence for limiting fragmentation in AA collisions  • dNch/d at mid-rapidity • Au+Au 55% higher yield than pp at maximum RHIC energy • Au+Au data at 200 & 130 GeV are in good agreement with initial state parton saturation model predictions and the two-component fits to the data (hard + soft) • Smooth evolution of yields as a function of ln(sNN)

  21. …Some extra slides…

  22. 12 8 4 0 6 0 -6 -4 2 -2 4 1 – Counting Hit Pads N(h,b) • Merge energies shared in adjacent pads DE2 DE1 + + DE3 DE (“MIP”) • Reject pads hit by background • particles h Not fromvertex Si From vertex

  23. 2 – Calculating Acceptance A(ZVTX) • Simple ray-tracing: 3-vectors drawn from a given ZVTX to random values of |h|<6,0<f<2p. • For a given [h, ZVTX ], the ratio of numbers vectors that hit an active pad to total thrown gives the acceptance A(ZVTX)

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