Results from PHOBOS at RHICDavid Hofman

University of Illinois at Chicago

For the Collaboration

European Physical SocietyHEP2005 International Europhysics Conference

on High Energy Physics EPS (July 21st-27th 2005) in Lisbon, Portugal

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What PHOBOS has studied so far…

• Relativistic Heavy Ion Collisions of – Au+Au

• √sNN = 19.6, 55.9, 62.4, 130.4 200.0 GeV

– Coming soon: • Cu+Cu (√sNN = 22.5, 62.4 and 200.0 GeV)

• “Benchmark” Collisions of – d+Au

• √sNN = 200.7 GeV

– Coming soon: • p+p (√sNN = 200, 410 GeV)

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What PHOBOS has measured so far…

• Centrality dependence of primary charged particle production vs.– (dN/d out to |<5.4)

– – R (flow)

– pT (0.2 – ~5 GeV/c)

• Identified charged particles– antiparticle/particle ratios

– low pT yields of ,K,p (0.03 – 0.2 GeV/c)

– Spectra for pT = 0.3 – ~4 GeV/c

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What PHOBOS has observed so far…

• 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.

“White Paper”: nucl-ex/0410022, Nucl. Phys. A 757, 28

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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 BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY

NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

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The PHOBOS Detector (2005)

NIM A 499 (2003) 603

TOF

T0 Trigger Counter

• Triggering & Centrality Determination• 4- Multiplicity Detector• Magnetic Spectrometer + ToF

Spectrometer

pCAL

Paddle Trigger Counter

SpecTrig

ZDC

Vertex

Octagon Rings

SpecCal

Properties of this “State of Matter”

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High Energy Density & Approaching a Baryon-Free Environment

PHOBOS nucl-ex/0405003Energy Density Estimate:

Largest Uncertainty is the Time to Equilibration.

Conservative estimates put eq ~ 2 fm/c & RHIC > 3 GeV/fm3

Energy Density ~6x energy density inside nucleons (20x for nuclei)

B ~ 27 MeV at full RHIC energy, a factor ~10x smaller than at SPS.

Particle Ratios:

Approaching Unity

PHOBOS PRC 67, 021901R

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Strongly Interacting Matter

• Elliptic flow reaches hydro limit• Suppressed high-pT production• No enhanced low-pT production

PHOBOS nucl-ex/0407012

FLOW

PHOBOS PRC 70, 051901(R)low-pT

00

0.5

1.0

1.5

RA

A, R

dA

u

Au+Au 0-6%

d+Au 0-20%

high-pT PHOBOS PRL 91, 072302

200 GeV Au+Au, 0-50%

hydro

200 GeV Au+Au

pT (GeV/c)

Gradual Transition to this “State of Matter”

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Smooth Evolution of Mid-rapidity dNch/dEnergy Centrality

200 GeV (measured UA5)

19.6 GeV (interpolated ISR)pp pp

90 % C.L.

PHOBOS nucl-ex/0410022 PHOBOS PRC 70, 021902(R)

Au+Au data

Au+Au & Pb+Pb data

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Au+Au 0-40% central

Smooth Evolution of Bulk Elliptic Flow IENERGY

v 2v 2

PHOBOS PRL 94, 122303 PHOBOS PRL 94, 122303

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Smooth Evolution of Bulk Elliptic Flow IICentrality

PHOBOS nucl-ex/0407012 PHOBOS nucl-ex/0407012

Au+Au @ 200 GeV

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Smooth RAA Evolution I

PHOBOS PRL 94, 082304

ENERGY

RA

A

0

1

2

pT

62.4 GeV200 GeV

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Smooth RAA Evolution II

CENTRALITY

Red = 62.4 GeVBlue = 200 GeV PHOBOS PRL 94, 082304 & PLB 578, 297

line for illustration only

“Simple” Scaling Behaviors

1) Extended Longitudinal Scaling

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p + p inel.

dN

/d

beamy

Reminder: Limiting Fragmentation in p+pData: UA5 (Alner et al.),Z.Phys.C33, 1 (1986)

Ansatz: At high collision energy, d2N/dy’dpT and particle mix, reach a limiting value and become independent of energy around beam rapidity (i.e. in the fragmentation region).

Ansatz: Benecke, Chou, Yang, Yen, Phys. Rev. 188, 2159 (1969)

Shift to Rest Frame of Beam

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dN/d: Extended Longitudinal ScalingAu+Au

PHOBOS PRL 91, 052303 & preliminary 62.4

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v2 flow: Extended Longitudinal ScalingAu+Au

PHOBOS PRL 94, 122303

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dN/d: Extended Longitudinal Scalingd+Au

dAu & pEmulsion per incident nucleon and approx. same Npart

PHOBOS nucl-ex/0409021 (accepted PRC-RC)

“Simple” Scaling Behaviors

1) Extended Longitudinal Scaling

2) The Importance of Npart

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Npart Participants

Ncoll Binary Collisions

Npart/2 ~ A

L~A1/3

Ncoll ~ A4/3

Participants (Npart) and Collisions (Ncoll)

b

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Collisions per Participant Pair

Glauber Monte Carlo

Au+Au

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Extracting <Nch> in Au+Au and d+Au

PHOBOS arXiv:nucl-ex/0409021 (accepted PRC-RC)

PHOBOS PRL 91, 052303

Utilize PHOBOS broad coverage (and the observed extended longitudinal scaling) to extract <Nch>

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PHOBOS PRL 91, 052303

Npart scaling for Nch in Au+Au

Total charged particle yield per participant pair is flat with centrality Reduction at mid-rapidity balanced by increase at larger rapidity

PHOBOS arXiv:nucl-ex/0301017

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Npart scaling for Nch in d+Au

arXiv:nucl-ex/0409021 (accepted PRC-RC)

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NpartRAA at 62.4 and 200 GeV

PHOBOS 62.4 GeV200 GeV

PHOBOS PRL 94, 082304

Ncoll

Ncoll

(Nco

ll sc

aled

)

Yields normalized by Npart are less centrality-dependent

Au+Au

Ncoll

“Simple” Scaling Behaviors

1) Extended Longitudinal Scaling

2) The Importance of Npart

3) Universality of Total Charged Particle Production

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Nch production in pp, dA, AA and ee

PHOBOS nucl-ex/0301017 & nucl-ex/0410022

“Match” for d+Au & p+p

e+e-

200 GeV

130 GeV

20 GeV

“Match” for Au+Au & e+e-

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Universal Curve for Nch vs (s) at RHIC Energies

PHOBOS nucl-ex/0301017 & nucl-ex/0410022

When pp and dA “corrected” for energy loss to leading baryon

Au+Au

PHOBOS d+Au (@ √s/2)

Factorization of CollisionEnergy and Geometry

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Factorization of Energy and Centrality Dependence for Mid-rapidity dNch/d

PHOBOS PRC 70, 021902R

PHOBOS PRC 65, 061901R

Preliminary: 62.4 GeV

NNpartNNpartmid sfNfsNR 21,

Au+Au Collisions

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Factorization: pT Dependence

TNNpartTNNpartmid psfNfpsNR ,,, 21

Centrality factorization appears to be pT independent

Au+Au: Ratio 200/62.4 GeV

lines for illustration only

PHOBOS Data PRL 94, 082304 & PLB 578, 297

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RAA Evolution with Centrality… Universal?

PHOBOS 62.4 GeV200 GeV

Npart

Ncoll

Ncoll

PHOBOS PRL 94, 082304

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RAA Evolution with Centrality… Universal?

PHOBOS 62.4 GeV200 GeV

Npart

TNNTpartNPC

NAA psfpNRR partpart ,,

• Energy-independent

• Weak function of Npart, pT

• Energy-dependent

• Strong function of √s, pT

Ncoll

Ncoll

PHOBOS PRL 94, 082304

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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.