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STAR Physics @ RNC

This program focuses on studying the properties of the quark-gluon plasma (QGP), nucleon spin structure, and cold nuclear matter. It aims to understand the QCD phase structure and investigate the collectivity of partons in high energy nucleus-nucleus collisions. The program utilizes the STAR detector at the Nuclear Science Division of LBNL for precision measurements and discoveries in the field of high-energy physics.

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STAR Physics @ RNC

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  1. STAR Physics @ RNC 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 Program

  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 Hadronic stage Initial hard scatterings Partonic 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 Two most significant discoveries originated in RNC: “Jet Quenching” - Significant suppression in particle yield at high pT in central heavy ion collisions STAR: PRL. 91 (2003) 072304 “Partonic Collectivity” - Strong collective flow, even for multi-strange hadrons (f, W) - Flow driven by Number-of-Constituent-Quark (NCQ) in hadrons v2 STAR: PRL. 99 (2007) 112301 Formation of strongly-coupled Quark Gluon Plasma (sQGP)!

  5. HI Physics Focus 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) • Partonic energy loss • – Jets, high pT correlations • 2) Study QCD phase structure - Beam Energy Scan • Turn-off of sQGP signatures • – elliptic flow of identified particles • – Rcp of charged hadrons • Search for critical point • – high moments of net-proton multiplicity

  6. Partonic Collectivity STAR, QM 2012 • Precision measurements of 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. H. Masui, Md. Nasim (Oct. – Dec. 2011)

  7. Charm production cross section STAR, PRD 86 (2012) 072013 STAR, QM 2011, QM 2012 pQCD X.D., Y. Zhang (now at USTC) • Heavy quarks – Sensitive to degree of thermalization of early system. • consistent with the pQCD calculation in p+p. • Nbin scaling from p+p to central Au+Au with significantly improved precision. • - Charm quarks predominantly created from initial hard scatterings.

  8. Open Charm Hadrons in Au+Au collisions STAR, QM2012 STAR, QM2011 BW fit BW – Blast Wave model 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 earlier than light hadrons. • Modification in low pT - indication of strong charm-medium interactions. • Large suppression at pT > 3 GeV/c • - indication of large charm quark energy loss in the sQGP medium.

  9. J/y Production STAR, QM2012 C. Powell • Quarkonium production – color screening in sQGP. • consistent with a shadowing model + cold nuclear absorption in d+Au. • Blast wave prediction with same b (radial flow velocity) as light hadrons different from J/y 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) Au+Au 200 GeV 0-80% 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 • Low mass region (LMR) enhancement persists from 19.6 – 200 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 via Di-electrons 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 • Search for the phase boundary • 2) Search for the critical point BES Phase-I RNC initiated and is now leading the BES program at RHIC

  15. Inclusive Charged Hadron v2 H. Masui, A. Schmah STAR, PRC 86 (2012) 054908 • 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. Rcp of Charged Hadrons STAR, QM 2012 E. Sangaline jet-quenching off • Significant change in the charged hadron Rcp at low energies. • Similar behavior in HIJING with no partonic energy loss at low energies. • - Cronin effects • Hadronic interactions play a significant role at √sNN<= 11.5 GeV.

  18. 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. • A non-monotonic behavior could be indicative of the existence of QCD critical point. X.F. Luo (now at CCNU)

  19. Higher Moments of Net-protons STAR QM 2012 variance skewness kurtosis • Systematic measurements in various energies and various centralities. • Deviation from Poisson expectation. • - can be due to other correlation sources • Current precision cannot allow a firm conclusion on the energy dependence. • Future precision measurements at low energies. X.F. Luo (now at CCNU) H.G. Ritter

  20. Spin Physics at STAR Nucleon spin structure 25-30% of total spin • RNC focus • Gluon polarization • – Jet double spin asymmetry • Strange quark polarization • – Hyperon (L) longitudinal spin transfer • Transverse spin • – Forward single spin asymmetries (p0, h)

  21. Gluon Polarization – Jet Asymmetries STAR PRL 97 (2006) 252001, PRL 98 (2008) 232003 PRD 86 (2012) 032006, SPIN 2012 2009 data 2006 data • Sizable gluon polarization for gluon light-cone momenta at the level of 5-20% of the proton momentum and at hard scales. • Numerous sustained RNC contributions to this program.

  22. Longitudinal Hyperon Spin Transfer STAR, SPIN 2012, DNP 2012 Non-relativistic naive quark model, all quarks contribute equally to DLL, Non-relativistic naive quark model, only strange quarks contribute DLL, Deep-Inelastic-Scattering like contributions to DLL. R. Cendejas (now at Penn. State. Univ.) E. Sichtermann • New sensitivity to strange (anti-)quark polarization and polarized fragmentation. • Unique at RHIC and complementary to deep-inelastic lepton-nucleon scattering.

  23. Transverse Spin Phenomena L. Eun STAR, PRD 86 (2012) 051101(R) • AN of p, h – QCD origin of large asymmetries, OAM contribution in nucleon spin. • NLO pQCD describes the η production cross section at forward rapidity. • - η/π0ratio consistent with the expectation from fragmentation. • The transverse asymmetry AN for the η might be larger than π0 AN.

  24. Synergies with Theory Group STAR, PRL 91 (2003) 172302 Song, PRL 106 (2011) 192301 • Jet quenching and partonic energy loss • Hydrodynamic flow • Correlation / fluctuation • Heavy flavor • Dileptons • Spin Steinheimer, arXiv: 1207.2791 STAR, PRD 86 (2012) 072013 Yuan, PRD 84 (2011) 034019

  25. Future Upgrades Near term 2013- TPC SSD 1) Heavy Flavor Tracker (HFT) RNC leading the Pixel subproject see Jim Thomas’s talk next IST PXL 2) Muon Telescope Detector (MTD) Mid term 2017- Subsystem upgrade at forward rapidity expanding pA/eA physics program

  26. 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

  27. Future STAR Physics Focus @ RNC • 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) • Full jet reconstruction in heavy ion collisions • In preparation of BES-II • pA/eA program • – nPDF / evolution from cold nuclear matter to sQGP

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