Transcript Wang

Search for local parity violation
with STAR ZDC-SMD
Gang Wang (UCLA)
for STAR Collaboration
Gang Wang (WWND2010)
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Parity violation
Looking into a mirror,
you see someone else…
It’s a parity violation?!
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
Parity transformation:
A spatial inversion of the coordinates.


x  x
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…
Gang Wang (WWND2010)
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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.
dN 

d

 1  2a  sin   RP
Gang Wang (WWND2010)
Kharzeev, PLB633:260 (2006)
Kharzeev, McLerran, Warringa, NPA803:227 (2008)

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Charge separation in strong interactions

dN 
 1  2a  sin    RP
d

A direct measurement of the P-odd quantity “a” should yield zero.
S. Voloshin, PRC 70 (2004) 057901
Non-flow/non-parity effects:
largely cancel out
Directed flow: vanishes Gang Wang (WWND2010)
if measured in a symmetric rapidity range
P-even quantity:
still sensitive to charge
separation
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STAR detector
Gang Wang (WWND2010)
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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 of
Shower Max Detector
Transverse
plane of
ZDC
Gang Wang (WWND2010)
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Approach
cos(1   2   east   west )
cos(1   2  2 RP ) 
cos( east   west )
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
a1  sin(   RP )  sin(   full ) / EP_Res full
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)
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Results with different event planes
STAR Preliminary
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)
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Cross-check for charge combinations
STAR Preliminary
The + + and – – combinations are consistent with each other.
Gang Wang (WWND2010)
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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 Rn after n steps?
The expectation is E(Rn)=0.
E(| Rn |) 
But the absolute distance is not expected to be 0: lim
n 
2n

Compared with going in one fixed direction,
the random-walk distance is diluted by a factor proportional to n1/2.
Our measurement of PV isGang
likeWang
Rn2(WWND2010)
, so the dilution factor ~ n ~ N10ch.
Dilution effect
Non-zero Bin  Bout
Radial flow?
Weaker B field
STAR Preliminary
arXiv:0909.1717;
Phys. Rev. Lett. 103(2009)251601
Thin medium
The factor Npart is used to compensate for the dilution effect.
Gang Wang (WWND2010)
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Systematic check: v1{ZDC-SMD}
S. Voloshin, PRC 70 (2004) 057901
If v1 (η) is not antisymmetric around η= 0,
then this term won’t vanish.
STAR Preliminary
Gang Wang (WWND2010)
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Systematic check: v1{ZDC-SMD}
S. Voloshin, PRC 70 (2004) 057901
v1 (η) crosses
zero for both
charges in the
TPC region.
STAR Preliminary
Gang Wang (WWND2010)
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Systematic check: a1{ZDC-SMD}
S. Voloshin, PRC 70 (2004) 057901
STAR Preliminary
Gang Wang (WWND2010)
The average
magnitude of
<a1> is smaller
than 10-4, so the
corresponding
contribution to
the correlator,
<a1><a1>, will
be safely
negligible.
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Systematic check: η gap
STAR Preliminary
The same-sign correlation approaches zero
when the η gap increases.
Gang Wang (WWND2010)
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Systematic check: pT gap
STAR Preliminary
The non-zero same-sign correlator for pT gap > 50 MeV
indicates that we are safe from HBT or Coulomb effects.
Gang Wang (WWND2010)
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Systematic check: average pT
STAR Preliminary
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)
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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)
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Back-up slides
Gang Wang (WWND2010)
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Systematic check: EP resolution
Gang Wang (WWND2010)
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