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 oLattice QCD predicts 1 st order phase transition  At critical point, transition becomes cross-over Can we find it? oScan beam energies (BES): Vary T and μ B simultaneously… Schematic representation

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

4 The STAR experiment oTPC 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 oExcellent PID in STAR for primary tracks: dE/dx: Proton/pion separation to p~1 GeV/c TOF: Proton/pion separation to p~3 GeV/c

6 V0 and cascade reconstruction oWeak decays reconstructed in the STAR TPC V0 finding Cascade finding oNo momentum limit for PID. oTop Au+Au energy highest values (stats limited) –V0 p T ~ 9 GeV –Cascade p T ~ 5 GeV Au+Au 7 GeV CPOD 2011

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

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

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

10 Freeze out parameters oUse 2 models to determine freeze-out properties. 1.Blast wave model Obtains T kin and Fit proton, kaon, pion spectra (PRC 70 (2004) ) 2.Thermal model Obtains T chem and μ B Uses Grand Canonical ensemble (Comp. phys. Comm. 180 (2009) 84) Fit proton, kaon, pion yields QM & CPOD 2011

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

12 Freeze out parameters oFirst observation: T chem and μ B depend on centrality Stronger dependence for μ B  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 at AGS/SPS CM energy < 22.4 GeV, yields over 4π PRC 60, (1999) , 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 oExtensive strange particle spectra o(Anti) Lambda corrected for Cascade feed-down CPOD 2011

15 Hyperons yields CPOD 2011

16 Hyperons ratios oAnti-particle/particle ratios increase for peripheral events Lower baryon transport oConsistent with μ B decreasing for peripheral events CPOD 2011

17 Strange baryon/meson ratios oMid-p T ratios get higher at lower energy –More baryon stopping? oCentrality dependence for Au+Au 39 GeV –Breaks at lower energies? CPOD 2011

18 Strange baryon/meson ratios CPOD 2011 oMid-p T ratios get lower at lower energies oRatios still rise from low to mid-p T at lower energies

19 Nuclear modification factor oNo K 0 s suppression for Au+Au 7.7 and 11.5 GeV oBaryon-meson splitting in Au+Au 39 reduces for lower energies. CPOD 2011

20 Summary oResults from spectra and yields critical in characterizing state of matter in BES. oBulk Production Particle ratios, T kin and β depend on energy and centrality First observation of T chem and μ B centrality dependence oStrangeness production Anti particle/particle ratios depends on centrality Baryon/meson differences disappear in Au+Au 7.7 and 11.5 GeV

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