Results from PHOBOS at RHIC David Hofman University
Results from PHOBOS at RHIC David Hofman University of Illinois at Chicago For the Collaboration European Physical Society HEP 2005 International Europhysics Conference on High Energy Physics EPS (July 21 st-27 th 2005) in Lisbon, Portugal HEP 2005
What PHOBOS has studied so far… • Relativistic Heavy Ion Collisions of – Au+Au • √s. NN = 19. 6, 55. 9, 62. 4, 130. 4 200. 0 Ge. V – Coming soon: • Cu+Cu (√s. NN = 22. 5, 62. 4 and 200. 0 Ge. V) • “Benchmark” Collisions of – d+Au • √s. NN = 200. 7 Ge. V – Coming soon: • p+p (√s. NN = 200, 410 Ge. V) HEP 2005 2
What PHOBOS has measured so far… • Centrality dependence of primary charged particle production vs. – h (d. N/dh out to |h|<5. 4) – f. R (flow) – p. T (0. 2 – ~5 Ge. V/c) • Identified charged particles – antiparticle/particle ratios – low p. T yields of p, K, p (0. 03 – 0. 2 Ge. V/c) – Spectra for p. T = 0. 3 – ~4 Ge. V/c HEP 2005 3
What PHOBOS has observed so far… “White Paper”: nucl-ex/0410022, Nucl. Phys. A 757, 28 • We have created a state of matter at RHIC with high energy-density, that is nearly net-baryon free and interacting very strongly. • Transition to this high-energy state of matter does not create abrupt changes in observables at RHIC energies. • The data exhibit many “simple” scaling behaviors. • The data exhibit a remarkable factorization of collision energy and geometry. HEP 2005 4
The Collaboration (June 2005) Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, Bolek Wysłouch ARGONNE NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF MARYLAND BROOKHAVEN NATIONAL LABORATORY MASSACHUSETTS INSTITUTE OF TECHNOLOGY UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF ROCHESTER HEP 2005 5
The PHOBOS Detector (2005) ZDC T 0 Trigger Counter • • • Triggering & Centrality Determination 4 -p Multiplicity Detector Magnetic Spectrometer + To. F Spec. Trig TOF p. CAL Spec. Cal Paddle Trigger Counter Vertex Octagon Spectrometer Rings NIM A 499 (2003) 603 HEP 2005 6
Properties of this “State of Matter” HEP 2005
High Energy Density & Approaching a Baryon-Free Environment PHOBOS nucl-ex/0405003 Energy Density Estimate: Largest Uncertainty is the Time to Equilibration. Conservative estimates put teq ~ 2 fm/c & e. RHIC > 3 Ge. V/fm 3 Energy Density ~6 x energy density inside nucleons (20 x for nuclei) PHOBOS PRC 67, 021901 R Particle Ratios: Approaching Unity m. B ~ 27 Me. V at full RHIC energy, a factor ~10 x smaller than at SPS. HEP 2005 8
Strongly Interacting Matter FLOW PHOBOS nucl-ex/0407012 ro d hy • Elliptic flow reaches hydro limit • Suppressed high-p. T production • No enhanced low-p. T production 200 Ge. V Au+Au, 0 -50% RAA, Rd. Au high-p. T low-p. T PHOBOS PRL 91, 072302 PHOBOS PRC 70, 051901(R) d+Au 0 -20% 1. 5 1. 0 0. 5 0 200 Ge. V Au+Au 0 -6% 0 p. T (Ge. V/c) HEP 2005 9
Gradual Transition to this “State of Matter” HEP 2005
Smooth Evolution of Mid-rapidity d. Nch/dh Energy Centrality PHOBOS nucl-ex/0410022 PHOBOS PRC 70, 021902(R) Au+Au data Au+Au & Pb+Pb data 90 % C. L. 200 Ge. V (measured UA 5) 19. 6 Ge. V (interpolated ISR) HEP 2005 11
Smooth Evolution of Bulk Elliptic Flow I ENERGY Au+Au 0 -40% central PHOBOS PRL 94, 122303 v 2 PHOBOS PRL 94, 122303 h HEP 2005 12
Smooth Evolution of Bulk Elliptic Flow II Centrality Au+Au @ 200 Ge. V PHOBOS nucl-ex/0407012 HEP 2005 PHOBOS nucl-ex/0407012 13
Smooth RAA Evolution I ENERGY PHOBOS PRL 94, 082304 RAA 2 62. 4 Ge. V 200 Ge. V 1 0 p. T HEP 2005 14
Smooth RAA Evolution II CENTRALITY Red = 62. 4 Ge. V Blue = 200 Ge. V PHOBOS PRL 94, 082304 & PLB 578, 297 line for illustration only HEP 2005 15
“Simple” Scaling Behaviors 1) Extended Longitudinal Scaling HEP 2005
Reminder: Limiting Fragmentation in p+p Data: UA 5 (Alner et al. ), Z. Phys. C 33, 1 (1986) Ansatz: Benecke, Chou, Yang, Yen, Phys. Rev. 188, 2159 (1969) Shift to Rest Frame of Beam p+p inel. d. N/dh¢ Ansatz: At high collision energy, d 2 N/dy’dp. T and particle mix, reach a limiting value and become independent of energy around beam rapidity (i. e. in the fragmentation region). HEP 2005 17
d. N/dh: Extended Longitudinal Scaling Au+Au PHOBOS PRL 91, 052303 & preliminary 62. 4 HEP 2005 18
v 2 flow: Extended Longitudinal Scaling Au+Au PHOBOS PRL 94, 122303 HEP 2005 19
d. N/dh: Extended Longitudinal Scaling d+Au PHOBOS nucl-ex/0409021 (accepted PRC-RC) d. Au & p. Emulsion per incident nucleon and approx. same Npart HEP 2005 20
“Simple” Scaling Behaviors 1) Extended Longitudinal Scaling 2) The Importance of Npart HEP 2005
Participants (Npart) and Collisions (Ncoll) Ncoll Binary Collisions L~A 1/3 Ncoll ~ A 4/3 Npart Participants b Npart/2 ~ A HEP 2005 22
Collisions per Participant Pair Au+Au Glauber Monte Carlo HEP 2005 23
Extracting <Nch> in Au+Au and d+Au PHOBOS PRL 91, 052303 Utilize PHOBOS broad h coverage (and the observed extended longitudinal scaling) to extract <Nch> PHOBOS ar. Xiv: nucl-ex/0409021 (accepted PRC-RC) HEP 2005 24
Npart scaling for Nch in Au+Au PHOBOS PRL 91, 052303 PHOBOS ar. Xiv: nucl-ex/0301017 Total charged particle yield per participant pair is flat with centrality Reduction at mid-rapidity balanced by increase at larger rapidity HEP 2005 25
Npart scaling for Nch in d+Au ar. Xiv: nucl-ex/0409021 (accepted PRC-RC) HEP 2005 26
Npart (Ncoll scaled) RAA at 62. 4 and 200 Ge. V Au+Au PHOBOS PRL 94, 082304 PHOBOS Ncoll 62. 4 Ge. V 200 Ge. V Ncoll Yields normalized by Npart are less centrality-dependent HEP 2005 27
“Simple” Scaling Behaviors 1) Extended Longitudinal Scaling 2) The Importance of Npart 3) Universality of Total Charged Particle Production HEP 2005
Nch production in pp, d. A, AA and ee PHOBOS nucl-ex/0301017 & nucl-ex/0410022 e+ e 200 Ge. V 130 Ge. V 20 Ge. V “Match” for d+Au & p+p HEP 2005 “Match” for Au+Au & e+e- 29
Universal Curve for Nch vs (s) at RHIC Energies When pp and d. A “corrected” for energy loss to leading baryon PHOBOS nucl-ex/0301017 & nucl-ex/0410022 Au+Au PHOBOS d+Au (@ √s/2) HEP 2005 30
Factorization of Collision Energy and Geometry HEP 2005
Factorization of Energy and Centrality Dependence for Mid-rapidity d. Nch/dh Au+Au Collisions PHOBOS PRC 65, 061901 R PHOBOS PRC 70, 021902 R Preliminary: 62. 4 Ge. V HEP 2005 32
Factorization: p. T Dependence PHOBOS Data PRL 94, 082304 & PLB 578, 297 Au+Au: Ratio 200/62. 4 Ge. V lines for illustration only Centrality factorization appears to be p. T independent HEP 2005 33
Npart RAA Evolution with Centrality… Universal? PHOBOS PRL 94, 082304 Ncoll PHOBOS HEP 2005 62. 4 Ge. V 200 Ge. V 34
Npart RAA Evolution with Centrality… Universal? PHOBOS PRL 94, 082304 Ncoll PHOBOS • • Energy-independent Weak function of Npart, p. T HEP 2005 • • 62. 4 Ge. V 200 Ge. V Energy-dependent Strong function of √s, p. T 35
What has PHOBOS learned so far? • We have created a state of matter at RHIC with high energy-density, that is nearly net-baryon free, and that is strongly interacting. • Transition to this high-energy state of matter does not create abrupt changes in observables at RHIC energies. • The data exhibits “simple” scaling behaviors and factorizations that unite the data. → suggests strong global constraints and illustrates the importance of the collision geometry in the initial state and the very early evolution of the colliding system. • Waiting for Cu+Cu results! → test these features in a smaller system. HEP 2005 36
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