1. 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

2. Outline  Motivations  Recombination model  Kaon production   production  Production of  and   Summary and discussion

3. 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 systems  Dependence on the cms energy  Possible signal for QGP formation  Good test for production mechanism

4. 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

5. Parton distribution (log scale) (recombine)(fragment) p p 1 +p 2 pq meson momentum higher yieldheavy penalty Why Recombination?

6. Different implementations Duke group etc: 6-dimensional phase space using Wigner function and density matrix Oregon group: one-dimensional momentum space using phenomenological recombination function

7. Basic formulas F: joint parton distributions R: recombination functions

8. Ingredients Soft partons are assumed having exponential p T distributions Hard parton spectra are calculated from pQCD, considering shadowing effect etc Shower parton distributions are known from fitting fragmentation data Recombination functions are known from former studies Energy loss effect can be taken into account phenomenologically

9. 3. Kaon Production Soft partons: known for light quarks; assumed to be exponential in p T 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 p T spectrum

11. C=23.2 GeV -1, T=0.317GeV C s, T s and ξdetermined from fitting Kaon spectrum

12. C s =15.5 GeV -1, T s =0.323 GeV

13. 4. ΛProduction Leading parton contribution to Λspectrum? Recombination function is assumed as Statistical factor for Λ is ¼

14. ΛSpectrum No leading parton contribution With leading parton contribution

15. Λ/K ratio

16. 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 quarks  loosely bound states

17. Assume valon distributions are Strange parton distributions are the same as for other hadrons

18. Φ spectrum

19. Ω spectrum

20. Ω/Φ ratio

21. 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 p T <4GeV/c, thermal quark recombination dominates for K and Λ Shower parton contribution is important for intermediate p T for K and Λ

22. 6.Summary and discussions For Φ and Ω,up to p T =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