Strange Baryon Production at RHICHui Long

Department of PhysicsUniversity of California, Los Angeles

1. Introduction

2. Strangeness measurement and results

> results of bulk properties

( yield, freeze-out T, event anisotropy )

> results from intermediate PT ( <10 GeV/c) on

Nuclear modification factor of Λ and Ks.

3. Summary

(p, d,…Au)100 GeV/nucleon

PHOBOS BRAHMS

STARPHENIX

Introduction : Physics Goal of RHIC

Identify and study the properties of the matter with partonic degree of freedom:

- nuclear effects in the intermediate pT regioninitial conditions;parton energy loss due to interaction of dense matter.

- bulk propertiescollision dynamics;collective motion with the partonic degree of freedom;early thermalization (c-quark).

Strange particles can be good probes to both !!!

(GeV/c)tP

0 0.5 1 1.5 2

-2dy

(GeV

/c)

tN

/dP

2 d t1/

Pev

t1/

Nπ

1/2

10-2

10-1

1

10

102

103

2×

21×

(min-bias)Λ

(min-bias)Λ

(central)Λ

(central)Λ

Strange Baryon Measurement at RHIC ( 130 GeV)

Λ p + π-

Ξ− Λ + π-

Ω− Λ + κ-

-2 )2

dy

(GeV

/c⊥

N/d

m2 d ⊥

mπ1/

(2ev

t1/

N

10-6

10-5

10-4

10-3

10-2

10-1

1

-Ξ(a)

)2- m (GeV/c⊥m

+Ξ(b)

1 2

+Ω+-Ω(c)

[0-10%][0-10%] [10-25%] (/5) [25-75%] (/10)

01 201 20

STAR

PHENEX

Strange Baryon Yield at Mid-rapidity (130 GeV)

Ω+⎯Ω

STAR Preliminary

Nearly linear dependence on number of charged particles

Ratio of Particle Yield and Statistic Model:..T, µb γs

( integrated yield dominated by low pt particles)

One of the bulk properties: Tch ~ 176 MeV(Chemical freeze-out: particle’s yield remain unchanged since )

ρ = tanh−1 βr

R

β s

E.Schnedermann et al, PRC48 (1993) 2462

dnmT dmT

∝ r dr mT K1mT cosh ρ

T⎛ ⎝

⎞ ⎠

0

R

∫ I0pT sinh ρ

T⎛ ⎝

⎞ ⎠

where

βT =βs (r/R)n

Strange Baryon and Hydro-dynamic Fit (130 GeV)1 σ, 2σ , 3 σ contours

Ξ s behave differently !! Higher Tfo-- >Freeze-out earlier

Tfo ~ Tch -- transverse flow at/before chemical freeze-out, maybe at partonic stage ?

Hydro-dynamic fit: T, β ( Thermal + Transverse flow )

Thermal freeze-out: particle pT distribution stable since

βΤ

Event Anisotropy– What’s Elliptic Flow ?

( )( )3 2

31

1 1 2 cos2 n r

nt t

d N d NE v nd p p dp dy

ϕπ

∞

=

⎛ ⎞= + −Ψ⎜ ⎟

⎝ ⎠∑

coordinate spacey

x

12 , tan ( )cos2( ) y

xrv

pp

ϕϕ −== −ΨMomentum space

py

pxEffected by pressure gradient, surface emission pattern, final re-scattering.

Features of Elliptic Flow at RHIC

Low pt : hydro-like collective behaviorHigh pt : lower than hydro-limit and the trend

consistent with expectation from jet-quenching due to parton energy loss.

Strange Hadron (Ks and Λ ) V2 (200 GeV)

Hydro behavior at pt<2 GeV

, V2(Λ) <V2(K)

When pt> 2.5GeV, V2(Λ) >V2(K) !!!

--Energy Loss ?--Coalescence ?

Partonic dof relevant ?

Strange Particle v2 and Coalescence

V2 ~ n

-> Ξ , Ω v2 = Λ v2

Partonic collectivityat RHIC ?

Z Lin, PRL, 89,202302(02)S. Voloshin, nu-ex/0210014R. Fries et al. nucl-th/0301087D. Molnar et al. nucl-th/0302014

Strange Baryon in Intermediate PT : Nuclear Modification

Npart: Number of participants number of incoming nucleons (participants) in the overlap region

ησηddpdT

ddpNdpRT

NNAA

TAA

TAA //)( 2

2

=

<Nbinary>/σinelp+p

N-N cross sectionNuclear Modification Factor:

If no “effects”:R < 1 in regime of soft physicsR = 1 at high-pt where hard

scattering dominates

Nbin: Number of binary collisions number of equivalent inelastic nucleon-nucleon collisions

Suppression of High pT Charged Hadrons

Preliminary √sNN = 200 GeV

1.0

1.0

1.0

Strong Suppressionin central Au+Au

N-Binary Scalingin peripheral Au+Au

Similar suppression pattern for π0 and for 130 GeV Au+Au dataand also observed by PHENIX

Used UA1 p+p 200 GeV data (pT<6 GeV/c) as reference

Rcp of Strange Particles

Different pt dependencesbetween Rcp(Λ) and Rcp(K)

At pt~5 GeV/c, all the Rcpapproach each other.

0-2 GeV, Flow effects ?

2-4 GeV, Coalescence ?

>5GeV, Fragmentation ?

partonicmatter

Summary (I)

bulk properties

Yield and Freeze-out T

-almost linear increase with charged hadrons ;

- earlier freeze-out and high freeze-out T of multi-

strange baryon Ξ - partonic radial flow ?

Event anisotropy

- hydro behavior of v2 at pt< 2GeV/c;

- heavier particle has larger saturated v2 at pt>2 Gev/c; ? Coalescence model ? Partonic Collectivity ?

Summary (II)

Nuclear Effects

-Suppression of charged hadron at pt<12 GeV/c ;

-Rcp dependent on particle type at pt< 5 GeV/c;

<- flow ( 0- 2 GeV/c) ?

<- Coalescence (2- 4 GeV/c) ? ( partonic matter ?)

<- fragmentation ( > 5 GeV/c) ?

Multi-strange v2 and Rcp measurement in AA and dA coming soon !!!