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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

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 detector
STAR Detector

  • Large & uniform acceptance at mid-rapidity (exploring forward upgrades)

  • Excellent particle identification

  • Fast data acquisition

High energy nucleus nucleus collisions
High Energy Nucleus-Nucleus Collisions


Initial hard








Nuclear modification factor (RAA)

Elliptic flow (v2) = 2nd Fourier coefficient

Sensitive to the early stage properties

Characterize the medium effect

Rhic discoveries
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


STAR: PRL. 99 (2007) 112301

Formation of strongly-coupled

Quark Gluon Plasma (sQGP)!

Physics focuses of rnc
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

Partonic collectivity
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)

Charm production cross section
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).

Open charm hadrons in au au collisions
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.

J y production
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)

J y v 2 probe charm collectivity
J/y v2 – Probe Charm Collectivity

STAR, QM2011

L. Grandchamp et al. NPA 790, 415 (2002)


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

Dielectrons at au au 200 gev
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

Energy dependent dielectron production
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

Future thermal radiation
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)

Beam energy scan
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

Inclusive charged hadron v 2
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

Identified particle v 2
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.

Higher moments of net protons
Higher Moments of Net-protons


STAR, PRL 105 (2010) 022302


First proof-of-principle measurement


  • 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)

Higher moments of net protons1
Higher Moments of Net-protons

STAR QM 2012




  • 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

Star physics focus in future
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

Future rnc focuses
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