Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Anti-particle to particle ratios in p+p, Cu+Cu and Au+Au collisions at RHIC

Gábor I. VeresEötvös Loránd University, Budapest

Massachusetts Institute of Technology, Cambridge

Quark Matter 2006 Conference, Shanghai, November 19, 2006

for the collaboration

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Collaboration

Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel,

Wit Busza (Spokesperson), Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Christof

Roland, Gunther Roland, Joe Sagerer, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, 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

9 ongoing PhD thesis studies

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Outline

Particle ratios:

PHOBOS detector and PID methods

Particle spectra: • Au+Au PID spectra at 62.4 GeV• low-pT particle yields

• baryon/hadron ratios• mT scaling

• centrality dependence of the net proton yield Summary

• ratios in central Au+Au vs. s• Cu+Cu ratios vs. centrality at 200 and 62 GeV• comparison of p+p, Cu+Cu and Au+Au ratios vs. Npart

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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MotivationParticle ratios:

• are influenced by baryon pair production, baryon number transport, andfinal state interactions

• show how close the system is to ideal baryon-free plasma

• provide input to quantitative analysis of chemical equilibrium in thermal models z

-xy

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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The detector1m

Spectrometer

Time of Flight

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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pT (GeV/c)0.03 0.3 3.0

Stoppingparticles

dE/dx TOF

Particle ID from low to high pT

Eloss (MeV)

PHOBOS PID Capabilities

1 2 3 4 50p (GeV/c)

30

40

50

60

70

1/v

(ps/

cm)p+p

K +K + -

++-

p (GeV/c)

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Analysis techniques• Particles identified via dE/dx

in the spectrometer

• Monte Carlo studiesto estimate corrections

acceptance in y and pT :

)()(

)()(

pp

papa

pp

pp

raw

raw

acceptancefficiency,corrections

PID cutsK

pd

e

0 < y < 1.50.2 < pT< 0.8 GeV/c

p [GeV/c]

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Controlling systematic uncertainties

)()( BBpapa

Geometrical acceptance drops out of the ratios:

Ratios measured independently for different: • bending directions • spectrometer arms

)()( BBpapa

Z Z

B+B–

h+

h–h+

h+ h+

h–

h–h–

by changing the polarity of the magnetic field

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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pT [GeV/c]

Abs

orpt

ion

coef

ficie

nt

p

p_

GEANT Monte Carlo(FLUKA+Geisha)/2

Corrections

Absorption• GEANT used• 4% for p/p• 1.5% for K–/K+

_

Secondary particles• GEANT used• 1% for p/p, 2% for –/+

Feed-down• daughters of weakly decaying particles (,, –)• HIJING particle yields are used• track DCA cut (3.5 mm!) keeps correction small• 2% for p/p, <1% for K–/K+ and –/+

_

_

DCA {DCA {

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Errors on the ratios

• Statistical errors: 1% (tens of millions of events analyzed)

• Systematic errors (90% C.L.):

beam orbit variations 2% 2% 2%PID selection cuts 4% 4% 1%track quality cuts 3% 3% 3%vertex cuts 1% 1% 1%DCA cuts 2% 2% 1%trigger selection cuts 1% 1% 1%absorption correction 3% 1% 1% feed-down correction 3% 0% 0%correction for secondaries 1.5% 0% 2%Total: 7.5% 6% 4.5%

p K

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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New results at 62.4 GeV fit smoothly into the energy evolution of antiparticle/particle ratios

Results: energy dependence

arXiv:nucl-ex/0610001submitted to PRCstat.errors only

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Cu+Cu ratios, all 3 species vs. Npart

• Integrated over pT and y

• p/p ratios: little, if any, centrality dependence (still not understood theoretically)

_

• Similarly to Au+Au, strong collision energy dependence

200 GeV

62.4 GeV

y0

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Comparison of systems at 200 GeV

• p/p ratios do not depend strongly on

- system size- centrality

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• The study is being currently extended to 62.4 GeV

y0

p+p: Phys.Rev. C71, 021901 (2005)

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Ratios of different species

Ratios of different species (like p/) are of significant interest:they are used to study the baryon excess observed at higher RHIC energies in the 2 – 4 GeV/c pT range.

For that, identified spectra of all species were analyzed.

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Proton spectra are harder thanthe meson spectra

Time-of-Flight measurementextends the pT reach to 3 GeV/cfor protons

y0

Smooth evolution with centrality

arXiv:nucl-ex/0610001submitted to PRC

Identified particle pT -spectra,

Au+Au at 62.4 GeV

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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Particle production at very low pT

Low pT spectra carry information about radial flow, possible anomalousenhancement at very low pT, quantum mechanical effects, scaling laws

• Unique low-pT coverage of PHOBOS

T = 1016 MeVsurface = 0.720.02

y0

PHOBOS Au+Au 62.4 GeVarXiv:nucl-ex/0610001submitted to PRC

T = 1026 MeVsurface = 0.760.02

T = 1036 MeVsurface = 0.780.02

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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mT -scaling in d+Au vs. Au+Au at 200 GeV

Lack of mT scaling in heavy ion collisions

arXiv:nucl-ex/0610001submitted to PRCJ.Phys.G 30 S1143-S1147 (2004 ) PRC 70 (2004) 051901(R)

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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(Anti)proton/hadron ratiosfor the two charge signs

Baryon excess approached at about pT3 GeV/c

y0

p>++K+

arXiv:nucl-ex/0610001submitted to PRC

PHOBOS

ArXiv:nucl-ex/0610001submitted to PRC

y0

Smooth energy dependenceof pT where p/+1

p+ c

ross

ing

Central A+A

Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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y0

arXiv:nucl-ex/0610001submitted to PRC

Net protons (p–p)_

The net proton yield is proportional to Npart!p/p centrality independent p and p Npart as well

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Gábor I. Veres Quark Matter 2006, Shanghai, November 14-20, 2006

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SummaryNew results from PHOBOS:

• antiparticle/particle ratios in Cu+Cu collisions• p(p)/h± ratios, identified spectra in 62.4 GeV Au+Au collisions

What have we learned?

• Centrality-independence:• p/p ratios• net proton, p and p yield per participant

(still not fully understood theoretically)• baryon excess in Au+Au collisions at 62.4 GeV• mT-scaling holds in d+Au but violated in Au+Au

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