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Identified particle production in the Beam Energy Scan from STAR. Anthony Timmins for the STAR Collaboration. The Beam energy scan The STAR experiment Pions, protons, and kaons. Freeze out parameters Strange and multi-strange hyperons Summary. Beam energy scan.

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Identified particle production in the Beam Energy Scan from STAR


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    1. Identified particle production in the Beam Energy Scan from STAR Anthony Timmins for the STAR Collaboration • The Beam energy scan • The STAR experiment • Pions, protons, and kaons. • Freeze out parameters • Strange and multi-strange hyperons • Summary

    2. Beam energy scan • Lattice QCD predicts 1st order phase transition • At critical point, transition becomes cross-over • Can we find it? • Scan beam energies (BES): • Vary T and μB simultaneously… Schematic representation

    3. Beam energy scan • Determine onset of de-confinement • Lower beam energy, lower energy density • Get an experimental handle on εc • Key signatures: • Strangeness production • Baryon/meson differences J. Phys. G 32 (2006) S105-S114

    4. The STAR experiment • TPC and TOF used for subsequent analyses • Fixed acceptance wrt beam energy • Advantage over SPS • Energies scanned: • Run 8: 9.2 GeV (no TOF) • Run 10: 7.7, 11.5, 39, 62.4, 200 GeV • Run 11: 19, 27 GeV

    5. Particle identification • Excellent PID in STAR for primary tracks: • dE/dx: Proton/pion separation to p~1 GeV/c • TOF: Proton/pion separation top~3 GeV/c

    6. V0 and cascade reconstruction • Weak decays reconstructed in the STAR TPC • V0 finding • Cascade finding • No momentum limit for PID. • Top Au+Au energy highest values (stats limited) • V0 pT ~ 9 GeV • Cascade pT ~ 5 GeV Au+Au 7 GeV CPOD 2011

    7. Pions, protons, kaons at Au+Au 39 GeV • BES spectra obtained with TPC and TOF: • Consistent with dE/dx in overlapping range QM & CPOD 2011

    8. Pions, protons and kaons (all energies) QM & CPOD 2011 • Kaon and proton yields increase relative to pions with decreasing energy • Larger baryon transport to mid rapidity.

    9. Pions, protons and kaons (all energies) QM & CPOD 2011 • Increase in anti-particles relative to pions with increasing energy

    10. Freeze out parameters QM & CPOD 2011 • Use 2 models to determine freeze-out properties. • Blast wave model • Obtains Tkin and <β> • Fit proton, kaon, pion spectra (PRC 70 (2004) 044907) • Thermal model • Obtains Tchem and μB • Uses Grand Canonical ensemble (Comp. phys. Comm. 180 (2009) 84) • Fit proton, kaon, pion yields

    11. Freeze out parameters QM & CPOD 2011 • Kinetic freeze temperature and expansion velocity depend on centrality and beam energy.

    12. Freeze out parameters • First observation: • Tchem and μB depend on centrality • Stronger dependence for μB • Centrality offers further dial in critical point search. • Result holds when strange particles included (not shown) CPOD 2011

    13. Kaon and pions in Cu+Cu 22.4 GeV • Ratios higher in lighter systems with similar <Npart> at AGS/SPS • CM energy < 22.4 GeV, yields over 4π • PRC 60, (1999) 044904, NPA 715 (2003) 474c • Not the case for RHIC at lower energy • Top RHIC energy: • Yields of strange and non strange particles higher in Cu+Cu • Ratio the same SQM 2011

    14. Strange and multi-strange hyperons • Extensive strange particle spectra • (Anti) Lambda corrected for Cascade feed-down CPOD 2011

    15. Hyperons yields • dN/dy shown for central data • STAR results closer to NA49 • Data from central collisions: • NA49: PRC78,034918 • NA57: PLB595,68; JPG32, 427 • STAR: PRL86, 89, 92, 98, PRC83 CPOD 2011

    16. Hyperons ratios • Anti-particle/particle ratios increase for peripheral events • Lower baryon transport • Consistent with μB decreasing for peripheral events CPOD 2011

    17. Strange baryon/meson ratios CPOD 2011 • Mid-pT ratios get higher at lower energy • More baryon stopping? • Centrality dependence for Au+Au 39 GeV • Breaks at lower energies?

    18. Strange baryon/meson ratios CPOD 2011 • Mid-pT ratios get lower at lower energies • Ratios still rise from low to mid-pT at lower energies

    19. Nuclear modification factor • No K0s suppression for Au+Au 7.7 and 11.5 GeV • Baryon-meson splitting in Au+Au 39 reduces for lower energies. CPOD 2011

    20. Summary • Results from spectra and yields critical in characterizing state of matter in BES. • Bulk Production • Particle ratios, Tkin and β depend on energy and centrality • First observation of Tchem and μB centrality dependence • Strangeness production • Anti particle/particle ratios depends on centrality • Baryon/meson differences disappear in Au+Au 7.7 and 11.5 GeV

    21. Outlook… • Run 11 Au+Au 19 and Au+Au 27 GeV data on the way! Statistical uncertainties only DNP 2011