strangeness and bulk freeze-out properties at rhic

24 June 2007

Strangeness in Quark Matter 2007

STARSTAR

2S0Q0M7

Strangeness and bulk freeze-out properties at RHIC

Aneta Iordanova

STARSTAR

2S0Q0M7

24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007

Outline

Motivation STAR Experiment Analysis Techniques Results Summary

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Motivation

Identified particle spectra in heavy-ion collisions at different center-of-mass energies and system size provide a unique tool to explore the QCD phase diagram. system size dependence of the freeze-out parameters at RHIC.

Bulk properties: - Kinetic freeze-out properties from spectral shapes

• Tkin at kinetic freeze-out, transverse radial flow (β) - Chemical freeze-out properties from particle ratios

• Tch at chemical freeze-out, strangeness and baryon production

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STAR Experiment

Time Projection Chamber Measures charged particle

momenta and energy loss within ||<1

Full azimuthal acceptance

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Analysis Technique

Exploit the ionization energy loss (dE/dx)

Distribution is normalized by the theoretical expectation for different particle types; e.g.

z()=ln[(dE/dx)/(dE/dxTh)].

z() distribution is sliced into p=50MeV/c for

|y|<0.1 (mid-rapidity) 6 centrality bins - corresponding to

the top 60% of the cross-section. √sNN=62.4GeV and 200 GeV

Consistent analysis technique is used for all low pT measurements for different center-of-mass energies and colliding systems.

Raw yields are extracted for different particles from multi-Gaussian fits.

Cu+Cu 200 GeV0-10% central

0.50<pT<0.55 GeV/c

Cu+Cu 200 GeV

Minimum Bias

pion

kaon

electron

proton

normalized

STAR Preliminary

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Results – Particle Spectra

Raw data are corrected for: Single particle (track

finding) efficiency Energy Loss in the TPC Pions/protons corrected

for background

Corrected spectra are shown for mT-m0 Central 0-10% data Mass dependence is

observed in the slope of the spectra shapes

STAR Preliminary

10% central, Cu+Cu200 GeV62.4 GeV

–

–

p

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Results – Particle Ratios

K–/– ratio Systematically higher

in the smaller system p/– ratio

Same for Au+Au and Cu+Cu

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Fitting the data – Statistical Model

The model has 4 fit parameters: chemical freeze-out

temperature, Tch baryon chemical

potential, μB strangeness chemical

potential, μS strangeness

suppression, γS

0)(

2

2 12),( TSBE

Ss

ii iSiBi

i

e

dppgTn

STAR Preliminary

10% central, Cu+Cu @ 62.4 GeV10% central, Cu+Cu @ 200 GeV

Fit uses only data from , K and p

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Chemical freeze-out properties

Strangeness suppression factor consistent within errors

with Au+Au results for the central data

But appears to be systematically higher for the smaller system

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Chemical freeze-out temperature Universal chemical

freeze-out temperature, Tch, is observed for all studied systems.

5% central10% centralMin-Bias

STAR Preliminary

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Chemical freeze-out temperature Similar systematic

dependence is observed for different ion species as a function of centrality.

STAR Preliminary

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Fitting the data – Blast –wave model

The model assumes a boosted thermal source in transverse and longitudinal directions.

3 fit parameters are extracted: flow velocity, β kinetic freeze-out

temperature, Tkin

shape of the flow profile, n

(PR C48 (1993) 2462)

R r R

n

sr R

rrr

)()(

r 1tanh

kin

T

kin

TR

TTT T

mK

T

pIrdrm

dmm

dn coshsinh1

0

0

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Fit Results

Blast wave fits are performed in each centrality bin simultaneously for π±, K±, p and p-bar.

For the case of pions, the range below 0.5 GeV/c is excluded in order to reduce effects from resonance contributions.

10% central, Cu+Cu @ 62.4 GeV10% central, Cu+Cu @ 200 GeV

STAR Preliminary

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Particle mean-pT

Particle mean-pT:

increases with centrality (Nch). consistent with an increase in

radial flow with centrality.

Remarkable similarities as a function of Nch are observed.

for both Cu+Cu and Au+Au systems

at both center-of-mass energies, 200 and 62.4 GeV.

Are the bulk properties entropy driven? * Bose-Einstein fits over the entire fiducial spectra range are used for the case of pions.

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STAR Preliminary

Kinetic freeze-out properties

All particle spectra for p±, K±, p and p-bar in Cu+Cu and Au+Au systems appear to be described by a common set of freeze-out parameters.

Cu+Cu 0-10% Au+Au 0-5%

Kinetic Parameters 62.4 GeV 200 GeV 62.4 GeV 200 GeV

flow velocity 0.48±0.06 0.52±0.07 0.54±0.01 0.59±0.05

Tkin (MeV) 115±7 112±9 93.8±4.1 89±12

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Kinetic freeze-out properties

STAR Preliminary STAR Preliminary

These extracted freeze-out parameters are found to be similar for an equivalent Nch

for both Cu+Cu and Au+Au systems at both center-of-mass energies, 200 and 62.4 GeV.

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Summary

STAR has enlarged the variety of hadron spectra measurements at RHIC by providing new results for Cu+Cu at two different center-of-mass energies, √s=200 and 62.4 GeV.

Thermal models used for studying kinetic and chemical freeze-out properties point to a systematic dependence on the number of charged particles produced Independent of collision system and energy

This multi-dimensional systematic study has revealed the importance of the initial collision geometric overlap.

strangeness and bulk freeze-out properties at rhic

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