Particle Production in Au+Au

Collisions at 9.2GeV

Jiayun Chen

(for Collaboration)

Institute of Particle Physics, CCNU, Wuhan, 430079, P.R.China

Brookhaven National Lab, Upton, NY,11973, USA

http://drupal.star.bnl.gov/STAR/starnotes/public/sn0493

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Outline

• Motivation

• STAR preliminary 9.2 GeV results Particle identification in TPC Charged multiplicity Azimuthal anisotropy PID spectra and ratios ： energy, centrality Charged pion interferometry

• Summary and outlook

Lessons from 200GeV

• Strong high pT suppression in hadron production.

• Number of Constituent Quark scaling in v2 measurements

Indicate the existence of sQGP.

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Phys. Rev. Lett. 91, 072304 (2003)

Phys. Rev. Lett. 92, 052302 (2004)Phys. Rev. C 72, 014904 (2005)Phys. Rev. Lett. 95, 122301(2005)Phys. Rev. Lett. 92, 112301(2004)

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MotivationQCD theory predicts:• Low B & High T :Cross-over• High B & Low T:1st order transition• Mid B & Mid T :Critical Point

Plan : Access to phase diagram• T and B vary with √sNN

Signatures Locating the phase boundaries Locating the critical point

Aim of this talk is to discuss the results from successful data taking in STAR with Au+Au collisions at 9.2 GeV. Demonstrate our readiness for the future Beam Energy Scan program at RHIC.

NSAC 2007 Long-range Plan

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STAR Detector

2.5<|η|<4.0

|η|<1.5

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Collisions @ 9.2 GeV STAR TPC display: Au+Au at 9.2GeV

1. ~ 3000 good events collected at ~ 0.6 Hz in year 2008

2. Data taking period lasted for < 5 hr.3. In the future, ToF will further

enhance the PID

Non-central Central

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Collider Acceptance

Fix

ed

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Charged Multiplicity Collision Centrality Selection

Track selection for various analysis

•With more events STAR will map out centrality (system-size) in detail.•Changes in fluctuations with system-size can indicate the presence of a 1st order phase transition.

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1cos , tan ( )ynx

pv n

p

This is believed to be due to pressure gradients in the dense medium which lead to collective motion.Directed flow (v1) at forward rapidities describes the “side-splash”, the foremost early collective motion of the system;Elliptic flow (v2) demonstrated the development of collectivityProvide insights into the EOS.

Coordinate-SpaceAnisotropy

Momentum-SpaceAnisotropy

interactions

pressure gradient pressure gradient collective flow collective flow

Azimuthal Anisotropy

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Directed Flow v1

two methods to analyze the data‘standard’ event plane method - v1{FTPC,EP1} - v1{BBC,EP1}

mixed event plane method -v1{EP1,EP2}

1 2

1 1 21 2

1 1 2 2

cos 2{ , }

cos( 2 ) Re ( )

FTPC TPC

FTPC FTPC TPC TPCv EP EP

s

1 1 1 1{ } cos( ) / Re ( )v EP s

Results from two methods are consistent.

STAR PreliminarySTAR Preliminary

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Directed Flow v1

• At mid-rapidity, all the results have comparable values. At forward rapidity, the trend of v1 from low energy is different from high energies. This is due to early longitudinal collision dynamics.

• V1 values lie on a common trend. STAR : PRL 92 (2004) 062301 PRL101 （ 2008 ） 252301NA49: PRC68(2003)034903

STAR Preliminary

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Elliptic Flow

• STAR and NA49 results seem to be consistent.• STAR 9.2GeV v2 nicely fits into the observed trends.

NA49 : PRC 68 (2003) 034903AGS : PLB 474 (2000) 27STAR : PRC 77 (2008) 054901 : PRC 75 (2007) 054906, PRC 72 (2005) 014904PHOBOS : PRC 72 (2005) 051901 : PRL 98 (2007) 242302 PHENIX : PRL 98 (2007) 162301

STAR Preliminary STAR Preliminary

Lokesh Kumar QM2009, SQM2008

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pT Spectra

The slope of the identified hadron spectra follow the order <k<p

These spectra can be further characterized by looking at the dN/dy and <pT> or <mT>

We measure within our pT acceptance~ 82 % of total produced~ 47 % of total K produced~ 75 % of total p producedSTAR Preliminary

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Yield &<mT> vs. Energy

Yield and <mT> beam energy dependence are consistent with the published data

Assuming a thermo-dynamic system : T ~ <mT> - m

entropy ~ dN/dy log(√sNN)

NA49 : PRC 66 (2002) 054902, PRC 77 (2008) 024903 PRC 73 (2006) 044910STAR : PRC 79 (2009) 034909, arXiv: 0903.4702E802(AGS) : PRC 58 (1998) 3523, PRC 60 (1999) 044904 E877(AGS) : PRC 62 (2000) 024901 E895(AGS) : PRC 68 (2003) 054903

STAR Preliminary

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Particle Ratios vs. centrality K-/K+ & K-/- are lower in 9.2

GeV compared to that in 200 and 62.4 GeV;

K+/ + in 9.2 GeV is comparable to that in 200 and 62.4 GeV

--K+ production is dominated by associated production in low energy.

p/ in 9.2 GeV is larger compared to 200 and 62.4 GeV

-- shows large baryon stopping at low energy;

Ratios seem to be almost independent of collision centrality for all energies studied.

STAR : PRC 79 (2009) 034909

p

K K

STAR Preliminary

STAR Preliminary

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Particle Ratio vs. Energy

NA49 : PRC 66 (2002) 054902, PRC 77 (2008) 024903, PRC 73 (2006) 044910

E802(AGS) : PRC 58 (1998) 3523, PRC 60 (1999) 044904, PRC 62 (2000) 024901,

PRC 68 (2003) 054903

p/p

K/K+

K/_

STAR Preliminary

STAR : PRC 79 (2009) 034909, arXiv: 0903.4702

STAR preliminary

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Pion Interferometry

Source parameters :

Centrality : 0 - 60%Error bars for Au+Au 9.2 GeV are statistical Systematic errors expected to be < 10 % for all radii KT=[0.15,0.25](GeV/c)

-

All are consistent with the previously observed trends

STAR : PRC 71 (2005) 044906, PRL 87 (2001) 082301PHENIX : PRL 88 (2002) 192302, PRL 93(2004) 152302

E802 : PRC 66 (2002) 054906CERES : NPA 714 (2003) 124 E895 :PRL 84 (2000 ) 2798PHOBOS : PRC 73 (2006) 031901

NA44 : PRC 58 (1998) 1656 E866 : NPA 661 (1999) 439NA49 : PRC 77 (2008) 064908WA97 : JPG 27 (2001) 2325

Rout(fm) Rside(fm)

5.05±0.96 3.52±0.56

Rlong(fm)

3.25±0.86 0.355±0.083

D. Das for STAR Collaboration WWND 09

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Summary

Azimuthal Anisotropy (v1 and v2) results are similar to those obtained at SPS from collisions at similar energies

Identified particle spectra obtained from Au+Au collisions at 9.2 GeV. Centrality and beam energy dependence of the hadron yields and ratios studied.

In central collisions anti-proton to proton ratio ~ 0.01 shows significant baryon stopping in these collisions

In central collisions K-/K+ ~ 0.4 shows associated production for K+

p/ ratio is higher and K-/- is lower at 9.2 GeV compared to 200 GeV at all collision centralities studied

Pion interferometry results follow the established beam energy trends

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STAR BES Program

Key measurements:

(1) PID hadron spectra, ratios, v2 … (2) Fluctuations: - Net-p Kurtosis - K/ - <pT>, <N> …

See Daniel Cebra’s talk ” STAR plans for the RHIC low energy run”

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• The results from the lowest beam energy collisions at RHIC demonstrate STAR experiment’s readiness to collect high quality data for the proposed Critical Point Search Program.

• Large and uniform acceptance for all beam energies in a collider set up, excellent particle identification (TPC+TOF) and higher statistics will lead to significant advance in the Critical Point search: Differential analyses of pT and number correlations for identified particles

-- vs and to identify the source and nature of observed non-statistical fluctuations System-size dependence of fluctuations (1st order phase transition?) Charge separation of K/ and P/ fluctuations Higher moments of net-charge and net-proton distributions Azimuthal HBT

Critical Point Search Program will allow us to explore the QCD phase diagram.

Outlook