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Search for local parity violation with STAR ZDC-SMD

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

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Search for local parity violation with STAR ZDC-SMD

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  1. Search for local parity violation with STAR ZDC-SMD for STAR Collaboration Gang Wang (UCLA) 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!) • 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, Zhitnitsky, NPA 797 67 (2007) Kharzeev, McLerran, Warringa, NPA 803 227 (2008) Fukushima, Kharzeev, Waringa, PRD 78, 074033 Gang Wang (WWND2010)

  3. 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 (WWND2010)

  4. 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 (WWND2010)

  5. STAR detector Gang Wang (WWND2010) 5

  6. STAR ZDC-SMD • 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 SMD is 8 horizontal slats & 7 vertical slats located at 1/3 of the depth of the ZDC ZDC side view Scintillator slats ofShower Max Detector Transverse plane of ZDC Gang Wang (WWND2010)

  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 2004 and 2007. Gang Wang (WWND2010)

  8. STAR Preliminary Results with different event planes Lost in the medium? arXiv:0909.1717; Phys. Rev. Lett. 103(2009)251601 The correlator using ZDC EP in Run7 is consistent with the correlator using TPC EP in Run4. Gang Wang (WWND2010)

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

  10. 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 (WWND2010)

  11. STAR Preliminary Non-zero Radial flow? Dilution effect Weaker B field STAR Preliminary Thin medium arXiv:0909.1717; Phys. Rev. Lett. 103(2009)251601 The factor Npart is used to compensate for the dilution effect. Gang Wang (WWND2010)

  12. STAR Preliminary S. Voloshin, PRC 70 (2004) 057901 Systematic check: v1{ZDC-SMD} If v1 (η) is not anti-symmetric around η= 0, then this term won’t vanish. Gang Wang (WWND2010)

  13. STAR Preliminary S. Voloshin, PRC 70 (2004) 057901 Systematic check: v1{ZDC-SMD} v1 (η) crosses zero for both charges in the TPC region. Gang Wang (WWND2010)

  14. 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 (WWND2010)

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

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

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

  18. 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 v1, s1,η gap, pT gap and average pT. • Further works on the systematic checks and interpretation of the correlation are ongoing… Summary Gang Wang (WWND2010)

  19. Back-up slides Gang Wang (WWND2010)

  20. Systematic check: EP resolution Gang Wang (WWND2010)

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