STAR Physics

X. DongNov. 27th, 2012 Director’s Review

STAR Physics

Xin DongNuclear 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

2

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


High Energy Nucleus-Nucleus Collisions

3

Time

Initial hardscatterings

Partonic stage

Hadronicstage

Freeze-out

Nuclear modification factor (RAA)

€

RAA =d2N AA /dpT dy

TAA d2σ /dpT dy

Characterize the medium effect

Elliptic flow (v2) = 2nd Fourier coefficient

Sensitive to the early stage properties


Formation of Strongly-coupled QGP

4

ST

AR

: PR

L. 99 (2007) 112301

Formation of strongly-coupled Quark Gluon Plasma (sQGP)!

v2

ST

AR

: PR

L. 91 (2003) 072304

“Jet Quenching”

Significant suppression in particle yield at high pT in central heavy ion collisions

“Partonic Collectivity”

Significant v2 of multi-strange hadrons (, )Number-of-Constituent-Quark (NCQ) scaling


Physics Focuses of RNC

5

1) Study sQGP Properties• Systematic investigation of partonic collectivity – Identified particle v2

• Chiral properties - Dielectron production • Thermalization - Heavy Quark Production (D-meson, J/)

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


Recorded Datasets

6


Partonic Collectivity

7

Hiroshi Masui, Md. Nasim (Oct. – Dec. 2011)

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()<v2(Ks), v2()<v2()

Precision identified particle v2 data provide constraints to study the sQGP properties.


Charm production cross section

8

X.D., Y. Zhang (now at USTC)

STAR, QM 2011, QM 2012STAR, PRD 86 (2012) 072013

• 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).

X. DongNov. 27th, 2012 Director’s Review9

Open Charm Hadrons in Au+Au collisions

• 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.

Model curves: M. He, et al. arXiv: 1204.4442, private communicationsP. Gossiaux, et al. arXiv: 1207.5445, private communications

Y. Zhang (now at USTC)

STAR, QM2011 STAR, QM2012


J/ Production

10

STAR, QM2012 C. Powell

Systematic measurements in p+p, d+Au and Au+Au collisions:

• J/ production in d+Au collisions consistent with EPS09 + abs(3mb).• In Au+Au collisions, BW predictions with same (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/ v2 – Probe Charm Collectivity

11

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

20-60%

Initial production vs. charm quark coalescence High pT low pT

Peripheral Central

• Disfavor regeneration from thermalized charm quarks at pT > 2 GeV/c• To probe charm collectivity cleanly Low pT J/ v2 in (more-) central collisions

H. QiuPaper in Collaboration reviewTo be submitted to PRL soon.

STAR, QM2011


Dielectrons at Au+Au 200 GeV

12

• 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 cannot reproduce the excess observed in data.

- in-medium modification

STAR, QM2011 J. Zhao


Energy Dependent Dielectron Production

13

• Dielectron mass spectra over a broad energy range.

• LMR enhancement persists down to 19.6 GeV.

• Theoretical calculations of in-medium broadening with similar baryon densities from 19.6 - 200 GeV reproduce LMR excesses consistently.

- Suggestive of (partial) restoration of chiral symmetry

In-medium broadeningR. Rapp: private communications

P. HuckSTAR, QM2012


Future: Thermal Radiation

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, v2, (M, pT)

J. ZhaoSTAR, QM2011


Beam Energy Scan

15

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 v2

16

STAR, PRC in press H. Masui, A. Schmah

• 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 v2

17

• 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.

mT-m0 (GeV/c2)

STAR Preliminary

STAR Preliminary

A. Schmah Papers well advanced in the collaboration, to be released soon.

STAR, QM 2012


Higher Moments of Net-protons

18

X.F. Luo (now at CCNU)

STAR, PRL 105 (2010) 022302

• Higher moments - more sensitive to Critical Point induced fluctuations.

• Moments of conserved quantities – susceptibilities from Lattice QCD.

• Net-protons – good proxy for net-baryons.

• Moment products – cancel the volume effect.

varianceskewness

kurtosis

First proof-of-principle measurement


Higher Moments of Net-protons

19

• 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 QM 2012varianceskewness

kurtosis


Proton Spin Structure

20


STAR Physics Focus in Future

21

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

BES-I

HFT/MTD: Heavy Flavor / Dileptons

BES-II (√sNN ≤ 20GeV)

pA/eA program

Precision measurements on HF and dileptons:Quantify the sQGP properties (hot QCD)

Precision measurements on focused energiesMap out the QCD phase structure

Precision measurements on pA and eAStudy QCD in cold matter


Future RNC Focuses

22

1) Heavy quark production:

a) Heavy quark collectivity: the degree of thermalization

b) Heavy quark energy loss: parton energy loss mechanism

c) Heavy quark correlations: deep insight into the medium properties

2) Di-lepton production: cross section, v2, (M, pT)

3) In preparation of BES-II

4) pA/eA ….


Documents

STAR Physics