A New Method for Detecting a Possible Parity
Violating Signal in Heavy Ion Reactions at RHIC
N. N. Ajitanand
Nuclear Chemistry, SUNY Stony Brook
For the PHENIX Collaboration
ICPAQGP Goa, India
December 6-10 2010
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Local Parity Violation at RHIC
It has been proposed [1] that in heavy ion collisions, metastable domains may be
created in which parity and CP symmetries are violated.
A combination of a net chirality of quarks within a domain and the extremely
strong magnetic field in a heavy ion collision could lead to the manifestation of
parity violation as a separation of charges along the angular momentum vector of
the collision system [2].
L or B
1.D. Kharzeev, R. D. Pisarski, and M. H. G. Tytgat,
Phys. Rev. Lett. 81,
512 (1998).
2.D. Kharzeev and R. D. Pisarski, Phys. Rev. D 61,
111901 (2000).
N. N. Ajitanand ICPAQGP 2010
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Azimuthal distribution w.r.t the reaction plane
N(φ ) = N0 (1+2v2cos(2φ)+2v4cos(4φ)+ 2a1sin(φ))
a1>0
a1<0
B
B ++
-φ
φ
ΨRP
-Positive charges favor B
++
ΨRP
Negative charges favor B
Charge seperation along B field results from parity breaking term a1
Detection of azimuthal charge-asymmetry is
a pre-requisite for evidence of Parity Violation
3
Charged Particle Tracks
|η|<0.35, π/2 x 2 arms
Central Arm Spectrometer (DC,PCs)
Mid Reaction Plane
1.0<|η|<2.8, 0<φ<2π
Reaction Plane Detector (RXN)
Charge asymmetry determined for
azimuthal distribution of charged
hadrons in the central arms w.r.t
Reaction Plane determined by
RXN
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The two-particle correlation – an extension
of the STAR method
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Two-particle
correlation Method
L or B
Y
s1
1 c2
 ,   , 
v1= 0 ,
Bin
≈
Bout
Averaging is performed over all pairs in
a given event and then over all events
with a given centrality
Signal proportional to square of ‘a1’
2
c1
Reactio
n Plane
s2
acceptance correction accounted for
with event mixing
reaction plane resolution correction
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X
Two-particle correlation Results
Signal grows with centrality and pT
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Two-particle correlation Results
Relatively good agreement between PHENIX & STAR
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A new multi-particle correlation method
arXiv:1009.5624 [nucl-ex]
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Multi-particle correlation: Construction of mixed
event
Event with p positively charged hadrons and n negatively charged
hadrons
1
2
3
4
----
p
1
2
3
4
----
n
p positives
n negatives
Construction of statistically matched mixed event
1
2
3
4
----
p
p random choices
1
2
3
4
----
n
n remaining
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S  sin( )
Evaluate real event averages of S
S ph 
= average S over the p positively charged hadrons in the event
S nh 
= average S over the n negatively charged hadrons in the event
Evaluate mixed event averages of S
S ph
= average S over p randomly chosen hadrons in the same event
S nh
= average S over n remaining hadrons in the same event
S

C  S  
N S
N
p
h
p
 Snh 
h
p
 S
h
n
,

S  S p  S n
The new multi-particle correlator
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Simulated Results
Properties of Cp
a1 > 0 in all events
N
N
S
h
p
 Snh 

(Blue points)
N
S
h
p
 Snh

(Red points)
S
Numerator shifted to the right
Cp shows positive slope
N. N. Ajitanand ICPAQGP 2010
Cp
S
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Simulated Results
Properties of Cp
a1 < 0 in all events
N
N
S
h
p
 Snh 

(Blue points)
N
S
h
p
 Snh

(Red points)
S
Cp
Numerator shifted to the left
Cp shows negative slope
S
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Simulated Results
Properties of Cp
a1
changes sign from
event to event
N
N
S
h
p
 Snh 

(Blue points)
N
S
h
p
 Snh

(Red points)
S
Cp
Numerator slightly broader
Cp concave and symmetric
S
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Simulated Results
Properties of Cp
Cp is an in-event correlator
 Several benefits
Flow : Yes
Parity violating signal : No
Decay : No
Cp
Flat response to flow
S
Cp
Jet : Yes
Parity violating signal : No
Decay : No
Flat response to jets
Cp insensitive to flow and jets
S
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Simulated Results
Properties of Cp
Cp is an in-event correlator
 Several benefits
Flow : Yes
Parity violating signal : Yes
Decay : No
Cp
Concave response to parity
violation
S
Cp
Flow : Yes
Parity violating signal : No
Decay : Yes
Convex response to decays
S
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Concavity also observed for more central events
Simulation fits favor a lower value of a1
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Summary
• PHENIX measurements of azimuthal chargeasymmetry presented for two separate methods
 Azimuthal charge-asymmetry observed by both
methods
Extracted signals in general agreement with STAR
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