1. Energy Dependence of ϕ -meson Production and Elliptic Flow in Au+Au Collisions at STAR Md. Nasim (for the STAR collaboration) NISER, Bhubaneswar, India Critical Point and Onset of Deconfinement 11 – 15 March 2013, Napa Valley, USA Outline:  Motivation  STAR Experiment  Results  Summary

2. Motivation 2 LHC Experiments The RHIC Beam Energy Scan (BES)  BES Motivations Search for phase boundary Search for Critical point  Observables of de-confinement a) Strange hadron dynamics N(Ω)/N( ϕ ) and R CP ( ϕ ) b) Strange hadron collectivity Elliptic flow (v 2 ) of ϕ 3/13/13CPOD 2013

3. Why  -meson ? 3 J. Phys. G 36, 064022 (2009), AMPT model In the hadronic case, no number- of-quark scaling and v 2 (  ) << v 2 of hadrons Phys. Rev. C 79, 067901 (2009) Phys. Rev. C 87, 014903 (2013) 3/13/13CPOD 2013 STAR Collaboration : Nuclear Physics A 757 (2005 )

4. STAR Experiment 4 ToF BBC PMD EMC Barrel EMC End Cap TPC HLT FTPC √s NN (GeV) Good MB events in Million 7.7~ 4 M 11.5~ 12 M 19.6~ 36 M 27~ 70 M 39~ 130 M 62.4~ 67 M dN evt / (N evt dN ch ) Uncorrected N ch  Collisions: Au+Au  Collisions centrality from uncorrected dN ch /dη in |η| < 0.5 3/13/13CPOD 2013

5. Particle Identification and v 2 measurement method 5 η-sub event plane method: STAR, Phys. Rev. C 77, 054901 (2008) 3/13/13CPOD 2013 Au+Au 39 GeV |η| < 0.9

6. ϕ -meson p T spectra 6  meson transverse momentum distribution can be well described by a Levy function STAR preliminary 3/13/13CPOD 2013 Statistical error

7. 7  R CP ≥ 1 at intermediate p T for √s NN ≤ 19.6 GeV. → no suppression K 0 S R CP : 0-05%/40-60%  R CP : 0-10%/40-60% and 0-05%/40-60% for 200 GeV 3/13/13CPOD 2013 Nuclear Modification Factor (R CP ) Statistical error

8. Strange Quark Dynamics 8  Intermediate p T Ω/ ϕ ratios: Indication of separation between ≥ 19.6 and 11.5 GeV.  Derived strange quark p T distributions show a trend of separation between ≥ 19.6 and 11.5 GeV. Change of Ω production mechanism ? Parton recombination fails at 11.5 GeV ? Hwa & Yang, Phys. Rev. C 75, 054904 (2007), Phys. Rev. C 78, 034907 (2008). STAR preliminary 3/13/13CPOD 2013 STAR preliminary Statistical + Systematic error STAR preliminary

9. Strange Quark Collectivity 9 Mass: proton ~  At low p T, v 2 (  )/v 2 (p) decreases with decreasing beam energies → Indicating less partonic collectivity with decreasing beam energy. STAR Collaboration: arXiv: 1301.2348 and arXiv:1301.2347, Phys. Rev C 87, 014903 (2013 ) STAR preliminary Au+Au, √s NN = 200 GeV At low p T, v 2 (  )/v 2 (p) > 1.0 → strange quark collectivity comparable to light quark. → proton v 2 possibly affected by hadronic re-scattering. 3/13/13CPOD 2013 Statistical error only Au+Au 200 GeV Hydro + cascade Phys. Rev. C 77, 044909 (2008)

10. NCQ scaling: ϕ -meson v 2 10  ϕ meson v 2 deviates from other particles ~ 2σ at the highest p T data in 7.7 and 11.5 GeV collisions. → Small or zero v 2 for ϕ meson implies hadronic interactions are more important at lower energies. More data for 7.7 and 11.5 GeV are needed for clear conclusion. Ref: B. Mohanty and N. Xu: J. Phys. G 36, 064022(2009) and Md. Nasim, B. Mohanty, N. Xu, Phys. Rev C 87, 014903 (2013 ) STAR Collaboration: arXiv: 1301.2348 and arXiv:1301.2347 ( PRL in press) 3/13/13CPOD 2013 Statistical error only

11. Summary 11  STAR preliminary ϕ meson spectra and elliptic flow in Au+Au collisions at √s NN = 7.7-62.4 GeV have been presented.  R CP ≥ 1 at intermediate p T for √s NN ≤ 19.6 GeV. → No suppression.  Intermediate p T Ω/ ϕ ratios and derived strange quark p T distribution show a indication of separation between ≥ 19.6 GeV and 11.5 GeV. → Suggests change of production mechanism. Indication of phase transition?  At low p T, v 2 (  )/v 2 (p) decreases with decreasing beam energies. → Indicating less partonic collectivity with decreasing beam energy. Top RHIC Energy (√s NN =200GeV) : At low p T, v 2 (  )/v 2 (p) > 1.0. → Could be the effect of hadronic re-scattering.  ϕ meson v 2 deviates from other particles ~ 2σ at the highest p T data in 7.7 and 11.5 GeV collisions. → It may indicate hadronic interactions are more important at lower energies. 3/13/13CPOD 2013

12. Back-Up 123/13/13CPOD 2013

13. 133/13/13CPOD 2013 ϕ and proton v 2 v 2 (p)/v 2 ( ϕ ) > 1 ~ mass effect < 1 ~ hadronic re-scattering effect Hydro + cascadeHydro T. Hirano et al. Phys. Rev. C 77, 044909 (2008)