STAR Physics Xin Dong Nuclear Science Division LBNL

STAR Physics Xin Dong Nuclear Science Division / LBNL 1) QCD in cold nuclear matter STAR 2) QCD in hot nuclear matter Physics • s. QGP properties Progaram • QCD phase structure 3) QCD in nucleon spin structure Nov. 27 th, 2012 Director’s Review X. Dong

STAR Detector • Large & uniform acceptance at mid-rapidity (exploring forward upgrades) • Excellent particle identification • Fast data acquisition Nov. 27 th, 2012 Director’s Review X. Dong 2

High Energy Nucleus-Nucleus Collisions Time Initial hard scatterings Partonic stage Hadronic stage Freeze-out Observables Nuclear modification factor (RAA) Elliptic flow (v 2) = 2 nd Fourier coefficient Characterize the medium effect Nov. 27 th, 2012 Sensitive to the early stage properties Director’s Review X. Dong 3

RHIC Discoveries STAR: PRL. 91 (2003) 072304 “Jet Quenching” Significant suppression in particle yield at high p. T in central heavy ion collisions “Partonic Collectivity” v 2 STAR: PRL. 99 (2007) 112301 Significant v 2 of multi-strange hadrons (f, W) Number-of-Constituent-Quark (NCQ) scaling Formation of strongly-coupled Quark Gluon Plasma (s. QGP)! Nov. 27 th, 2012 Director’s Review X. Dong 4

Physics Focuses of RNC 1) Study s. QGP Properties • Systematic investigation of partonic collectivity – Identified particle v 2 • 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 s. QGP signatures – elliptic flow of identified particles • Search for critical point – high moments of net-proton multiplicity Nov. 27 th, 2012 Director’s Review X. Dong 5

Partonic Collectivity STAR, QM 2012 Precision measurements on identified particle v 2 from high statistics Au+Au 200 Ge. V. • 0 -30%: baryon-meson grouping / NCQ scaling holds. • 30 -80%: Multi-strange hadron v 2 deviate from NCQ scaling at m. T-m 0>1 Ge. V/c 2. v 2(f)<v 2(Ks), v 2(X)<v 2(L) Precision identified particle v 2 data provide constraints to study the s. QGP properties. Hiroshi Masui, Md. Nasim (Oct. – Dec. 2011) Nov. 27 th, 2012 Director’s Review X. Dong 6

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) p. QCD calculation. • Charm cross section in Au+Au follows the Nbin scaling (significantly improved uncertainty). Nov. 27 th, 2012 Director’s Review X. Dong 7

Open Charm Hadrons in Au+Au collisions STAR, QM 2012 STAR, QM 2011 BW fit Y. Zhang (now at USTC) Model curves: M. He, et al. ar. Xiv: 1204. 4442, private communications P. Gossiaux, et al. ar. Xiv: 1207. 5445, private communications • D-mesons freeze out at different stage than light hadrons. • Small hump structure in D 0 RAA in low p. T – similar in transport model calculations. - indication of strong charm-medium interactions (caveat: Cronin effect) • RAA in Au+Au collisions suppressed at p. T > 3 Ge. V/c. - indication of large charm quark energy loss in the s. QGP medium. Nov. 27 th, 2012 Director’s Review X. Dong 8

J/y Production STAR, QM 2012 C. Powell Systematic measurements in p+p, d+Au and Au+Au collisions: • J/y production in d+Au collisions consistent with EPS 09 + sabs(3 mb). • In Au+Au collisions, BW predictions with same b (radial flow velocity) as light hadrons different from data in the low p. T - Small (or zero) radial flow velocity - Other production mechanisms (e. g. regeneration) Nov. 27 th, 2012 Director’s Review X. Dong 9

J/y v 2 – Probe Charm Collectivity STAR, QM 2011 L. Grandchamp et al. NPA 790, 415 (2002) 20 -60% H. Qiu Paper in Collaboration review To be submitted to PRL soon. Initial production vs. charm quark coalescence High p. T low p. T Peripheral Central • Disfavor regeneration from thermalized charm quarks at p. T > 2 Ge. V/c • To probe charm collectivity cleanly Low p. T J/y v 2 in (more-) central collisions Nov. 27 th, 2012 Director’s Review X. Dong 10

Dielectrons at Au+Au 200 Ge. V STAR, QM 2011 J. Zhao • Clean penetrating probe to study various stage of HI collisions • Goals: o In-medium vector mesons o 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 Nov. 27 th, 2012 Director’s Review X. Dong 11

Energy Dependent Dielectron Production P. Huck STAR, QM 2012 In-medium r broadening R. Rapp: private communications • Dielectron mass spectra over a broad energy range. • LMR enhancement persists down to 19. 6 Ge. V. • Theoretical calculations of in-medium r broadening with similar baryon densities from 19. 6 - 200 Ge. V reproduce LMR excesses consistently. - Suggestive of (partial) restoration of chiral symmetry Nov. 27 th, 2012 Director’s Review X. Dong 12

Future: Thermal Radiation J. Zhao STAR, QM 2011 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 Ge. V. Suggestive of either charm modifications or other sources. - Future measurements to quantify the correlated charm contribution. -– QGP thermal radiation: Cross section, RAA, v 2, a(M, p. T) Nov. 27 th, 2012 Director’s Review X. Dong 13

Beam Energy Scan 0) Turn-off of s. QGP signatures 1) Search for the signals of phase boundary 2) Search for the QCD critical point BES Phase-I Nov. 27 th, 2012 Director’s Review X. Dong 14

Inclusive Charged Hadron v 2 H. Masui, A. Schmah STAR, PRC in press • p. T > 2 Ge. V/c, consistent v 2(p. T) from 7. 7 Ge. V to 2. 76 Te. V • p. T < 2 Ge. V/c, v 2 rises with increasing collision energy Large collectivity / Particle composition Nov. 27 th, 2012 Director’s Review X. Dong 15

Identified Particle v 2 STAR, QM 2012 STAR Preliminary m. T-m 0 (Ge. V/c 2) A. Schmah Papers well advanced in the collaboration, to be released soon. • Significant difference between baryon-antibaryon v 2 at lower energies. • No clear baryon/meson grouping for anti-particles at <=11. 5 Ge. V. NCQ scaling (s. QGP signature observed at 200 Ge. V) is broken! Hadronic interactions play a significant role at √s. NN<= 11. 5 Ge. V. Nov. 27 th, 2012 Director’s Review X. Dong 16

Higher Moments of Net-protons STAR, PRL 105 (2010) 022302 First proof-of-principle measurement variance skewness 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) Nov. 27 th, 2012 Director’s Review X. Dong 17

Higher Moments of Net-protons STAR QM 2012 variance skewness kurtosis • Systematic measurements in various energies and various centralities. • Deviation from Poisson baseline. • Ur. QMD shows monotonic behavior. • Future precision measurements at low energies. X. F. Luo (now at CCNU) H. G. Ritter Nov. 27 th, 2012 Director’s Review X. Dong 18

Synergies with Theory Group Nov. 27 th, 2012 Director’s Review X. Dong 19

Proton Spin Structure Nov. 27 th, 2012 Director’s Review X. Dong 20

STAR Physics Focus in Future 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 BES-I HFT/MTD: Heavy Flavor / Dileptons BES-II (√s. NN ≤ 20 Ge. V) p. A/e. A program Precision measurements on HF and dileptons: Quantify the s. QGP properties (hot QCD) Precision measurements on focused energies Map out the QCD phase structure Precision measurements on p. A and e. A Study QCD in cold matter Nov. 27 th, 2012 Director’s Review X. Dong 21

Future RNC Focuses 1) Heavy quark production: a) Heavy quark collectivity: - the degree of thermalization b) Heavy quark RAA and correlations: - parton energy loss mechanism / medium properties 2) Di-lepton production: cross section, RAA, v 2, a (M, p. T) 3) In preparation of BES-II 4) Initiate p. A/e. A program a) – n. PDF / evolution from cold nuclear matter to s. QGP Nov. 27 th, 2012 Director’s Review X. Dong 22

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