Centrality Dependence of Freeze-out Parameters From Centrality Dependence of Freeze-out Parameters From the Beam Energy Scan at STARthe Beam Energy Scan at STAR

Sabita Das (for the STAR collaboration)

Institute of Physics, Bhubaneswar, India

Quark Matter 2012, Washington D.C, USA Sabita Das04/20/23 1

OutlineOutline

Motivation

Introduction - Chemical freeze-out - Kinetic freeze-out

Experimental set-up

Particle Identification method

Results on freeze-out parameters

Summary

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 2

STAR BES proposal: arXiv:1007.2613

- To map the QCD phase diagram- To search the possible QCD phase boundary- To search the possible QCD critical point

The STAR data from BES are used to extract the freeze-out parameters , B and <> from identified particle spectra and ratios

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 3

MotivationMotivation

QCD Phase Diagram

The main goals of RHIC BES program

IntroductionIntroduction

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 4

Chemical Freeze-out : Inelastic collision ceases Particle ratios get fixed

★THERMUS : Statistical thermal model Ensemble used – Grand Canonical and Strangeness Canonical

€

ni =Tmi

2gi2π 2

(±1)k+1

kk=1

∞

∑ ekμ iT

⎛

⎝ ⎜

⎞

⎠ ⎟K2

kmiT

⎛

⎝ ⎜

⎞

⎠ ⎟

To consider incomplete strangeness equilibration:

Extracted thermodynamic quantities: ch, B, s and γS •Thermus, S. Wheaton & Cleymans, Comput. Phys. Commun. 180: 84-106, 2009.€

ni →niγ SSi

For Grand Canonical: Quantum numbers (B, S, Q) conserved on average

For Strangeness Canonical: Strangeness quantum number (S) conserved exactly

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 5

Kinetic Freeze-out : Elastic collision ceases Transverse momentum spectra get fixed Blast Wave : Hydrodynamic inspired model

€

dN

pTdpT∝ rdrmT I0

pT sinhρ(r)

Tkin

⎛

⎝ ⎜

⎞

⎠ ⎟

0

R

∫ ×K1

mT coshρ(r)

Tkin

⎛

⎝ ⎜

⎞

⎠ ⎟

Extracted thermodynamic quantities: kin and <β>

E. Schnedermann et al., Phys. Rev. C 48, 2462 (1993)

IntroductionIntroduction

Particle spectra are fitted simultaneously

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das

The Solenoid Tracker At RHIC (STAR)The Solenoid Tracker At RHIC (STAR)

6

Data (2010): Au+Au 39GeV, 11.5 GeV and 7.7 GeV. Particles used : π p, Λ, Ξ and K0

S

TPCTPCTOFTOF

Full azimuthal particle identification.

Particle IdentificationParticle Identification

€

z = logdE /dx( )

meas.

dE /dx( )theory

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

€

m2 = p2 c2t 2

L2 −1 ⎛

⎝ ⎜

⎞

⎠ ⎟

p = momentum, t = time-of-flightc = velocity of light, L = path length

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das

TPC TPC+TOF

Coun

ts

Coun

tszπ m2 (GeV2/c4)

7

STAR Preliminary

STAR Preliminary

STAR Preliminary

STAR Preliminary

arXiv:1201.4203v1

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 8

Particle SpectraParticle Spectra

arXiv:1201.4203v1 arXiv:1203.5183

STAR Preliminary STAR Preliminary

STAR PreliminarySTAR Preliminary STAR Preliminary

STAR Preliminary

Chemical Freeze-out Chemical Freeze-out

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das

Particles used : π, K, p, Λ Ξ and K0

s

Ensemble used: Grand Canonical

Fit parameters: ch s and γS

(strangeness saturation factor)

9

STAR Preliminary

STAR Preliminary

STAR Preliminary

STAR Preliminary

Particles used in the fit: π, K, p, Λ Ξ and K0s

Chemical freeze-out temperature increases with increase in collision energy. Baryon chemical potential decreases with increase in collision energy.

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 10

Chemical Freeze-out Chemical Freeze-out

Au+Au 200 GeV : Phys. Rev. C 83 (2011) 24901

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das

Particles used in the fit : π, K, p, Λ Ξ and K0s

Strangeness chemical potential decreases with increase in collision energy

Strangeness saturation factor increases from peripheral to central collisions for all energies

11

Chemical Freeze-out Chemical Freeze-out

Au+Au 200 GeV : Phys. Rev. C 83 (2011) 24901

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 12

Chemical Freeze-out: Tch vs. B Chemical Freeze-out: Tch vs. B

Andronic: NPA 834 (2010) 237 Cleymans: PRC 73 (2006) 034905Au+Au 200 GeV : Phys. Rev. C 83 (2011) 24901

STAR Preliminary

Particles used : π, K, p, Λ Ξ and K0

s

Ensemble used: Grand Canonical and Strangeness Canonical

Fit parameters: ch s and γS

(strangeness saturation factor)

We observe a centrality dependence of chemical freeze-out parameters (Tch, B) at lower energies. For peripheral collisions: Tch (SCE) > Tch (GCE)

Blast-Wave Model is used to fit the spectra Two main parameters kin and

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 13

Kinetic Freeze-out: Blast-Wave Kinetic Freeze-out: Blast-Wave

STAR Preliminary

STAR Preliminary

STAR Preliminary

STAR Preliminary For central collisions: Low temperature and high flow velocity

For peripheral collisions: High temperature and low flow velocity

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 14

Kinetic Freeze-out: Tkin vs. <> Kinetic Freeze-out: Tkin vs. <>

Blast-Wave: Higher kinetic temperature corresponds to lower value of average flow velocity and vice-versa.

STAR Preliminary

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 15

New measurements for BES energies (39, 11.5 and 7.7 GeV) at RHIC extend B range from 20 - 400 MeV of the QCD phase diagram

Chemical Freeze-out: Thermus model and particle ratios

First observation of centrality dependence of chemical freeze-out parameters at lower energies - Central collisions: GCE and SCE ~ Tch

- Peripheral collisions: Tch (SCE) > Tch (GCE) and 2/ndf (SCE) > 2/ndf (GCE) at low energies

Kinetic Freeze-out: Blast-Wave model and Particle pT spectra

- Central collisions: Low Tkin and high <> - Peripheral collisions: High Tkin and low <>

SummarySummary

STAR Preliminary

THANK YOU

16Quark Matter 2012, Washington D.C, USA Sabita Das

Back up

04/20/23 Quark Matter 2012, Washington D.C, USA Sabita Das 17

STAR Preliminary

STAR Preliminary

STAR Preliminary