Charge Asymmetry Correlations to Search for Local Parity Violation

1. Charge Asymmetry Correlations to Search for Local Parity Violation Fuqiang Wang Purdue University

2. Outline • Local parity violation in strong interactions and chiral magnetic effect • Charge separation and correlation • Charge correlator measurements by STAR • Possible physics background • Possible explanation by cluster particle correlations • Charge asymmetry correlation measurements • What could be the underline physics • Potential measurements to confirm/refute LPV • Summary

3. QCD vaccu Energy of gluonic field is periodic in NCSdirection (~ a generalized coordinate) NCS= -2 -1 0 1 2 The volume of the box is 2.4 by 2.4 by 3.6 fm. The topological charge density Animation by Derek Leinweber Instantons and sphalerons are localized (in space and time) solutions describing transitions between different vacua via tunneling or go-over-barrier Topological transitions have never been observed directly (e.g. at the level of quarks in DIS). An observation of the spontaneous strong parity violation would be a clear proof for the existence of such physics.

4. Chiral magnetic effect • Metastable QGP domain • Chiral symmetry restoration, massless quarks • Topological charge (e.g. Q=-1): quarks L  R (by mom. flip)maximal parity violation. • Strong magnetic field, spin locked: + charge spin up– charge spin down • Single-handed quarks: momentum & spin parallel • + charge move up, – charge move down. Only Landau ground state is occupied. Thermal motion (~200 MeV) not important. Need large B, and ~massless quarks.

5. Magentic field Energy density in B field: Short duration: r/g ~ 0.1 fm. B drops quickly with time. Larmor radius: Much larger than lifetime of large B: r/g. The large B unlikely has an effect.

6. Charge separation • If all work: • QGP domain • Chiral symmetry • Large B and sizeable effect Kharzeev et al. estimate: Charge asymmetry ~ 1%.

7. STAR detector

8. Parity violation observable Voloshin, PRC 70 (2004) 057901 Use a third particle Small PV observable

9. Initial expectation vs data + + STAR: Au+Au @ 62 GeV Like-sign + -<cos(fa+fb-2y)> Charge correlation Unlike-sign Initial (naïve) theory expectations —

10. Back-to-back charge suppression + + X — — Medium interaction

11. STAR 3-particle correlator measurement What’s measured What one’s after: <cos(fa+fb-2y)> Assumption: 3-particle correlation is negligible. <cos(fa+fb-2y)> ≈ <cos(fa+fb-2fc)> / v2,c

12. Published STAR data 0909.1717, 0909.1739 Unlike-sign suppressed by medium? Like-sign consistent with PV. <cos(fa+fb-2fc)> / v2,c

13. Possible physics background Observable is parity-even, subject to physics backgrounds. Resonance decay effect: Order of magnitude smaller than measurement. Unlikely source of background. But how about clusters of particles: each cluster can have many particle pairs.

14. Scott Pratt

15. Scott Pratt

16. Cluster model FW, PRC 81, 064902 (2010). SA: Small-Angle pairs BB: Back-to-back pairs v2 < 0 v2 > 0

17. Needed v2,SA and v2,BB

18. Issue 1 with cos(fa+fb-2yRP) SA: Small-Angle pairs BB: Back-to-back pairs Cannot be distinguished!

19. Issue 2 with cos(fa+fb-2yRP) There are more particles in-plane than out-of-plane, therefore <cos*cos> has more weight than <sin*sin>.

20. New Variable: Charge Asymmetry Correlation UP RIGHT LEFT • 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 EP DOWN RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

21. Analysis: to avoid self-correlation EP RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

22. Stat. fluc. + detector effects in ‹A2› STAR preliminary Adding-pmethod EP Stat. fluct. + Detector effects A RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

23. Subtracting stat+det effects STAR preliminary • Symbols are data (TPC left/right, positive/negative charges). • Curves: net effect of statistical fluct. and detector non-uniformity. • Stat+det effects are actually wider than data in ‹A±2›. • Physical processes narrow charge asymmetry distributions same-sign back-to-back pairs. RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

24. Charge Asym. Correl. Results STAR preliminary • Oppo-sign aligned; ‹A+A-›UD>‹A+A-›LR LPV expects: ‹A+A-›UD<‹A+A-›LRContradicts LPV expectations. + — + d+Au + • Same-sign back-to-back in central, unexpected from only LPV. Data: ‹A2›UD>‹A2›LRLPV expects: ‹A2›UD>‹A2›LR + — + + RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

25. The observable: UD-LR Like-sign: consistent w/ LPV Unlike-sign: contradicts LPV

26. pT dependence 20-40% centrality • Opposite-sign charges are more aligned at high-pt. • Same-sign charges are more preferentially back-to-back at high-pt. STAR preliminary RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

27. UD-LR vs pt • UD-LR increases with pt, ~linearly. • Qualitatively similar between same-sign and opposite-sign. 20-40% 0-20% STAR preliminary STAR preliminary RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

28. UD-LR compared to correlator Asymmetry correlations can be expanded into harmonic terms. One term is the correlator. • Charge asymmetry correlations have been divided by EP resolution. • “Corrected” same-sign charge asymmetry correlation and correlator are similar: higher order terms are negligible. • “Corrected” opposite-sign charge asymmetry correlation and correlator are different: higher order terms are important. STAR preliminary RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

29. What could be the underlying physics? Possible causes: correlation overlaid with v2(FW, PRC 81, 064902 (2010)) • Charge balance (S. Pratt): correlator ~ -v2/N*‹cos2DfBF›, may affect ‹A+A-›. • Momentum conservation (S. Pratt): correlator ~ v2/N, may affect both ‹A+A-› and ‹A2›. • Path-length dependent jet-quenching, opacity. • Try to investigate those effects experimentally, by studying UD-LR • vs event-by-event high-pT v2 (indicative of jet-quenching) and bulk v2. RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

30. UD–LR vs Event-by-Event Anisotropy 20-40% centrality 20-40% centrality STAR preliminary STAR preliminary • Same trend in high-pt v2. • Stronger jet-quenching out-of-plane enhance UD asymmetry for both ‹A2UD› and ‹A+A-›UD? • ‹A2›, ‹A+A-› opposite trend in low-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 RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

31. The Central Problem We do NOT know exactly what the background is!

32. UD-LR vs wedge size 20-40% centrality STAR preliminary RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang

33. Summary • Chiral magnetic effect is predicted by QCD. It generates charge separation. • Charge correlator:cos(a+b-2y)LS<0: consistent with local parity violation.cos(a+b-2y)US≈0: unexpected from LPV only, but can be explained by back-to-back suppression. • New variable: charge asymmetry correlations are reported. • Opposite-sign charges are preferentially aligned.Same-sign charges are preferentially back-to-back in central collisions. • UD – LR and cos(a+b-2y) are related:‹A2›UD > ‹A2›LR& cos(a+b-2y)LS<0: qualitatively similar.‹A+A-›UD > ‹A+A-›LR: contradicts LPV expectations. • Detailed studies of charge asymmetry correlations, hopefully providing new insights: • UD-LR v.s. E-by-E high-pT and low-pT v2’s • Dependence on wedge size • My hunch: observed data are some kind of particle/cluster correlations; the data do not demand LPV. LPV effect, if exists, must be orders of magnitude smaller than the observed data. RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang