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Measurement of Multiplicity Asymmetry in Au+Au and d+Au Collisions

This study by Quan Wang from Purdue University, part of the STAR Collaboration, examines the correlation between positive and negative particles in Au+Au and d+Au collisions, focusing on local parity violation (LPV) and charge asymmetry observables. The analysis details, results, and summary are discussed, revealing insights into LPV effects, charge asymmetry correlations, and the chiral magnetic effect in the presence of large magnetic fields. Data from MinBias 200GeV Au+Au and d+Au collisions are analyzed to investigate various observables such as ‹A2›, ‹A+A-›, and differences between UD and LR asymmetries. The findings suggest significant charge imbalance correlations and unexpected trends in asymmetry measurements, warranting further investigation into the underlying physics mechanisms.

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Measurement of Multiplicity Asymmetry in Au+Au and d+Au Collisions

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  1. Measurement of Multiplicity Asymmetry Correlation Between Positive and Negative Particles in Au+Au and d+Au Quan Wang, Purdue University For the STAR Collaboration • Motivation: Local Parity Violation (LPV) • Charge asymmetry observables • Analysis details • Results • Summary

  2. Motivation and Observables UP RIGHT LEFT EP DOWN • LPV effects in UD. LR is null-reference. • LPV expectations: • A+UD and A-UDare anti-correlated • ‹A+A-›UD<‹A+A-›LR • Additional dynamical fluctuation broadens A±UDdistributions • ‹A±2›UD>‹A±2›LR • Chiral magnetic effect: • LPV + large magnetic field • charge separation along the system angular momentum. • Kharzeev et al. NPA 803 (2008) 227 APS apr meeting

  3. Analysis Details • Data set • Y4 MinBias 200GeV Au+Au data (8.5M events) • Y3 MinBias 200GeV d+Au data • Cuts: standard STAR event and track cuts • To avoid self-correlation effect: • EP: η<0; Asymη>0 • EP: η>0; Asymη<0 η<0 η>0 APS apr meeting

  4. Stat. Fluc. Effect in ‹A2› STAR preliminary Stat. fluc. effect Thanks to V. Koch. APS apr meeting

  5. Results: ‹A2›-(‹N›+1)/‹N›2 and ‹A+A-› • Strong charge asymmetry correlations present. • UD is always larger than LR. STAR preliminary STAR preliminary • LPV expects: ‹A2›UD>‹A2›LR • LPV expects: ‹A+A-›UD<‹A+A-›LR Data consistent with LPV expectation Data inconsistent with LPV expectation APS apr meeting

  6. Difference UD-LR STAR preliminary Observations: • ‹A2›UD>‹A2›LR consistent with LPV expectation • ‹A+A-›UD>‹A+A-›LR inconsistent with LPV expectation • Similar centrality dep. • Possible causes: correlation overlaid with v2(F. Wang, 0911.1482) • Charge balance (S. Pratt): ~-v2/N*‹cos2DfBF›, only in ‹A+A-›. • Momentum conservation (S. Pratt): ~ v2/N, in both ‹A+A-› and ‹A2›. • Path-length dependent jet-quenching, opacity. • Try to investigate those effects experimentally, by studyingUD-LR vs E-by-E bulk v2, and high-pT v2 (indicative of jet-quenching). APS apr meeting

  7. UD–LR vs Event-by-Event Anisotropy 20-40% centrality 20-40% centrality STAR preliminary STAR preliminary (Nin-Nout)/(Nin+Nout) at low pT (pT<2GeV/c) (Nin-Nout)/(Nin+Nout) at high pT (pT>2GeV/c) ‹A2› and ‹A+A-› opposite trend in low-pT v2! Same trend in high-pt v2. • More b-to-b same-charge pairs in-plane reducing ‹A2LR›? → Increasing trend • More opposite-charge pairs in-plane enhancing ‹A+A-›LR? → Decreasing trend • Stronger jet-quenching out-of-plane enhance UD correlation for both ‹A2UD› and ‹A+A-›UD? APS apr meeting

  8. Summary • Charge asymmetry correlations are all positive. • ‹A2›UD>‹A2›LR: consistent with LPV expectation‹A+A-›UD > ‹A+A-›LR: inconsistent with LPV expectation • UD-LR v.s. E-by-E v2: • ‹A2› and ‹A+A-› have same trend with high-pT v2 • ‹A2› and ‹A+A-› have opposite trend with low-pT v2 • Physics mechanisms need further investigations APS apr meeting

  9. Backup APS apr meeting

  10. LR-UD Compared to PV Correlator LR-UD: including all possible terms, cos(a+b-2y) is only one term. • Asymmetry correlation ≠ correlator. • Higher order effects may be significant.. • Different observables. • Same data. Numerical difference due to different observables Charge asymmetry correlation = correlator + higher order terms • Correlator calculated in the same h region (half the TPC) as Asym correl. • Qualitatively same as LPV papers (full TPC). STAR preliminary APS apr meeting

  11. Stat. Fluc. Effect in ‹A2› APS apr meeting

  12. High pT “v2” sensitive to jet-quenching • “v2”=(Nin-Nout)/(Nin+Nout) • Larger “v2” indicates more jet-quenching effect. N events Out of plane Jet-quenching In plane In plane EP Out of plane -1 +1 High-pt v2 APS apr meeting

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