1. Di-electron elliptic flow in
200 GeV Au+Au collisions at STAR
Xiangli Cui (for the STAR Collaboration)
University of Science and Technology of China (USTC)
Brookhaven National Laboratory (BNL)
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2. Outline Introduction Electron identification Analysis technique
Centrality, mass and pT dependence of v2
Summary
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3. Introduction Low mass region (Mll <1.1 GeV/c2):
NA60: Eur. Phys. J.C 61(2009)
R. Rapp, et al .,Phys.Rev. C63 (2001)
Low mass region (Mll <1.1 GeV/c2):
In-medium modifications of vector mesons.
Chiral symmetry restoration?
Intermediate mass region
(1.1< Mll <3.0 GeV/c2 ):
QGP thermal radiation.
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4. Introduction
Rupa Chatterjee , et al., Phys Rev C 75, (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
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5. STAR detector 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
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6. Electron identification
TOF velocity cut to remove slow hadrons
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
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7. Invariant mass distribution
Au+Au
efficiency uncorrected
efficiency uncorrected
pTe>0.2
|ηe|<1
|yee|<1
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 background
Mee>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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8. 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 ϕ
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9. Analysis technique v2T : Signal + Background v2, v2B: Background v2,
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
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10. Unlike-sign and background v2
Au+Au
In each mass bin:
Calculated the v2T, v2B 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
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11. Cocktail v2 and pT input (Mee<1.1GeV/c2)
(Ph.D thesis)
Phys. Rev. C 80, (2009)
components : π0, η, ω, ϕ
Input: flat rapidity (-1,1);
pT: Tsallis function;
φ:1+ 2v2cos(2φ).
Phys.Rev. C 79,051901(2009)
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12. Cocktail simulation results (Mee<1.1GeV/c2)
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.
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13. Mee dependence of v2 in Au+Au 200 GeV
pTe>0.2 (GeV/c)
|ηe|<1, |yee|<1
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
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14. PT dependence of v2 in Au+Au 200 GeV
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 ϕKK within error
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15. π0 Dalitz decay v2 and pT Input in different centralities (Mee<0
π0 Dalitz decay v2 and pT Input in different centralities (Mee<0.14 GeV/c2 )
PHYSICAL REVIEW C 80, (2009)
Phys. Rev. Lett. 97 (2006)
components : π0
Input: flat rapidity (-1,1);
pT: MT scaling;
φ: 1+2v2cos(2φ).
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16. Centrality dependence of v2 (Mee<0.14 GeV/c2 )
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.
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17. 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
Thanks!
Thermal Radiation Workshop
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