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) Thermal Radiation Workshop Xiangli Cui

Outline Introduction Electron identification Analysis technique Centrality, mass and pT dependence of v2 Summary Thermal Radiation Workshop Xiangli Cui

Introduction Low mass region (Mll <1.1 GeV/c2): NA60: Eur. Phys. J.C 61(2009) 711-720 R. Rapp, et al .,Phys.Rev. C63 (2001) 054907. 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. Thermal Radiation Workshop Xiangli Cui

Introduction 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

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 Thermal Radiation Workshop Xiangli Cui

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 Thermal Radiation Workshop Xiangli Cui

Invariant mass distribution Au+Au 0-80%@200GeV 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. Thermal Radiation Workshop Xiangli Cui

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 Thermal Radiation Workshop Xiangli Cui

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 Thermal Radiation Workshop Xiangli Cui

Unlike-sign and background v2 Au+Au 0-80%@200GeV 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 Thermal Radiation Workshop Xiangli Cui

Cocktail v2 and pT input (Mee<1.1GeV/c2) http://drupal.star.bnl.gov/STAR/files/Bai_Yuting.pdf (Ph.D thesis) Phys. Rev. C 80, 054907 (2009) components : π0, η, ω, ϕ Input: flat rapidity (-1,1); pT: Tsallis function; φ:1+ 2v2cos(2φ). Phys.Rev. C 79,051901(2009) Thermal Radiation Workshop Xiangli Cui

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. Thermal Radiation Workshop Xiangli Cui

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 Thermal Radiation Workshop Xiangli Cui

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 Thermal Radiation Workshop Xiangli Cui

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, 054907 (2009) Phys. Rev. Lett. 97 (2006) 152301 http://arxiv.org/abs/1203.2644v1 components : π0 Input: flat rapidity (-1,1); pT: MT scaling; φ: 1+2v2cos(2φ). Thermal Radiation Workshop Xiangli Cui

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. Thermal Radiation Workshop Xiangli Cui

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 Xiangli Cui