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STAR Physics

STAR Physics. Xin Dong Nuclear Science Division / LBNL. 1) QCD in cold nuclear matter 2) QCD in hot nuclear matter sQGP properties QCD phase structure 3) QCD in nucleon spin structure. STAR Physics Progaram. STAR Detector.

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STAR Physics

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  1. STAR Physics Xin Dong Nuclear Science Division / LBNL • 1) QCD in cold nuclear matter • 2) QCD in hot nuclear matter • sQGP properties • QCD phase structure • 3) QCD in nucleon spin structure STAR Physics Progaram

  2. STAR Detector • Large & uniform acceptance at mid-rapidity (exploring forward upgrades) • Excellent particle identification • Fast data acquisition

  3. High Energy Nucleus-Nucleus Collisions Time Initial hard scatterings Partonic stage Hadronic stage Freeze-out Observables Nuclear modification factor (RAA) Elliptic flow (v2) = 2nd Fourier coefficient Sensitive to the early stage properties Characterize the medium effect

  4. RHIC Discoveries “Jet Quenching” Significant suppression in particle yield at high pT in central heavy ion collisions STAR: PRL. 91 (2003) 072304 “Partonic Collectivity” Significant v2 of multi-strange hadrons (f, W) Number-of-Constituent-Quark (NCQ) scaling v2 STAR: PRL. 99 (2007) 112301 Formation of strongly-coupled Quark Gluon Plasma (sQGP)!

  5. Physics Focuses of RNC • 1) Study sQGP Properties • Systematic investigation of partonic collectivity • – Identified particle v2 • Chiral properties / thermal radiation • – Dielectron production (energy dependence) • Thermalization • – Heavy Quark Production (D-meson, J/y) • 2) Study QCD phase structure - Beam Energy Scan • Turn-off of sQGP signatures • – elliptic flow of identified particles • Search for critical point • – high moments of net-proton multiplicity

  6. Partonic Collectivity STAR, QM 2012 • Precision measurements on identified particle v2 from high statistics Au+Au 200 GeV. • 0-30%: baryon-meson grouping / NCQ scaling holds. • 30-80%: Multi-strange hadron v2 deviate from NCQ scaling at mT-m0>1 GeV/c2. • v2(f)<v2(Ks), v2(X)<v2(L) • Precision identified particle v2 data provide constraints to study the sQGP properties. Hiroshi Masui, Md. Nasim (Oct. – Dec. 2011)

  7. Charm production cross section STAR, QM 2011, QM 2012 STAR, PRD 86 (2012) 072013 X.D., Y. Zhang (now at USTC) • Charm cross section in p+p consistent with the upper bound of the FONLL (Fixed-Order-Next-to-Leading-Logrithm) pQCD calculation. • Charm cross section in Au+Au follows the Nbin scaling (significantly improved uncertainty).

  8. Open Charm Hadrons in Au+Au collisions STAR, QM2012 STAR, QM2011 BW fit Model curves: M. He, et al. arXiv: 1204.4442, private communications P. Gossiaux, et al. arXiv: 1207.5445, private communications Y. Zhang (now at USTC) • D-mesons freeze out at different stage than light hadrons. • Small hump structure in D0 RAA in low pT – similar in transport model calculations. • - indication of strong charm-medium interactions (caveat: Cronin effect) • RAA in Au+Au collisions suppressed at pT > 3 GeV/c. • - indication of large charm quark energy loss in the sQGP medium.

  9. J/y Production STAR, QM2012 C. Powell • Systematic measurements in p+p, d+Au and Au+Au collisions: • J/y production in d+Au collisions consistent with EPS09 + sabs(3mb). • In Au+Au collisions, BW predictions with same b (radial flow velocity) as light hadrons different from data in the low pT • - Small (or zero) radial flow velocity • - Other production mechanisms (e.g. regeneration)

  10. J/y v2 – Probe Charm Collectivity STAR, QM2011 L. Grandchamp et al. NPA 790, 415 (2002) 20-60% Initial production vs. charm quark coalescence High pT low pT Peripheral Central H. Qiu Paper in Collaboration review To be submitted to PRL soon. • Disfavor regeneration from thermalized charm quarks at pT > 2 GeV/c • To probe charm collectivity cleanly • Low pT J/y v2 in (more-) central collisions

  11. Dielectrons at Au+Au 200 GeV J. Zhao STAR, QM2011 • Clean penetrating probe to study various stage of HI collisions • Goals: • In-medium vector mesons • Thermal radiation • Data show an enhancement at low mass region compared to the hadron cocktails. • - charm = PYTHIA*Nbin (0.96 mb), real contribution in Au+Au is an open question • Vacuum r cannot reproduce the excess observed in data. • - r in-medium modification

  12. Energy Dependent Dielectron Production P. Huck STAR, QM2012 In-medium r broadening R. Rapp: private communications • Dielectron mass spectra over a broad energy range. • LMR enhancement persists down to 19.6 GeV. • Theoretical calculations of in-medium r broadening with similar baryon densities from 19.6 - 200 GeV reproduce LMR excesses consistently. • - Suggestive of (partial) restoration of chiral symmetry

  13. Future: Thermal Radiation J. Zhao STAR, QM2011 • Transverse momentum spectra slopes: to disentangle charm and QGP radiation • Au+Au result seems to be higher than p+p and PYTHIA charm at ~ 2 GeV. • Suggestive of either charm modifications or other sources. • Future measurements to quantify the correlated charm contribution. • – QGP thermal radiation: Cross section, RAA, v2, a(M, pT)

  14. Beam Energy Scan • 0) Turn-off of sQGP signatures • 1) Search for the signals of • phase boundary • 2) Search for the QCD critical • point BES Phase-I

  15. Inclusive Charged Hadron v2 H. Masui, A. Schmah STAR, PRC in press • pT > 2 GeV/c, consistent v2(pT) from 7.7 GeV to 2.76 TeV • pT < 2 GeV/c, v2 rises with increasing collision energy • Large collectivity / Particle composition

  16. Identified Particle v2 STAR, QM 2012 STAR Preliminary STAR Preliminary mT-m0 (GeV/c2) A. Schmah Papers well advanced in the collaboration, to be released soon. • Significant difference between baryon-antibaryon v2 at lower energies. • No clear baryon/meson grouping for anti-particles at <=11.5 GeV. • NCQ scaling (sQGP signature observed at 200 GeV) is broken! • Hadronic interactions play a significant role at √sNN<= 11.5 GeV.

  17. Higher Moments of Net-protons variance STAR, PRL 105 (2010) 022302 skewness First proof-of-principle measurement kurtosis • Higher moments • - more sensitive to Critical Point induced fluctuations. • Moments of conserved quantities • – related to susceptibilities from Lattice QCD. • Net-protons • – good proxy for net-baryons. • Moment products • – cancel the volume effect. X.F. Luo (now at CCNU)

  18. Higher Moments of Net-protons STAR QM 2012 variance skewness kurtosis • Systematic measurements in various energies and various centralities. • Deviation from Poisson baseline. • UrQMD shows monotonic behavior. • Future precision measurements at low energies. X.F. Luo (now at CCNU) H.G. Ritter

  19. Synergies with Theory Group

  20. Proton Spin Structure

  21. STAR Physics Focus in Future Precision measurements on HF and dileptons: Quantify the sQGP properties (hot QCD) Precision measurements on focused energies Map out the QCD phase structure Precision measurements on pA and eA Study QCD in cold matter

  22. Future RNC Focuses • Heavy quark production: • Heavy quark collectivity: • - the degree of thermalization • Heavy quark RAA and correlations: • - parton energy loss mechanism / medium properties • Di-lepton production: cross section, RAA, v2, a (M, pT) • In preparation of BES-II • Initiate pA/eA program • – nPDF / evolution from cold nuclear matter to sQGP

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