RHIC PHENOMENOLOGY AS SEEN BY Wit Busza QCD in the RHIC Era UCSB, April 2002

The PHOBOS Collaboration Birger Back, Alan Wuosmaa Mark Baker, Donald Barton, Alan Carroll, Nigel George, Stephen Gushue, George Heintzelman, Burt Holzman, Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak Maarten Ballintijn, Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane, Judith Katzy, Piotr Kulinich, Heinz Pernegger, 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 Willis Lin, ChiaMing Kuo Joshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter Richard Bindel, Edmundo Garcia, 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

PHOBOS Detector   4  Multiplicity : Two-arm Spectrometer:

Number of Participants pA: AA: Participants Npart Spectators Npart= +1

Estimating the Number of Participants Assumption: – Multiplicity is monotonic with Npart – Glauber model applicable Nucleons maintain same cross- section Model

Charged Particle Multiplicity 130 GeV AuAu 200 GeV AuAu 25-35%cent 0-6%cent 45-55%cent N tot = 4100 ±210 N tot = 4960 ±250  dN/d 

Total observed multiplicity  W. Thome et al., Nucl. Phys. B129 (1977) GeV AuAu 200 GeV AuAu 25-35%cent 0-6%cent 45-55%cent  dN/d  ISR data

Compilation of p-emulsion data Limiting fragmentation in pA, Ap and pp scattering pA Ap pp

Collision Viewed in Rest Frame of One Projectile UA5, Z.Phys.C33, 1 (1986) Limiting Fragmentation is seen

Reduction of Target Fragments with Centrality ? 200 GeV AuAu NA5 DeMarzo, et al (1984) From Barton et al pA pX pA pi-X XFXF

With one exception, the pp, pA and AA data in the fragmentation region are consistent with the following picture: 1.A “wall of gluons” strips the gluons from the target nucleons. This process is independent of the energy of the incident nucleus. 2.The number of remaining quarks is only weakly dependent on the thickness of the incident nucleus. 3.The quarks fragment into the particles detected in the fragmentation region. Some rescattering occurs.

Au+Au & pp at 200 GeV From W. Busza (1976) From Peter Steinberg

Most central RHIC : PHOBOS AuAu  s = 200 GeV SPS : EMU-13 PbPb  s = 17 GeV RHIC : PHOBOS AuAu  s = 130 GeV 200 GeV AuAu Collision Viewed in Center of Mass Frame

AuAu normalized to equivalent number of participants f pp (s) = (CDF/UA5) Central Au+Au p+p PRL 87 (2001) Total energy released ~2000GeV Max. initial overlap volume At 200 GeV initial released energy density

Centrality Dependence of dN/deta Naïve expectations: –Consider collision of two “tubes of nucleons” 16 or 36? 1 or 6?1 Note: = Avg number of collision each participant makes (~6 for central AuAu)

Au+Au & pp at 200 GeV From Peter Steinberg Au+Au & pp at 200 GeV

Azimuthal Angular Distributions dN/d(  R ) = N 0 (1 + 2V 1 cos (  R ) + 2V 2 cos (2(  R )) +... ) b (reaction plane) Look at emission patterns using Fourier expansion: extract V 2 components from the fits. “head on” view of colliding nuclei y x 

Centrality Dependence of v 2 Hydrodynamic model Elliptic flow, v 2 Normalized Paddle Signal Systematic error ~ SPS AGS Preliminary |  | < 1.0  s NN =130GeV Peripheral Collisions b Central Collisions b

V 2 (elliptical flow) vs  Averaged over centrality V 2 drops for |  | > 1.5 Dependence on  appears to be different than at lower energy.  V2V2 PHOBOS Preliminary STAR (PRL) SPS NA49 (QM99) rapidity Pion (b<11fm) PHOBOS Systematic error ~  s NN = 130 GeV  s NN = 17 GeV All Charged Min. bias

AuAu130 GeV Stat. Syst. Preliminary 200GeV AuAu

Energy Dependence of Baryo-chemical potential  B Nucl. Phy. A697: (2002)

Baryon Stopping From W.B and A.S.Goldhaber

PHOBOS web-site: Published 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, (2001) –Charged-particle pseudorapidity density distributions from Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, (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 |  in Au+Au Collisions at 130 GeV Phys. Rev. C65, (R)(2002) –Centrality Dependence of Charged Particle Multiplicity at |  <1 in Au+Au Collisions at 130 and 200 GeV Submitted to Phys. Rev. C (2002)

Conclusions based on PHOBOS Results Central rapidity density increases approximately logarithmically with energy. Why? It is lower than most pre-RHIC predictions. Initial Energy Density > 10GeV/fm 3 Eliptic Flow suggests high pressure is created. dN/dn is boost invariant for +- 2 units of rapidity about zero, but not eliptic flow. Why? Fragmentation of incident states essentially as expected. Rapidity distribution per participant for AuAu and p pbar have similar shape over entire pseudo rapidity range. The former is approximately 1.3 times the later. This scaling is not understood. Many features of AuAu multiparticle production are well reproduced by saturation model of Kharzeev et al. No surprises in particle ratios. To understand multiparticle production, need energy scan and species scan.