RHIC Physics Through the Eyes of PHOBOS Moriond, March 2003 RHIC Physics Through the Eyes of PHOBOS Wit Busza MIT

Relativistic Heavy Ion Collider

Why Collide Heavy Ions? From Frank Wilczek time

UA1, 900 GeV anti-proton proton Gold sNN = 130 GeV

Goal of Relativistic Heavy Ion Physics is to Obtain a Better Understanding of the Solutions of the QCD Lagrangian: QCD Phase Diagram Properties of QGP Mechanism of Particle Production Structure and Interactions of Relativistic Hadrons & Nuclei

What Are the Correct Variables When Looking at AA Collisions? Spectator Nucleons Participating Nucleons Npart= 7 Ncoll.= 10 Nquarks +gluons = ? Ninelastic= 1 In calculating Npart or Ncoll  taken to be nucleon-nucleon inelastic cross-section. A priori no reason for this choice other than that it seems to give a useful parameterization. Will the following be equivalent to the above?  inel ~ (R1+R2)2 ~ (A11/3 + A21/3)2 ~ A2/3 Npart ~ A2/3(A11/3+ A21/3) ~ A Ncoll ~ A2/3(A11/3 * A21/3) ~ A4/3

pA multiplicities were found to be proportional to Npart Busza et al., PRL 41(1978).285

In no rest frame is this picture correct In rest frame of one nucleus: Soft components overlap, “gluon saturation effects”, shadowing etc. In rest frame of the center of mass of the system: The use and relevance of Npart is far from obvious when the collision is viewed from different frames of reference

Central Collisions PHOBOS 19.6 GeV 130 GeV 200 GeV PHOBOS PHOBOS dN d  Peripheral Collisions 1. Is there an interesting state created in high energy hadronic (in particular AA) collisions?

Evidence that shortly after the collision a high energy density Evidence that shortly after the collision a high energy density* is created Number of Particles Produced at y=0 Total energy released ~2000GeV Max. initial overlap volume Initially released energy density >5GeV/fm3 Note: energy density inside proton ≈ 0. 5GeV/fm3 Energy of Collision * Strictly speaking it is the energy released in the transverse direction per unit volume

Evidence that at y≈0 this high energy density state has the quantum numbers of the vacuum A+A central collisions K–/K+ Ratio of antimatter to matter p/p Energy of collision

Evidence for interactions with the created state Peripheral Au+Au data Central Au+Au data D. Hardtke QM ‘02 Disappearance of back-to-back correlations in central Au+Au Away-side particles absorbed or scattered in medium Jets seen in peripheral Au+Au and p+p Azimuthal correlations Small angle (Df ~ 0) Back-to-Back (Df ~ p)

Evidence that the created state has a high pressure Phobos data for 130 and 200 GeV Azimuthal Angular Distributions Peripheral Central “head on” view of colliding nuclei Also, PHOBOS sees very few low Pt particles All this is direct evidence of collective effects

Evidence that most of the action ends very quickly after the collision PHOBOS preliminary h+ + h- 200 GeV Au+Au 0<h<1.5 (top 55%) v 2 17% scale error Evidence that most of the action ends very quickly after the collision v2 Elliptic Flow PHOBOS Au+Au Preliminary v2200 Final v2130 200 GeV 130 GeV <Npart>~190 130 GeV result: nucl-ex/0205021, submitted to PRL h

Evidence that the system may reach some kind of equilibriuim Event by Event Fluctuations NA49, PRL 86 (2001) 1965 NA49, Phys Lett B459 (1999) 679 From Gunther Roland/MIT

Further evidence that it may be reaching statistical equilibrium Gene Van Buren. QM’02 Particle ratios compared to statistical model STAR Preliminary

2. There are remarkable similarities between e+e-, pp & AA collisions Is this evidence that dynamics are dominated by the initial state interactions?

 dN d Collision viewed in rest frame of CM: 19.6 GeV 200 GeV 130 GeV Central Collisions PHOBOS 19.6 GeV 200 GeV PHOBOS 130 GeV PHOBOS dN d  Peripheral Collisions Collision viewed in rest frame of one nucleus: Limiting fragmentation

h 4 3 2 1 UA5 -6 -4 -2 h-ybeam Proton+Antiproton 24 31 45 53 63 ISR data Proton+Antiproton Limiting fragmentation: 24 31 45 53 63 4 2-4 900 GeV 3 546 GeV 5-9 h 200 GeV Total observed multiplicity 2 10-14 53 GeV 1 15-19 UA5 20-24 -6 -4 -2 h-ybeam W. Thome et al., Nucl. Phys. B129 (1977) 365.

Amazing similarity of AA and e+e- Au+Au (preliminary) e+e- Number of Particles Produced Eskola, QM ’01 Energy of Collision From P. Steinberg Au+Au dN/dh||h|<1

3. Some results inconsistent with naïve expectations: e.g. impact parameter dependence of the number of particles produced at the center of mass of the collision Data inconsistent with the following picture: PHOBOS Au+Au 200 GeV PRC 65 (2002) 061901R Slow quark 130 GeV Fast quark 19.6 GeV preliminary Au+Au yields normalized to corresponding pp value for all three energies pp 200GeV 130GeV 19.6GeV (PRELIMINARY) pp Errors from Au+Au only

“X-Ray” of Medium Using Jets 4. Direct study of the properties of the produced state “X-Ray” of Medium Using Jets Leading Particle Hadrons q Leading Particle Leading Particle Hadrons q Leading Particle

Charged Hadron Spectra Preliminary sNN = 200 GeV

Particle Production at high Pt Submitted to Phys.Lett. AuAu 200GeV Particle Production at high Pt Fast quark Ncollscaling pA AuAu Ncollscaling Relative Yield per participant PHOBOS Cronin effect data

“Quenching” of leading partons in pA collisions? Eichten et al. Baron et al. Skupic et al. W. Busza Nucl.Phys. A544 (1992) 49c

Summary pp, pA, AA collisions are magnificent laboratories for the study of QCD No doubt a very high energy density creates a fascinating medium. If it equilibrates, it does so quickly. If it is the QGP, the transition is almost certainly a cross-over Main difficulty in interpretation of data is the separation of the initial and final state interactions Data continues to surprise us Smoothness of data with energy Jet quenching Similarity of AA with e+ e- Why approx. Nparticipant scaling, even at high Pt?

Collaboration (Jan 2003) Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley, Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan Carroll, Patrick Decowski, Edmundo Garcia, Nigel George, Kristjan Gulbrandsen, Stephen Gushue, Clive Halliwell, Joshua Hamblen, George Heintzelman, Conor Henderson, David Hofman, Richard Hollis, Roman Holynski, Burt Holzman, Aneta Iordanova, Erik Johnson, Jay Kane, Judith Katzy, Nazim Khan, Wojtek Kucewicz, Piotr Kulinich, Chia Ming Kuo, Jang Woo Lee, Willis Lin, Steven Manly, Don McLeod, Jerzy Michalowski, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg, Michael Reuter, Christof Roland, Gunther Roland, Leslie Rosenberg, Joe Sagerer, Pradeep Sarin, Pawel Sawicki, Wojtek Skulski, Stephen Steadman, Peter Steinberg, George Stephans, Marek Stodulski, Andrei Sukhanov, Jaw-Luen Tang, Ray Teng, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Gábor Veres, Robin Verdier, Bernard Wadsworth, Frank Wolfs, Barbara Wosiek, Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER