Gang Wang (WWND2010)1 Search for local parity violation with STAR ZDC-SMD Gang Wang (UCLA) for STAR Collaboration

Gang Wang (WWND2010)2 Parity violation Looking into a mirror, you see someone else… It’s a parity violation?! Parity transformation: A spatial inversion of the coordinates. Origins of parity violation: 0. Fake parity violation (No mirror there… Doesn’t count!) 1.Global parity violation Occurs in weak interactions »Confirmed 2.Local parity violation Predicted in strong interactions »we are working on it… Kharzeev, PLB (2006) [hep-ph/ ] Kharzeev, Zhitnitsky, NPA (2007) Kharzeev, McLerran, Warringa, NPA (2008) Fukushima, Kharzeev, Waringa, PRD 78,

Gang Wang (WWND2010)3 P/CP invariance are (globally) preserved in strong interactions: neutron EDM (electric dipole moment) experiments: Θ<10 −11 Pospelov, Ritz, PRL83:2526 (1999) Baker et al., PRL97: (2006) In heavy-ion collisions, the formation of (local) meta-stable P-odd domains is not forbidden. Local P violation in strong interactions The strong magnetic field (B~10 15 T) could induce electric field (E~θB), and manifest the P-odd domains with charge separation w.r.t the reaction plane. Kharzeev, PLB633:260 (2006) Kharzeev, McLerran, Warringa, NPA803:227 (2008)

Gang Wang (WWND2010)4 A direct measurement of the P-odd quantity “a” should yield zero. S. Voloshin, PRC 70 (2004) Charge separation in strong interactions Directed flow: vanishes if measured in a symmetric rapidity range Non-flow/non-parity effects: largely cancel out P-even quantity: still sensitive to charge separation

Gang Wang (WWND2010)55 STAR detector

Gang Wang (WWND2010)6 STAR ZDC-SMD SMD is 8 horizontal slats & 7 vertical slats located at 1/3 of the depth of the ZDC New knowledge of the direction of the impact parameter vector Minimal, if any, non-flow/non-parity effects Worse resolution than from TPC… can be overcome with statistics ZDC side view Scintillator slats of Shower Max Detector Transverse plane of ZDC

Gang Wang (WWND2010)7 Approach With the EP from ZDC, the 3-particle non-flow/non-parity correlations (independent of the reaction plane) will be basically eliminated as a source of background. As a systematic check, I also calculate directly The results on the following slides are based on Au+Au collisions at 200 GeV, taken in RHIC run 2004 and 2007.

Gang Wang (WWND2010)8 STAR Preliminary Results with different event planes The correlator using ZDC EP in Run7 is consistent with the correlator using TPC EP in Run4. Lost in the medium? arXiv: ; Phys. Rev. Lett. 103(2009)251601

Gang Wang (WWND2010)9 Cross-check for charge combinations The + + and – – combinations are consistent with each other. STAR Preliminary

Gang Wang (WWND2010)10 Dilution effect In the quark-gluon medium, there could be multiple P-odd domains. The net effect is like a random walk, but one-dimensional. What do we know about the position R n after n steps? The expectation is E(R n )=0. But the absolute distance is not expected to be 0: Compared with going in one fixed direction, the random-walk distance is diluted by a factor proportional to n 1/2. Our measurement of PV is like R n 2, so the dilution factor ~ n ~ N ch.

Gang Wang (WWND2010)11 STAR Preliminary Dilution effect The factor N part is used to compensate for the dilution effect. Weaker B field Non-zero Radial flow? Thin medium STAR Preliminary arXiv: ; Phys. Rev. Lett. 103(2009)251601

Gang Wang (WWND2010)12 Systematic check: v 1 {ZDC-SMD} STAR Preliminary S. Voloshin, PRC 70 (2004) If v 1 (η) is not anti- symmetric around η= 0, then this term won’t vanish.

Gang Wang (WWND2010)13 Systematic check: v 1 {ZDC-SMD} v 1 (η) crosses zero for both charges in the TPC region. S. Voloshin, PRC 70 (2004) STAR Preliminary

Gang Wang (WWND2010)14 Systematic check: a 1 {ZDC-SMD} S. Voloshin, PRC 70 (2004) The average magnitude of is smaller than 10 -4, so the corresponding contribution to the correlator,, will be safely negligible. STAR Preliminary

Gang Wang (WWND2010)15 Systematic check: η gap STAR Preliminary The same-sign correlation approaches zero when the η gap increases.

Gang Wang (WWND2010)16 Systematic check: p T gap The non-zero same-sign correlator for p T gap > 50 MeV indicates that we are safe from HBT or Coulomb effects. STAR Preliminary

Gang Wang (WWND2010)17 Systematic check: average p T The SS correlation increases with the pair p T (p T < 1.6GeV/c). The statistical error is too big to tell for higher p T. STAR Preliminary

Gang Wang (WWND2010)18 Summary In heavy-ion collisions, the formation of (local) meta-stable P-odd domains manifests itself with charge separation w.r.t RP. P-even correlation, still sensitive to charge separation, has been measured, with EP from both STAR TPC and ZDC: consistent! The gross feature of the correlation meets the expectation for the picture of local Parity Violation with charge separation across the reaction plane suppressed by opacity. STAR has checked the possible effects on v 1, s 1, η gap, p T gap and average p T. Further works on the systematic checks and interpretation of the correlation are ongoing…

Gang Wang (WWND2010)19 Back-up slides

Gang Wang (WWND2010)20 Systematic check: EP resolution