Xiangli Cui (for the STAR Collaboration)

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Xiangli Cui (for the STAR Collaboration)

Di-electron elliptic flow in 200 GeV Au+Au collisions at STAR

University of Science and Technology of China (USTC)Brookhaven National Laboratory (BNL)

Thermal Radiation Workshop Xiangli Cui

Outline

IntroductionElectron identificationAnalysis techniqueCentrality, mass and pT dependence of v2

Summary

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Introduction

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Low mass region (Mll <1.1 GeV/c2):In-medium modifications of vector mesons.Chiral symmetry restoration?

R. Rapp, et al .,Phys.Rev. C63 (2001) 054907.

Intermediate mass region (1.1< Mll <3.0 GeV/c2 ):QGP thermal radiation.

NA60: Eur. Phys. J.C 61(2009) 711-720

Thermal Radiation Workshop Xiangli Cui

Introduction

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Rupa Chatterjee , et al., Phys Rev C 75, 054909 (2007)

The mass and pT dependences of di-lepton v2 could give very rich information on specific stages of the fireball expansion

Measurements of v2 of thermal di-lepton could distinguish partonic and hadronic radiation sources

Thermal Radiation Workshop Xiangli Cui

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Time Projection Chamber (0<ϕ<2π, |η|<1 )

Tracking – momentum Ionization energy loss (dE/dx, PID) Time Of Flight (0<ϕ<2π, |η|<0.9)

Timing resolution <100ps - significant improvement for PID

STAR detector

Electron identification

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Clean electron PID Au+Au collisions with a combination of TPC dE/dx and TOF velocity ✓ electron purity ~97% in Au + Au Min Bias collision.

Combine data sets of year 2010 and 2011 ~ 720M

TOF velocity cut to remove slow hadrons

Invariant mass distribution

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Au+Au 0-80%@200GeV

pTe>0.2

|ηe|<1

|yee|<1

efficiency uncorrectedefficiency uncorrected

Thermal Radiation Workshop Xiangli Cui

Background: Like-sign (same) or Unlike-sign (mixed); Signal: Unlike-sign(same) – Background;Signal + Background: Unlike-sign(same)

Mee<=0.7 GeV/c2 : Subtract the like-sign backgroundMee>0.7 GeV/c2 : Subtract the normalized mixed-event background (normalized to like-sign background at Mee(0.7,3) GeV/c2 and pT (0,4)) GeV/c.

We mix events which are in the same centrality bin (9), vertex z bin (10) and event plane angle bin (100), same magnetic field.

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Dominant particle contribution in different mass ranges

STAR Preliminary Mee(0-0.14) : π0+others

Mee(0.14,0.3): η+others

Mee(0.5,0.7) : charm+ρ+ others

Mee(0.7,0.9): ω+others

Mee(0.9,1.1): ϕ+others

Mee(1.1,2.9): charm+other

Thermal Radiation Workshop Xiangli Cui

ϕ

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)1()(

22)(

2BS

SBT

BS

SS

N

Nvv

N

Nv

ji rv /))(2cos( 22

Thermal Radiation Workshop Xiangli Cui

Background: Like-sign (same) or Unlike-sign (mixed).Signal + Background: Unlike-sign(same). Signal: Unlike-sign(same) – Background.

v2T : Signal + Background v2,

v2B: Background v2,

v2S: Signal v2

NS/N(S+B): Signal/(Signal + Background)rj : Resolution of event plane in centrality j<>: average over all di-electron pairs in all events

STAR Preliminary

Analysis technique

Unlike-sign and background v2

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)1()(

22)(

2BS

SBT

BS

SS

N

Nvv

N

Nv

In each mass bin:

Au+Au 0-80%@200GeV

Thermal Radiation Workshop Xiangli Cui

Calculated the v2T, v2

B and NS/N(S+B) , use above formula to get v2S

Background: Mee <=0.7 GeV/c2: Like-sign same event, Mee >0.7 GeV/c2 : Unlike-sign mixed-event

Cocktail v2 and pT input (Mee<1.1GeV/c2)

Phys. Rev. C 80, 054907 (2009)

http://drupal.star.bnl.gov/STAR/files/Bai_Yuting.pdf (Ph.D thesis)

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Phys.Rev. C 79,051901(2009)

components : π0, η, ω, ϕ

Input: flat rapidity (-1,1);

pT: Tsallis function;

φ:1+ 2v2cos(2φ).

Cocktail simulation results (Mee<1.1GeV/c2)

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1. Reconstruct e+e- pairs after they decay in the STAR simulators.

2. Same acceptance cuts applied as in data.

3.obtain each component v2 and yield.

4. The cocktail v2 obtained by combining each component v2 (weight by its yield).

5.The cocktail v2 and each component contribution (weighted by the yield) are shown in the plot.

Mee dependence of v2 in Au+Au 200 GeV

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Di-electron v2 is consistent with simulation using the measured meson v2 as input in Au+Au at 200 GeV.

Work in progress to include intermediate mass region v2

Thermal Radiation Workshop Xiangli Cui

pTe>0.2 (GeV/c)

|ηe|<1, |yee|<1

PT dependence of v2 in Au+Au 200 GeV

Au+Au 0-80%@200GeV

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Simulated v2 is consistent with measured di-electron v2 within uncertainties in 0-80% Au+Au collisions.

The v2 of di-electrons in the ϕ mass region is consistent with the results of ϕKK within error

π0 Dalitz decay v2 and pT Input in different centralities (Mee<0.14 GeV/c2 )

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http://arxiv.org/abs/1203.2644v1PHYSICAL REVIEW C 80, 054907 (2009)

Phys. Rev. Lett. 97 (2006) 152301

components : π0

Input: flat rapidity (-1,1);

pT: MT scaling;

φ: 1+2v2cos(2φ).Thermal Radiation Workshop Xiangli Cui

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Centrality dependence of v2 (Mee<0.14 GeV/c2 )

Thermal Radiation Workshop Xiangli Cui

For each centrality:

1. Reconstruct e+e- pairs after π0 decay in the STAR simulators.

2. Same acceptance cuts applied as in data.

3. obtain e+e- pairs v2

Di-electron v2 is consistent with simulated results for different centralities in Au+Au at 200 GeV.

Summary

For Mee <1.1 GeV/c2, the simulated cocktail v2 are consistent with the measured di-electron v2

For Mee <0.14 GeV/c2, the simulated e+e- v2 are consistent with the measured di-electron v2 in different centralities.

Work in progress to include intermediate mass region v2

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Thanks!