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Searching for Local Strong Parity Violation in STAR

This study examines the presence of local strong parity violation in heavy-ion collisions using the STAR detector. The formation of meta-stable P-odd domains and charge separation is investigated by measuring the P-even correlator with event planes from both TPC and ZDC. The results show consistency with the expectation for local parity violation. Additional systematic checks and interpretation of the correlation are ongoing.

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Searching for Local Strong Parity Violation in STAR

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  1. Gang Wang (APS2010)

  2. Search for Local Strong Parity ViolationUsing Reaction Plane Determined by Spectator Neutrons in STAR for STAR Collaboration Gang Wang (UCLA) Gang Wang (APS2010)

  3. 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!) • Global parity violation Occurs in weak interactions • Confirmed • Local parity violation Predicted in strong interactions • we are working on it… Kharzeev, PLB 633 260 (2006) [hep-ph/0406125]; Kharzeev, McLerran, Warringa, NPA 803 227 (2008); Kharzeev, Zhitnitsky, NPA 797 67 (2007); Fukushima, Kharzeev, Waringa, PRD 78, 074033. Gang Wang (APS2010)

  4. Local P violation in strong interactions 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., PRL97:131801 (2006) In heavy-ion collisions, the formation of (local) meta-stable P-odd domains is not forbidden. The strong magnetic field (B~1015 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 (APS2010)

  5. Charge separation in strong interactions S. Voloshin, PRC 70 (2004) 057901 Non-flow/non-parity effects: largely cancel out P-even quantity: still sensitive to charge separation Directed flow: vanishes if measured in a symmetric rapidity range A direct measurement ofthe P-odd quantity “a” should yield zero. Gang Wang (APS2010)

  6. STAR detector • Spectator neutrons are captured by STAR ZDC-SMDs • 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 n Gang Wang (APS2010)

  7. Approach With the EP from ZDC, the3-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 2007. Gang Wang (APS2010)

  8. STAR Preliminary Results with different event planes Lost in the medium? The correlator using ZDC EP is consistent with that using TPC EP. Gang Wang (APS2010)

  9. In the quark-gluon medium, there could be multiple P-odd domains. The net effect is like a random walk, but one-dimensional. Dilution effect What do we know about the position Rn after n steps? The expectation is E(Rn)=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 n1/2. Our measurement of PV is like Rn2, so the dilution factor ~ n ~ Nch. Gang Wang (APS2010)

  10. Non-zero Radial flow? Dilution effect Weaker B field STAR Preliminary STAR Preliminary Thin medium The factor Npart is used to compensate for the dilution effect. Gang Wang (APS2010)

  11. STAR Preliminary STAR Preliminary S. Voloshin, PRC 70 (2004) 057901 Systematic check: v1{ZDC-SMD} v1 (η) crosses zero for both charges in the TPC region. If v1 (η) is not anti-symmetric around η= 0, then this term won’t vanish. Gang Wang (APS2010)

  12. STAR Preliminary S. Voloshin, PRC 70 (2004) 057901 Systematic check: a1{ZDC-SMD} The average magnitude of <a1> is smaller than 10-4, so the corresponding contribution to the correlator, <a1><a1>, will be safely negligible. Gang Wang (APS2010)

  13. Systematic check: ηgap STAR Preliminary The same-sign correlation approaches zero when the η gap increases. Gang Wang (APS2010)

  14. STAR Preliminary Systematic check: pT gap The non-zero same-sign correlator for pT gap > 200 MeV/c indicates that we are safe from HBT or Coulomb effects. Gang Wang (APS2010)

  15. The formation of (local) meta-stable P-odd domains in heavy-ion collisions is predicted to lead to charge separation w.r.t the reaction plane. • P-even correlator has been measured with event planes from both STAR TPC and ZDC; and the results are consistent! • The gross feature of the correlator meets the expectation for the picture of local Parity Violation: charge separation, suppression of OS by opacity, weaker OS signal in central collisions, OS&LS symmetry in peripheral collisions ... • STAR has checked the possible effects on v1, s1,η gap, pT gap. • Further work on the systematic checks and interpretation of the correlation is ongoing… Summary Gang Wang (APS2010)

  16. Back-up slides Gang Wang (APS2010)

  17. Systematic check: EP resolution Gang Wang (APS2010)

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