Transcript Effects of Cluster Particle Correlations on Local Parity

Charge Asymmetry Correlations to
Search for Local Parity Violation
Fuqiang Wang
Purdue University
Outline
• Local parity violation in strong interactions and
chiral magnetic effect
• Charge separation and correlation
• Charge correlator measurements by STAR
– Possible physics background
– Possible explanation by cluster particle correlations
• Charge asymmetry correlation measurements
– What could be the underline physics
– Potential measurements to confirm/refute LPV
• Summary
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QCD vaccu
Energy of gluonic field is periodic in NCS
direction (~ a generalized coordinate)
Instantons and sphalerons are
localized (in space and time) solutions
describing transitions between different vacua
via tunneling or go-over-barrier
The volume of the box is 2.4 by 2.4 by 3.6 fm.
The topological charge density
Animation by Derek Leinweber
Topological transitions have never been observed
directly (e.g. at the level of quarks in DIS). An
observation of the spontaneous strong parity
violation would be a clear proof for the existence
of such physics.
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Chiral magnetic effect
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B ~  e / 2 [fm 2 ] ~ 1015 T (next slide)
Metastable QGP domain
Chiral symmetry restoration,
massless quarks
Topological charge (e.g. Q=-1):
quarks L  R (by mom. flip)
maximal parity violation.
Strong magnetic field, spin
locked: + charge spin up
– charge spin down
Single-handed quarks:
momentum & spin parallel
+ charge move up,
– charge move down.
 ~ eB / 4mq ~  e2 / 8mq [fm 2 MeV 1 ] ~ 100 1.4 / 40  200 MeV ~ 700 MeV
Only Landau ground state is occupied. Thermal motion (~200 MeV) not important.
Need large B, and ~massless quarks.
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Magentic field
A/ 2
ec  4 fm ~  e / 2 [fm 2 ] ~ 4 fm 2 ~ 1015 T
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4 A / 3 fm
eB ~  e2 / 2 [fm2 ] ~ 0.35 fm2 ~ 1.4 104 MeV 2
B ~ j  r ~  necr ~ 
Energy density in B field:
B2 ~  2e2 / 4 [fm4 ] ~ 3.5 GeV/fm3
Short duration: r/ ~ 0.1 fm.
B drops quickly with time.
Larmor radius:
rL ~  mq / eB ~ 2mq / e2 [fm2 ] ~10 /1.4 fm ~ 7 fm
Much larger than lifetime of large B: r/.
The large B unlikely has an effect.
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Charge separation
If all work:
• QGP domain
• Chiral symmetry
• Large B and sizeable effect
Kharzeev et al. estimate: Charge asymmetry ~ 1%.
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STAR detector
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Parity violation observable
Voloshin, PRC 70 (2004) 057901
Use a third particle
Small
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PV observable
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<cos(fa+fb2y)>
Charge correlation
Initial expectation vs data
STAR: Au+Au @ 62 GeV
++
Like-sign
+
Initial (naïve) theory
expectations
Unlike-sign
—
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Back-to-back charge suppression
++
X
—
—
Medium interaction
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STAR 3-particle correlator measurement
What’s
measured
What one’s after:
<cos(fa+fb2y)>
Assumption: 3-particle correlation is negligible.
<cos(fa+fb2y)> ≈ <cos(fa+fb2fc)> / v2,c
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Published STAR data
0909.1717, 0909.1739
Unlike-sign
suppressed by
medium?
Like-sign consistent
with PV.
<cos(fa+fb2fc)> / v2,c
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Possible physics background
Observable is parity-even, subject to physics backgrounds.
Resonance decay effect:
cos(fa  fb  2y )
reso.
 cos(fa  fb  2freso.  2(freso. y )
 cos(fa  fb  2freso. ) v2,reso. ~ v2,reso. /10
cos(fa  fb  2y ) ~
Nreso.
cos(fa  fb  2y )
N  N 
reso.
~
v2,reso.
10 N
~ 105
Order of magnitude smaller than measurement. Unlikely source of background.
But how about clusters of particles: each cluster can have many particle pairs.
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Scott Pratt
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Scott Pratt
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Cluster model
FW, PRC 81, 064902 (2010).
SA: Small-Angle pairs
v2 < 0
cos(fa  fb  2y )  0
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BB: Back-to-back pairs
v2 > 0
cos(fa  fb  2y )  0
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Needed v2,SA and v2,BB
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Issue 1 with cos(fa+fb-2yRP)
SA: Small-Angle pairs
BB: Back-to-back pairs
Cannot be
distinguished!
cos(a  b  2y )  0
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cos(a  b  2y )  0
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Issue 2 with cos(fa+fb-2yRP)
There are more particles in-plane than out-of-plane,
therefore <cos*cos> has more weight than <sin*sin>.
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New Variable: Charge Asymmetry Correlation
UP
RIGHT
LEFT
EP
DOWN
A ,UD 
A , LR 
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N  ,up  N  , down
N  ,up  N  , down
LPV effects in UD. LR is null-reference.
LPV expectations:
• A+UD and A-UD are anti-correlated
→ ‹A+A-›UD < ‹A+A-›LR
• Additional dynamical fluctuation
broadens A±UD distributions
→ ‹A±2›UD > ‹A±2›LR
N  ,left  N  , right
N  ,left  N  , right
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Analysis: to avoid self-correlation
EP
A ,UD 
A , LR 
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N  ,up  N  , down
N  ,up  N  , down
N  ,left  N  , right
N  ,left  N  , right
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Stat. fluc. + detector effects in ‹A2›
STAR preliminary
Adding-
method
EP
1
1

N
N
2
A
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Stat. fluct. +
Detector effects
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Subtracting stat+det effects
STAR preliminary
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Symbols are data (TPC left/right, positive/negative charges).
Curves: net effect of statistical fluct. and detector non-uniformity.
Stat+det effects are actually wider than data in ‹A±2›.
Physical processes narrow charge asymmetry distributions 
same-sign back-to-back pairs.
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Charge Asym. Correl. Results
STAR preliminary
• Oppo-sign aligned; ‹A+A-›UD > ‹A+A-›LR
LPV expects: ‹A+A-›UD < ‹A+A-›LR
Contradicts LPV expectations.
—
+
+
+—
d+Au
+
+
+
• Same-sign back-to-back in central,
unexpected from only LPV.
Data: ‹A2›UD > ‹A2›LR
LPV expects: ‹A2›UD > ‹A2›LR
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The observable: UD-LR
Like-sign:
consistent w/ LPV
Unlike-sign:
contradicts LPV
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pT dependence
20-40% centrality
STAR preliminary
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• Opposite-sign charges
are more aligned at
high-pt.
• Same-sign charges are
more preferentially
back-to-back at high-pt.
RHIC/AGS Users Meeting Local Parity Violation workshop -- Fuqiang Wang
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UD-LR vs pt
20-40%
STAR preliminary
0-20%
STAR preliminary
• UD-LR increases with pt, ~linearly.
• Qualitatively similar between same-sign and opposite-sign.
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UD-LR compared to correlator
Asymmetry correlations can be expanded into harmonic terms. One term is the correlator.
STAR preliminary
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Charge asymmetry correlations have been divided by EP resolution.
“Corrected” same-sign charge asymmetry correlation and correlator are similar: higher
order terms are negligible.
“Corrected” opposite-sign charge asymmetry correlation and correlator are different:
higher order terms are important.
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What could be the underlying physics?
Possible causes: correlation overlaid with v2 (FW, PRC 81, 064902 (2010))
– Charge balance (S. Pratt): correlator ~ -v2/N*‹cos2DfBF›, may affect ‹A+A-›.
– Momentum conservation (S. Pratt): correlator ~ v2/N, may affect both ‹A+A-› and ‹A2›.
– Path-length dependent jet-quenching, opacity.
Try to investigate those effects experimentally, by studying UD-LR
vs event-by-event high-pT v2 (indicative of jet-quenching) and bulk v2.
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UD–LR vs Event-by-Event Anisotropy
20-40% centrality
STAR preliminary
• Same trend in high-pt v2.
• Stronger jet-quenching
out-of-plane enhance UD
asymmetry for both ‹A2UD›
and ‹A+A-›UD?
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20-40% centrality
STAR preliminary
• ‹A2›, ‹A+A-› opposite trend in low-pT v2!
• More b-to-b same-charge pairs in-plane
reducing ‹A2LR›? → Increasing trend
• More opposite-charge pairs in-plane
enhancing ‹A+A-›LR? → Decreasing trend
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The Central Problem
We do NOT know exactly what the background is!
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UD-LR vs wedge size
20-40% centrality
STAR preliminary
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Summary
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Chiral magnetic effect is predicted by QCD. It generates charge separation.
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Charge correlator:
cos(a+b-2y)LS<0: consistent with local parity violation.
cos(a+b-2y)US≈0: unexpected from LPV only, but can be explained by back-to-back
suppression.
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New variable: charge asymmetry correlations are reported.
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Opposite-sign charges are preferentially aligned.
Same-sign charges are preferentially back-to-back in central collisions.
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UD – LR and cos(a+b-2y) are related:
‹A2›UD > ‹A2›LR & cos(a+b-2y)LS<0: qualitatively similar.
‹A+A-›UD > ‹A+A-›LR: contradicts LPV expectations.
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Detailed studies of charge asymmetry correlations, hopefully providing new insights:
– UD-LR v.s. E-by-E high-pT and low-pT v2’s
– Dependence on wedge size
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My hunch: observed data are some kind of particle/cluster correlations; the data do
not demand LPV. LPV effect, if exists, must be orders of magnitude smaller than the
observed data.
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