Recent Results from PHOBOS at RHIC Robert Pak

Recent Results from PHOBOS at RHIC Robert Pak Brookhaven National Laboratory for the PHOBOS Collaboration PANIC 2002 Osaka, Japan Sept. 30 th, 2002

Collaboration ARGONNE NATIONAL LABORATORYBirger Back, Alan Wuosmaa BROOKHAVEN NATIONAL LABORATORYMark 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 Mc. Leod, Rachid Nouicer, Michael Reuter, Joe Sagerer UNIVERSITY OF MARYLANDAbigail Bickley, Richard Bindel, Alice Mignerey, Marguerite Belt Tonjes UNIVERSITY OF ROCHESTERJoshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs Robert Pak

PHOBOS Apparatus p K+ + 4 Multiplicity Detector f 500 -5. 4 h Unrolled single event display 0 ke. V +5. 4 Robert Pak PID from Spec Arms

Kinematics: many ways to slice pz Rapidity: Generalized velocity Feynman x: scaled pz Pseudorapidity: ~ y: easier to measure Away from mid-rapidity: Robert Pak

Counts Centrality Determination more central Counts Paddle signal (a. u. ) • HIJING + GEANT • Glauber Calculation • Model Paddle Response Npart Robert Pak Number of participants is number of nucleons that interact

Particle Spectra 1/ (2 p. T)d 2 N/dydp. T ( ++ ) (K++K–) (p+p) 15% central PHOBOS preliminary 0. 2<y <1. 4 p. T [Ge. V/c] PHOBOS niche at low p Robert Pak

Why charged particle production? • Fundamental handle on the initial state • Upper bound on initial entropy density • Entropy does NOT require equilibrium • Initial gluon density • Constrains subsequent evolution • Freezeout entropy density Robert Pak

d. N/d Pseudorapidity Density Distributions 130 Ge. V 19. 6 Ge. V 200 Ge. V Most Central • Au+Au collisions at s = 19. 6, 130, 200 Ge. V • d. N/dh for |h| < 5. 4 over full azimuth • Top 50% of total cross section (Npart~ 65 - 360) • Data at 56 Ge. V only for central density at |h| < 1 Robert Pak Npart

Particle Density near Midrapidity (2001) (2000) Theory 1999 Theory 2000 Constraining the Models Robert Pak

Centrality Dependence of Central Density Two Component Fit: PHOBOS Au+Au 200 Ge. V 130 Ge. V _ pp 19. 6 Ge. V preliminary Data from PRC 65 061901 R (2002) Robert Pak s. NN x ---------200 0. 11± 0. 04 13119. 6 0. 15± 0. 07

Limiting Fragmentation Plotting d. N/dh vs. • Universal limiting distribution • Extent of limiting fragmentation region grows with beam energy • No broad boost invariant region d. N/d / Npart/2 PHOBOS Au+Au 200 Ge. V 130 Ge. V 19. 6 data allows extrapolation to extract Nch h = h - ybeam Robert Pak

Centrality Dependence of Nch Error band due to high- extrapolation Au+Au Total charged particle multiplicity per participant is centrality independent Robert Pak

Nch Remove the “Leading Proton” Effect Basile et al (1980 -1984) seff s p. QCD e+e- Calculation Robert Pak (A. Mueller, 1983)

Comparison of Nch vs. Energy Different systems converge at high energy Robert Pak

Total Multiplicity vs. Beam Energy e+e- Fit p+p p+X : Universality + ee Au+Au pp Robert Pak

Azimuthal Angular Distributions “head on” view of colliding nuclei b (reaction plane) y f x Look at emission patterns using Fourier expansion: extract v 2 components from the fits. d. N = N 0 (1 + 2 v 1 cos (f YR) + 2 v 2 cos (2(f YR)) +. . . ) d(f YR ) Robert Pak

v 2 vs. centrality Preliminary v 2200 Final v 2130 200 | | < 1 130 PHOBOS Au+Au <Npart> Suggests early thermalization Robert Pak

v 2 vs. transverse momentum PHOBOS preliminary h+ + h 200 Ge. V Au+Au track-weighted centrality averaging (top 55%) 0< <1. 5 17% scale error v 2 appears to saturate for p. T > 2 Similar to results from STAR and PHENIX Robert Pak

v 2 vs. pseudorapidity PHOBOS Au+Au Preliminary v 2200 <Npart>~190 No boost invariance over a broad region Robert Pak

v 2 vs. cf. Hydro Model 130 Ge. V result: nucl-ex/0205021, accepted in PRL Robert Pak

Conclusions • • Not boost invariant over broad h range Limiting fragmentation confirmed for AA Total yields in AA scale primarily with Npart Surprising similarity between AA and e+e- Robert Pak