Gang Wang (WWND2010) 1

Search for local parity violation with STAR ZDC-SMD

Gang Wang (UCLA)for STAR Collaboration

Gang Wang (WWND2010) 2

Parity violationLooking 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…

xx

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

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.

Local P violation in strong interactions

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)

RPad

dN

sin21

Gang Wang (WWND2010) 4

RPad

dN

sin21

A direct measurement of the P-odd quantity “a” should yield zero.

S. Voloshin, PRC 70 (2004) 057901

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.

)cos(

)cos()2cos( 21

21westeast

westeastRP

fullfullRPa EP_Res/)sin()sin(1

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:0909.1717;Phys. Rev. Lett. 103(2009)251601

Gang Wang (WWND2010) 9

Cross-check for charge combinations

The + + and – – combinations are consistent with each other.

STA

R P

reliminary

Gang Wang (WWND2010) 10

Dilution effectIn 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 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 Rn

2, so the dilution factor ~ n ~ Nch.

n

Rnn

2)||E(lim

Gang Wang (WWND2010) 11

STAR Preliminary

Dilution effect

The factor Npart is used to compensate for the dilution effect.

Weaker B field

Non-zero Radial flow?

Thin medium

outin BB

STAR Preliminary

arXiv:0909.1717;Phys. Rev. Lett. 103(2009)251601

Gang Wang (WWND2010) 12

Systematic check: v1{ZDC-SMD}

STAR Preliminary

S. Voloshin, PRC 70 (2004) 057901

If v1 (η) is not anti-symmetric around η= 0, then this term won’t vanish.

Gang Wang (WWND2010) 13

Systematic check: v1{ZDC-SMD}

v1 (η) crosses zero for both charges in the TPC r

egion.

S. Voloshin, PRC 70 (2004) 057901

STAR Preliminary

Gang Wang (WWND2010) 14

Systematic check: a1{ZDC-SMD} S. Voloshin, PRC 70 (2004) 057901

The average magnitude of <a1> is smaller than 10-4, so the corresponding contribution to the corr

elator, <a1><a1>, will be safely ne

gligible.STAR Preliminary

Gang Wang (WWND2010) 15

Systematic check: η gapST

AR

Prelim

inary

The same-sign correlation approaches zero when the η gap increases.

Gang Wang (WWND2010) 16

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.

STA

R P

reliminary

Gang Wang (WWND2010) 17

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.

STA

R P

reliminary

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 v1, s1,η gap, pT gap and average pT.

• 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