5-12 april 2008 winter workshop on nuclear dynamics star particle production at rhic aneta iordanova...

5-12 April 2008

Winter Workshop on Nuclear Dynamics

STARSTARParticle production at RHIC

Aneta Iordanova

for the STAR collaboration

25-12 April 2008

Aneta IordanovaUniversity of Illinois at Chicago

Winter Workshop onNuclear Dynamics

STARSTAR

Outline

Motivation Analysis Technique Freeze-out

ChemicalKinetic

Baryon/meson and strangeness production Scaling properties

35-12 April 2008



STARSTAR

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 particle production provide: 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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STARSTAR

Heavy ion collision evolution

Experimental Observation: the bulk of produced matter is “soft” (99%) to study the QGP → study hadronization and properties of produced

particles• species abundances (Chemical freeze-out and equilibration)• momentum distributions (Kinetic freeze-out)

Collision

HardPartonic

Scattering

Hot, DenseMatter, QGP?

HadronizationPhase

InelasticScattering

ElasticScattering

the current view

ChemicalFreeze-out

KineticFreeze-out

q

q

time: the system cools and expands

55-12 April 2008



STARSTAR

Mapping the QCD Phase Diagram

Lattice QCD predicts critical temperature for QGP phase boundary:

Tc ~ 170 MeV

c ~ 1 GeV/fm3

Experimentally derived freeze-out parameters from different experiments

Extend the measurements at RHIC over a broad range of

Energy Centrality System size

J.Stachel (Trento 2004)

65-12 April 2008



STARSTAR

STAR Experiment

Time Projection Chamber Measures charged particle

momenta and energy loss

within ||<1 Full azimuthal acceptance

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STARSTAR

Exploit the ionization energy loss (dE/dx)

Distribution normalized by the theoretical expectation for different particle types.

Normalized distribution sliced into p=50MeV/c for

|y|<0.1 (mid-rapidity) 6 centrality bins (60% of the cross-

section) Cu+Cu @ √sNN=62.4GeV and 200

GeV. Raw yields: extracted from multi-

Gaussian fits.

Consistent analysis technique for Different center-of-mass energies Colliding systems.

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

Low-pT particle identification

85-12 April 2008



STARSTAR

Chemical freeze-out

Statistical model approach: particle ratios described by 4 fit parameters: chemical freeze-out

temperature, Tch

baryon chemical potential, μB

strangeness chemical potential, μS

strangeness suppression, S

STAR Preliminary

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

Fits use only data from , K and p

2| |

( ) /20

( , )

2 1B s

i

i i isSi i

E B TS

N g p dp

V

T

e

Phys.Lett.B465(1999)15, arXiv:nucl-th/0405068

95-12 April 2008



STARSTAR

Chemical freeze-out

B decreases with increasing collision energy Approaching a net baryon

free system

Freeze-out temperature independent of initial conditions Collision energy Energy density Net baryon density

Strangeness suppression (s) approaches unity with increasing Nch

Chemical equilibrium

Approaching ‘net baryon free’

Universal Tch~160MeV

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STARSTAR

Transverse momentum spectra

Spectra Mass dependence Pressure build-up in the center, large pressure gradient

collective expansion Common expansion velocity?

Preliminary

Preliminary

115-12 April 2008



STARSTAR

PRC48 (1993) 2462

Kinetic freeze-out

Hydro-dynamically motivated “blast-wave” model fits

Gain insight into the dynamics of the collision.

Model assumes a boosted thermal source in transverse and longitudinal directions.

Can describe the data with a common set of fit parameters

Transverse flow velocity, β Kinetic freeze-out temperature,

Tkin

Studies show that resonance decays (at RHIC energies) do not affect the fit results significantly

n

sr R

rrr

)()( r 1tanh

kin

T

kin

TR

TTT T

mK

T

pIrdrm

dmm

dn coshsinh1

0

0

10% central, Cu+Cu @ 200 GeV

STAR Preliminary

125-12 April 2008



STARSTAR

STAR Preliminary

Tkin

Tch

Freeze-out properties

Extracted freeze-out parameters similar for the same number of

produced charged-particles, Nch• For all systems• For all colliding energies

135-12 April 2008



STARSTAR

STAR Preliminary

Tkin

Tch

Freeze-out properties

Tkin , ‹› with centrality information on system

expansion Explosive system, higher higher pressure gradients in

central events

Tch insensitive to centrality Insensitive to expansion and

cooling Close to TC

• QCD predicted TC~160-170 MeV

Coincides with hadronization

Thus, chemical freeze-out may probe the phase boundary

TTCC

145-12 April 2008



STARSTAR

Probing the phase boundary

Current systematics: Increasing collision

energy Using different collision

systems and centrality

Next step: RHIC low-energy scan

J.Stachel (Trento 2004)

155-12 April 2008



STARSTAR

Baryon/meson production

Low-pT Baryon < meson Centrality independent Ratio similar to p+p

Intermediate pT ratio enhanced relative to p+p Maximal at pT~2GeV/c Strong centrality

dependence centrality independent for

pT>5GeV/c (Cu+Cu) and pT>7GeV/c (Au+Au)

R.Hollis WWND07

Phys. Rev. Lett. 92 (2004) 112301 Phys. Rev. Lett. 97 (2006) 152301

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STARSTAR


Energy dependence

Centrality dependence of Anti-proton/ enhancement versus pT is similar in 200 and 62.4GeV

Phys. Lett. B 655 (2007) 104

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STARSTAR

Enhancement also evident in strangeness sector /K0 shows the same

systematic dependencies

/K

0 S


185-12 April 2008



STARSTAR

Strangeness production

PR C60 044904 (1999)E802 (AGS/BNL)

Nucl. Phys. A715 (2003) p474

NA49 (SPS/CERN)

Small system versus large system: Strangeness

enhancement reported at the AGS and SPS

• Relative to p+p• K/ ratio

195-12 April 2008



STARSTAR

Charged kaons are enhanced in the Cu+Cu system compared to the Au+Au At the same Npart

Kao

n d

N/d

y

Npart

Charged kaon enhancement

STAR Preliminary

205-12 April 2008



STARSTAR

K/ - Cu+Cu versus Au+Au

K-/- versus Npart no apparent strangeness

enhancement Relative to the spectra

reference

are “enhanced” in the same way as kaons

Question: if pions (and perhaps all species) are “enhanced”, is Npart the relevant variable for comparison?

Look at Nch as the comparison variable

Nch ~

STAR PreliminarySTAR Preliminary

215-12 April 2008



STARSTAR

Systematic study

similar values for all systems vs Nch

System size Center-of-mass energy

225-12 April 2008



STARSTAR

mean-pT vs Nch

STAR Preliminary

235-12 April 2008



STARSTAR

mean-pT vs Npart

STAR Preliminary

245-12 April 2008



STARSTAR

mean-pT vs Npart

STAR Preliminary

255-12 April 2008



STARSTAR

mean-pT vs Nch

All studied systems and energies are described better when using Nch,

STAR Preliminary

265-12 April 2008



STARSTAR

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.

The freeze-out properties (Tch, Tkin, S, at RHIC energies seem to scale by the number of produced charged hadrons at mid-rapidity determined at the initial stages of the collision and driven by the initial

energy density, Tch coincides with the LGT predicted Tc which is around 160 MeV.

In both Cu+Cu and Au+Au collisions, mid-rapidity baryon production at intermediate pT is enhanced compared to that of mesons indicating the coalescence process for hadronization.

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STARSTAR

High-pT and Nch

similar scaling trends? Ncoll relevant at higher pT scaling with Nch (at mid-rapidity) for high-pT data

STAR

Preliminary

STAR

Preliminary

Au+Au 200GeV

Au+Au 62.4GeVAu+Au 200GeV

Au+Au 62.4GeV

++- ++-

Phys. Lett. B 655 (2007) 104

5-12 april 2008 winter workshop on nuclear dynamics star particle production at rhic aneta iordanova...

Documents

chicago winter workshop

star collaboration slide

nuclear dynamics enhancement

rhic aneta iordanova

nuclear dynamics motivation

derived freeze

baryon production slide

ratio slide