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Charge dependent azimuthal correlations with the ALICE
detector at the LHC
Panos Christakoglou1, for the ALICE Collaboration
1Nikhef
25.06.2012
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1
Motivation
 Suggestions that heavy-ion collisions may form domains where the parity symmetry
in strong interaction is locally violated
 In non-central collisions, these domains may manifest themselves by a separation of
charge, above and below the reaction plane.
 The resulting charge separation is a consequence of two factors
o
o
the difference in numbers of quarks with positive and negative chiralities due to a non-zero
topological charge of the region,
the interaction of these particles with the extremely strong and short lived magnetic field
produced in such a collision (the Chiral Magnetic Effect-CME).
 The existence of the CME, is directly related to the Chiral Symmetry restoration and
to extreme B field values
o
~1018 Gaus, stronger than on the surface of a neutron star
• D. Kharzeev, Phys. Lett. B633, 260 (2006).
• D. Kharzeev and A. Zhitnitsky, Nucl. Phys. A797,
67 (2007).
• D. E. Kharzeev, L. D. McLerran and H. J. Warringa,
Nucl. Phys. A803, 227 (2008).
• K. Fukushima, D. E. Kharzeev and H. J. Warringa,
Phys. Rev. D78, 074033 (2008).
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Proposed tools: azimuthal correlations
S. Voloshin, Phys. Rev. C70, 057901 (2004)
3–particle
correlator
2–particle
correlator
(
)
(
)
cos (fa - fb ) = cos (fa -Y RP ) - (fb -Y RP ) =
cos (fa + fb -2Y RP ) = cos (fa + fb -2jg ) /v2g
cos (fa + fb -2Y RP ) = cos (fa -Y RP ) + (fb -Y RP ) =
cos (Dja -Djb ) = cos (Dja ) cos (Djb ) + sin (Dja ) sin (Djb )
cos (Dja +Djb ) = cos (Dja ) cos (Djb ) - sin (Dja ) sin (Djb )
correlations in-plane
1é
cos (Dja ) cos (Djb ) = ë cos (Dja +Djb ) + cos (Dja -Djb ) ùû
2
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correlations out-of-plane
1é
sin (Dja ) sin (Djb ) = ë cos (Dja -Djb ) - cos (Dja +Djb ) ùû
2
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ALICE: Experimental setup
Not shown: ZDC
~116m from I.P.
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Studies in ALICE: Analysis details
 Analysis of the Pb-Pb events recorded in
November/December 2010 during the
first LHC heavy-ion run
o
Event sample split in two sets having
different magnetic field polarities (results
used for the systematic uncertainties)
 Trigger conditions:
o
o
SPD, VZERO-A, VZERO-C (2 out of 3)
VZERO-A && VZERO-C
 The centrality is selected using the
magnitude of the VZERO signal
(~multiplicity) as the default estimator
o
o
Centrality bins: 0-5%, 5-10%, 1020%,…,60-70%
Different centrality estimators (TPC
tracks, SPD clusters) investigated
 Results used for the systematic uncertainty
 Due to the small magnitude of the
potential signal, we need to have the
acceptance corrections under control:
o
o
The TPC tracks provide a uniform
acceptance with minimal corrections
Disadvantage: contamination from
secondaries
 Investigated by varying the cut on the
distance of closest approach (results used
for the systematic uncertainty).
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Centrality dependence: Charge combinations
0.6 ´10
b
á cos(fa + f - 2YRP) ñ
-3
Correlations measured
with the cumulant
technique
0.4
0.2
0
-0.2
ALICE Pb-Pb @ sNN = 2.76 TeV
(+-)
(++)
(--)
-0.4
-0.6
0
10
20
30
40
50
60
70
centrality, %
 Clear charge asymmetry observed
 Results for (++) and (--) consistent (combined later as “Same charge”)
 The magnitude of the correlations between the same charged pairs is larger than the one of the
opposite charges (excluding the most peripheral collisions  due to large non-flow?)
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Different methods: event plane estimate from detectors
Qn,y = å w i × sin( nf i )
i
Qn,x = å w i × cos( nf i )
Event plane from charged particles at forward rapidity
i
æ Qn,y ö
Yn = atan2ç
÷/n
Q
è n,x ø
Event plane from charged particles at mid-rapidity
Event plane from the neutron spectators
Investigation with four independent methods
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Centrality dependence: Comparison of methods
´10-3
TPC (cumulants)
TPC (cumulants)
TPC (cumulants)
TPC
VZERO
TPC
VZERO
ZDC
0
b
á cos(fa + f - 2YRP) ñ
0.5 same opp.
-0.5
0
10
20
30
40
50 60 70
centrality, %
Very good
agreement
between
the four
methods
Charge
asymmetry
due to
correlation
wrt the
reaction plane
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3-particle correlator: LHC vs RHIC
Stat. error: error bars
Syst. error: shaded area
STAR Collaboration: Phys. Rev. Lett. 81, 251601 (2009)
STAR Collaboration: Phys. Rev. C81, 054908 (2010)
0.6 ´10
b
á cos(fa + f - 2YRP) ñ
-3
0.4
0.2
0
-0.2
-0.4
same opp.
ALICE Pb-Pb @ sNN = 2.76 TeV
ALICE Pb-Pb @ sNN = 2.76 TeV
STAR Au-Au @ sNN = 0.2 TeV
-0.6
0
10
20
30
40
50
60
70
centrality, %
 Magnitude of the effect seems to be similar to what is reported by STAR.
 Some models predict a much lower effect at LHC energies (see next slide)
o
Signal and background should both scale with the inverse of the square of the multiplicity
 The effect can be similar depending on the t0 of the magnetic field
o
o
25.06.2012
D. Kharzeev et al., Nucl. Phys. A803, (227) 2008
A. R. Zhitnitsky, arXiv:1201.2665 [hep-ph].
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3-particle correlator: Comparison with models
á cos(fa + f - 2YRP) ñ
-3
´
10
0.6
ácos(fa + f - 2fc)ñHIJING / v2{2}
0.4
b
0.2
b
b
á cos(fa + f - 2YRP) ñ
V.D. Toneev and V. Voronyuk, arXiv:1012.1508v1 [nucl-th]
0
ácos(fa + f - 2fc)ñHIJING / v2{2}
b
0.4
CME expectation (Toneev et al.)
0.2
0
-0.2
-0.2
-0.4
-3
´
10
0.6
same opp.
-0.4
ALICE Pb-Pb @ sNN = 2.76 TeV
STAR Au-Au @ sNN = 0.2 TeV
-0.6
0
10
20
30
40
same opp.
ALICE Pb-Pb @ sNN = 2.76 TeV
STAR Au-Au @ sNN = 0.2 TeV
-0.6
50
60
0
70
10
20
30


HIJING results between pairs of same and opposite
charge are consistent  combined into one point
HIJING points consistent with the (+-) data points
HIJING points scaled with the square of the multiplicity,
consistent with the idea of having the correlations
originating from emerging clusters (jets, resonances)
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50
60
70
centrality, %
centrality, %

40


The only available quantitative prediction for LHC
energies (@4.5 TeV)
According to the authors the magnitude should roughly
scale with 1/√s
o
Applied in the figure to convert the prediction to √sNN = 2.76
TeV
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2–particle correlations: Centrality dependence

7 ´10
-3
6
(+-)
(++)
(--)
5
b
á cos(fa-f ) ñ

Pb-Pb @ sNN = 2.76 TeV

4
3
(++) and (--) combined into one set of points
(“Same charge”).
Similarity to STAR: the magnitude of the
opposite charged pairs which is larger than the
same charged ones.
Difference with STAR:
o Sign of the same charged correlations
o Strength of the correlations
2
10
7 ´same
opp.
ALICE Pb-Pb @
6
1
-3
0
20
30
40
50 60 70
centrality, %
 Correlations between opposite
charges are positive and large
 Correlations of same charged pairs
are also positive and have a smaller
magnitude
 Results between (++) and (--) are
consistent
25.06.2012
STAR Au-Au @ sNN = 0.2 TeV
5
b
10
á cos(fa-f ) ñ
-1
0
sNN = 2.76 TeV
4
3
2
1
0
-1
0
10
20
30
40
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50 60 70
centrality, %
11
Decomposition
0.003
Pb-Pb @ sNN = 2.76 TeV
same opp.
á cos(D f ) cos(D f ) ñ
a
b
á sin(D fa) sin(D f ) ñ
b
0.002
0.001
0
0
10
20
30
40
50 60 70
centrality, %
 Similar magnitude for the cos terms for same and opposite charged pairs
 Higher magnitude for the sin terms for same than opposite charged pairs
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Background effects: flow fluctuations
 The orientation angle of the dipole
asymmetry shows a preference out-ofplane.
o
D. Teaney and L. Yan, arXiv:1010.1876v1 [nucl-th]
This results in a net v1 out of plave with a
small magnitude
 The magnitude of the correlations
depending on the freeze-out conditions
can give a potentially significant
contribution
o
The hydrodynamic calculation though
does not describe the charge separation!
 Baseline shift in our measurement?
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Background effects: initial state fluctuations (cont.)
Charge independent correlator
0.6 ´10
b
á cos(fa + f - 2YRP) ñ
-3
ácos(fa + f - 2fc)ñHIJING / v2{2}
b
0.4
CME expectation (Toneev et al.)
same+opp. mean
0.2
0
-0.2
-0.4
same opp.
ALICE Pb-Pb @ sNN = 2.76 TeV
STAR Au-Au @ sNN = 0.2 TeV
-0.6
0
10
20
30
40
50
60
70
centrality, %
P. Christakoglou (for the ALICE Collaboration), Phys. G G38, (2011) 124165
Paper at the last stage of the Collaboration review (will released soon after the workshop):
2- and 3-particle integrated correlator + differential analysis (3-particle correlator)
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Summary
 The possibility of observing parity odd domains was investigated by using
both a 2-particle and a 3-particle P-even correlator.
 The results from the 2-particle correlator studies show that the sign of the
correlations is the same regardless of the charge combination, contrary to
what was observed in STAR
o Need to take into account the different non-flow contributions
 The centrality dependence of the 3-particle correlator illustrates a
remarkable agreement in both the magnitude and the behavior with the
results reported by STAR in Au-Au collisions at √sNN = 0.2 TeV
o Hydro calculations indicate that the dipole asymmetry’s preferential out-of-plane
orientation might result into a v1 contribution out-of-plane, but the charge
asymmetry is not explained.
o Baseline shift from the fluctuations of the initial geometry?
 Theory was not clear about the possible energy dependence of the effect
o Significant need for quantitative (realistic) calculations of the CME effects for
both RHIC and LHC energies
Charge asymmetry is seen experimentally with a similar magnitude as at the highest
RHIC energy  Theory is challenged by the latest findings!
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Outlook (towards QM…and beyond)
 Charge conservation coupled to elliptic flow seems to describe the
difference of the 3-particle correlator for same and opposite charged pairs at
RHIC
o Look at the balance function wrt Ψ
S. Schlichting and S. Pratt, Phys. Rev. C83, 014913 (2011).
S. Pratt, S. Schlichting and S. Gavin, Phys. Rev. C84, 024909 (2011)
 Look at other correlators (e.g. double harmonics)
S. Voloshin, arXiv:1111.7241 [nucl-ex]
 Correlations between identified particles
 Chiral vortical effect studies
D. Kharzeev Phys. Rev. Lett. 106 (2011) 062301
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BACKUP
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