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System Size and Energy Dependence of - meson Production at RHIC

Motivation - meson analysis and results - meson from Cu+Cu 200 GeV and 62.4 GeV System size and energy dependence of - meson production at RHIC Summary. System Size and Energy Dependence of - meson Production at RHIC J.H. Chen a,b) for the STAR Collaboration

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System Size and Energy Dependence of - meson Production at RHIC

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  1. Motivation • -meson analysis and results • -meson from Cu+Cu 200 GeV and 62.4 GeV • System size and energy dependence of -meson production at RHIC • Summary System Size and Energy Dependence of -meson Production at RHIC J.H. Chena,b)for the STAR Collaboration a) Nuclear Physics division, Shanghai Institute of Applied Physics, b) Department of Physics and Astronomy, University of California at Los-Angeles

  2. 1. <pT>, -meson decoupled early; 2. N()/N(K), ruled out the K-coalescence; 3. Evolution in the centrality dependence;  Cu+Cu collisions provide unique data to further probe these features. -meson production from Au+Au <pT> f/K- STAR Col. Phys. Lett. B 612, 181 (2005), Phys. Rev. Lett. 99, 112301 (2007). Quark Matter 2008, Feb 4-10, India

  3. Since -mesons are made via coalescence of seemingly thermalized s quarks in central Au+Au collisions, the observations imply hot and dense matter with partonic collectivity has been formed at RHIC. STAR Col. Phys. Rev. Lett. 99, 112301 (2007). W and f mostly from bulk s quarks And bulk s quarks have collective elliptic flow ! Quark Matter 2008, Feb 4-10, India

  4. STAR TPC used to identify KaonviadE/dx in TPC gas STAR Detector • We used the high-statistics 200 GeV and 62.4 GeV Cu+Cu data to measure the -meson production at STAR: • ~10.6M 200 GeV events (0-54%); • ~8.8M 62.4 GeV events (0-60%). The STAR experiment • Event-mixing method used to estimate combinatorial background from uncorrelated K+K- pairs; • Final subtracted minv distribution fitted with Breit-Wigner + straight line. Quark Matter 2008, Feb 4-10, India

  5. 0-10% 10-19% 19-28% 28-36% 36-45% 45-54% STAR Preliminary • The  mass is consistent with PDG value at pT>0.7 GeV/c while a drop of ~2.5 MeV at lower pT • The  mass value from data and simulation are consistent and the drop at low pT is understood within detector effects  mass 0-10% 10-19% 19-28% 28-36% STAR Preliminary Quark Matter 2008, Feb 4-10, India

  6. Levy function describe well the data (exponential in central and power-law-like in peripheral) • Similar trend in Cu+Cu and Au+Au at the similar Npart at same collision energy STAR Preliminary spectra Quark Matter 2008, Feb 4-10, India

  7. STAR Preliminary dN/dy and <pT> • The dN/dy and <pT> are similar for Cu+Cu and Au+Au at similar Npart bin for the same collisions energy. • f yields from Au+Au and Cu+Cu collisions depend on the number of participant nucleons only, unlike Kaon and hyperons. see STAR Xiaobin Wang – parallel session VII [1] NA49 Col. Phys. Rev. Lett. 96, 052301 (2005); [2] E-802 Col. Phys. Rev. C 60, 044904 (1999). Quark Matter 2008, Feb 4-10, India

  8. Nuclear Modification factor STAR Preliminary Comparing central Au+Au and central Cu+Cu with peripheral Au+Au Collisions— 1) the yield and pT shape depend on number of participants only 2) in terms of number of binary collisons the Cu+Cu data is higher indicating given the same top 10% centrality Cu+Cu has less number of collisions per participant Quark Matter 2008, Feb 4-10, India

  9. Strangeness enhancement: yield relative to p+p -meson enhancement: -- between K/L and Xi -- 200 GeV data > 62.4 GeV, unlike hyperons -- could not be solely due to the canonical suppression, there could be dynamics effect STAR Preliminary Strangeness enhancement X K, L 200 GeV 62.4 GeV Quark Matter 2008, Feb 4-10, India

  10. Summary • We present a study of the system size and energy dependence of -meson production at RHIC. • The transverse momentum spectra in both shape and yields are similar in Cu+Cu and Au+Au for collisions with similar Npart • -meson enhancement shows a distinct collision centrality and energy dependence. The enhancement factor of the -meson production (yield per Npart) lies between those of K/L and X, and decreases from 200 GeV to 62.4 GeV data unlike hyperons. The -meson enhancement cannot solely due to canonical suppression of strangeness production, there could be dynamics effect. Quark Matter 2008, Feb 4-10, India

  11. A decreasing trend in N()/N(p) vs. pT would be an unambiguous signature for early decoupling of  mesons from the hadronic rescattering dynamics[1] • Fitting with Hirano’s model[1] yield: • p+p, ChiSquare/ndf = 6.01/8 • Au+Au w rescattering, ChiSquare/ndf = 11.42/8 • Au+Au w/o rescattering, ChiSquare/ndf = 29.14/8 • Au+Au, Tth=100 Mev, ChiSquare/ndf = 7.75/8 [1] Hydro calculation: arXiv:0710.5795v1 N()/N(p) Quark Matter 2008, Feb 4-10, India

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