Nuclear Modification of Identified Strange Particles at Moderate p T

1. Nuclear Modification of Identified Strange Particles at Moderate pT in Au+Au Collisions at at RHIC Hui Long University of California, Los Angeles For the STAR Collaboration • Outline: • 1. Motivations • 2. Invariant mass spectra and pT spectra of Ks, L+L, X+X • 3. Nuclear modification factor (RAA) of L+L, X+X ,Ks • 4. L/K and X/L • 5. Summary + + Hui Long, SQM2003

2. Motivations The measurements of moderate pT particle production at RHIC have inspired some new theoretical explanations for the particle production mechanisms/dynamics . *asuppression of charged hadrons and neutral pions in AA collisions with respect tobinary scaling Energy loss, gluon density saturation, final state re-scattering, etc. ?? *v2 saturation for charged hadrons  surface-emission ..?？ *different v2 saturation value for KS and L ( P. Sorensen’s talk) partonic state+string fragmentation, parton fragmentation or quark coalescence..?? With the measurements of strange particles, we can study the details of particle production dynamics, such as, the effects of baryon vs. meson, mass dependency and strangeness content. Hui Long, SQM2003

3. 0 Strange Particle Reconstruction: invariant mass spectra of Ks and L(L) star preliminary AuAu 5%, pt>0.4,|y|<1 minbias AuAu pt>0.4 , |y|<1 0-5% s= 5-8 MeV s= 1.9-4 MeV Hui Long, SQM2003

4. Strange Particle Reconstruction: X+X s~ 4 MeV Background from Event Rotating AuAu Pt>0.8 GeV/c |y|<1 Hui Long, SQM2003

5. Strange Particle Spectra : statistical errors only Hui Long, SQM2003

6. Strange Particle Spectra : X+X statistical errors only. AuAu star preliminary 0-5% 0-5%: dn/dy=4.5 ( 0.1) 40-60% Mt exponential fit 60-80% 0-10% ,130 GeV dn/dy=2.30+1.89= 4.29 ( 0.1)(0.8) Hui Long, SQM2003

7. : Nuclear Modification Factor: Hui Long, SQM2003

8. RAAof Strange Baryons and Mesons: A significant difference is seen between the pt dependence of KS and Λ RAA. For pt from 1.8-3.5 GeV/c in central collisions, Λ production approximately follows Nbin scaling. At higher pthowever,a suppression with respect to Nbin scaling is seen for both KS and Λ. Hui Long, SQM2003

9. RAA of Strange Baryons RAA~1 and no strong mass dependence for strange baryons in 1.8<pT<3.2 GeV/c. The absence of suppression of baryons may indicate production dynamics beyond parton energy loss and frag-mentation. *mass dependence in energy loss? *parton recombination? (v2 talk from P.Sorensen) Hui Long, SQM2003

10. Particle Ratios vs. pT: *Faster increase of L/K ratio in central collisions than peri-pheral collisions up to 3 GeV/c. *Fast drop in pT>3 GeV. *Nearly flat X/L compared to the L/K , indicating similar pT spectra of L and X. Strange baryon production dynamics is very different from the dynamics of strange meson. Recombination+Fragmentation (nucl-th/0301087) ? Hui Long, SQM2003

11. Summary: • The measurements of RAA show that the production of Kaons in central collisions is suppressed to a greater degree than strange baryons (L, X). The RAA of strange baryons is about 1 for pT between 1.8 and 3.2 GeV/c, indicating production dynamics beyond the framework of parton energy loss and fragmentation. • For pT~5.5 GeV/c, the RAA of Ks and L approach each other and the charged hadron RAA, perhaps indicating the pT scale where parton fragmentation may begin to dominate. • The pT dependence of the measured strange baryon productions is similar in central AA collisions.  similar v2 for strange baryons? • Strange baryon production at pT~3 GeV shows a strong enhancement in central collisions, with respect to strange meson production. Hui Long, SQM2003