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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 !!!
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