Production of strange particles at RHIC

via quark recombination C.B. Yang

Institute of Particle Physics, Wuhan, China

Collaborated with

Rudolph C. Hwa

University of Oregon

Outline

MotivationsRecombination modelKaon production productionProduction of and Summary and discussion

1. Motivations• Why we study strange particle production

Most strange particles are produced in the interactions

Relevant to the deconfinement and flavor equilibrium

Dependence on the colliding systemsDependence on the cms energyPossible signal for QGP formationGood test for production mechanism

2. Recombination model• Hadrons are formed by combining quarks• No gluon contribution is considered• This model is successful in explaining non-

strange particle production at RHIC in central and forward directions

• Fragmentation is interpreted as a quark recombination process

• Recombination of soft partons is more effective for producing hadrons with intermediate momentum

Parton distribution (log scale)

(recombine) (fragment)

p

p1+p2p q

meson momentum

higher yield heavy penalty

Why Recombination?

Different implementations

• Duke group etc: 6-dimensional phase spaceusing Wigner function and density matrix

• Oregon group:one-dimensional momentum spaceusing phenomenological recombination function

, ,..., ,...3 31 2 1 2

1 2 31 2 3

( , , ) ( , , )p

pdx xdx dx x xdNx F x x x R

dx x x x x x x

Basic formulas

F: joint parton distributionsR: recombination functions

Ingredients• Soft partons are assumed having exponen

tial pT distributions• Hard parton spectra are calculated from p

QCD, considering shadowing effect etc• Shower parton distributions are known fro

m fitting fragmentation data• Recombination functions are known from f

ormer studies• Energy loss effect can be taken into accou

nt phenomenologically

3. Kaon Production

• Soft partons: known for light quarks; assumed to be exponential in pT for strange quarks with two parameters

• Semihard shower parton distributions known already

• Recombination function for Kaon determined

• Fix the density of strange quarks in the medium by fitting the low pT spectrum

1 2 1 2 1 2

1 1 2 2

1 2 1 2 1 2

( , , ) ( , )

/ , /

1( , ) ( 1)

( 1, 1)

M M M

a bM

R p p p g y y G y y

y p p y p p

G y y y y y yB a b

0

1 1 1 11

2 2

{ }

( ) exp( / )

( ) ( ) ( / )

s s d s sd s

thq

i iik

F TT T S TS SS

dNT p p Cp p T

dp

S p dkkf k S p k

1, (3 1) / 2 2, 1Kg

0

1 1 1 11

2 2

( ) exp( / )

( ) ( ) ( / )

ths

s s s

ss i ii

k

dNT p p C p p T

dp

S p dkkf k S p k

C=23.2 GeV-1, T=0.317GeV

Cs , Ts and ξdetermined from fitting Kaon spectrum

Kaon spectrum

Cs=15.5 GeV-1, Ts=0.323 GeV

4. ΛProduction

• Leading parton contribution to Λspectrum?

• Recombination function is assumed as

1 2 3 1 2 3 1 2 3

1 2 3 1 2 3 1 2 3

( , , , ) ( , , )

/

1( , , ) ( ) ( 1)

( 1, 2) ( 1, 1)

i i

R p p p p g y y y G y y y

y p p

G y y y y y y y y yB B

•Statistical factor for Λ is ¼

• 1, (3 1) / 2 2

ΛSpectrum

No leading partoncontribution

With leading partoncontribution

Λ/K ratio

5. Production of and

They are a measure of the initial state– no rescattering with other hadrons– sensitive to the initial state

Their masses are almost the sum of that of constituent quarks

weak interactions among quarksloosely bound states

Assume valon distributions are

1 2 1 2

1 2 3 1 2 3

( , ) ( 1/ 2) ( 1/ 2)

( , , ) ( 1/ 3) ( 1/ 3) ( 1/ 3)

G y y y y

G y y y y y y

Strange parton distributions are thesame as for other hadrons

Φ spectrum

Ω spectrum

0.3, 0.008g g

Ω/Φ ratio

6.Summary and discussions

• K, Λ,Φ,Ω spectra can be understood in the framework of the recombination model with only three free parameters

• At low pT<4GeV/c, thermal quark recombination dominates for K and Λ

• Shower parton contribution is important for intermediate pT for K and Λ

6.Summary and discussions

• For Φ and Ω,up to pT=8GeV/c, thermal quark recombination dominates, but the shower contribution is visible in the Ω / Φ ratio

• Competition effect plays an important role in explaining the yields of Φ and Ω