Charge dependent azimuthal correlations with the ALICE detector at the LHC Panos Christakoglou1, for the ALICE Collaboration 1Nikhef 25.06.2012 [email protected] - P and CP odd.
Download ReportTranscript Charge dependent azimuthal correlations with the ALICE detector at the LHC Panos Christakoglou1, for the ALICE Collaboration 1Nikhef 25.06.2012 [email protected] - P and CP odd.
Charge dependent azimuthal correlations with the ALICE detector at the LHC
Panos Christakoglou 1 , for the ALICE Collaboration 1 Nikhef 25.06.2012
[email protected] - P and CP odd effects in hot and dense matter, BNL 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 the difference in numbers of quarks with positive and negative chiralities due to a non-zero topological charge of the region, o 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 ~10 18 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). 25.06.2012
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Proposed tools: azimuthal correlations
S. Voloshin, Phys. Rev.
C70
, 057901 (2004) 3 –particle cos ( f a + f b correlator 2 Y
RP
) = cos ( f a + f b 2 j g ) /
v
2 g cos ( f a + f b 2 Y
RP
) = cos ( ( f a -Y
RP
) + ( f b -Y
RP
) ) = cos ( D j a +D j b ) = cos ( ) cos sin ( ) a sin cos f ( ) b 2 –particle = correlator cos ( ( f a -Y
RP
) ( f b -Y
RP
) ) cos ( D j a -D j b ) = cos ( ) a cos = + sin ( ) a sin cos ( ) a cos correlations in-plane = 2 1 é é cos ( D j a +D j b ) + cos ( D j a -D j b ) é é sin ( ) sin correlations out-of-plane = 2 1 é é cos ( D j a -D j b ) cos ( D j a +D j b ) é é 25.06.2012
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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 Centrality bins: 0-5%, 5-10%, 10 20%,…,60-70% o 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 The TPC tracks provide a uniform acceptance with minimal corrections o 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 -3 Correlations measured with the cumulant technique 0.4
0.2
0 -0.2
-0.4
-0.6
ALICE Pb-Pb @ (+-) (++) (--) s NN = 2.76 TeV 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?) 25.06.2012
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Different methods: event plane estimate from detectors
Q n
,
y
= å
i w i
× sin ( )
Q n
,
x
= å
i w i
× cos ( ) Y
n
=
a
tan2 æ è
Q n
,
y Q n
,
x
ö ø /
n
Event plane from charged particles at mid-rapidity Event plane from charged particles at forward rapidity Event plane from the neutron spectators Investigation with four independent methods 25.06.2012
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Centrality dependence: Comparison of methods
´
10
same opp.
TPC (cumulants) TPC
0 10 20 30 40 50 60 70
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Stat. error: error bars Syst. error: shaded area
3-particle correlator: LHC vs RHIC
´ 10 STAR Collaboration: Phys. Rev. Lett.
81
, 251601 (2009) STAR Collaboration: Phys. Rev.
C81
, 054908 (2010) ALICE Pb-Pb @ ALICE Pb-Pb @ STAR Au-Au @ s s s NN NN NN = 2.76 TeV = 2.76 TeV = 0.2 TeV 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 t 0 o of the magnetic field D. Kharzeev et al., Nucl. Phys.
A803
, (227) 2008 o A. R. Zhitnitsky, arXiv:1201.2665 [hep-ph].
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3-particle correlator: Comparison with models
0.6
´ 10 -3 0.4
0.2
0 é cos( f a + f b - 2 f c ) é HIJING / v 2 {2} V.D. Toneev and V. Voronyuk, arXiv:1012.1508v1 [nucl-th] 0.6
´ 10 -3 é cos( f a + f b - 2 f c ) é HIJING / v 2 {2} 0.4
CME expectation (Toneev
et al.
) 0.2
0 -0.2
-0.4
-0.6
same opp.
ALICE Pb-Pb @ s NN = 2.76 TeV STAR Au-Au @ s NN = 0.2 TeV 0 10 20 30 40 50 60 70 centrality, % 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) -0.2
-0.4
-0.6
same opp.
ALICE Pb-Pb @ s NN = 2.76 TeV STAR Au-Au @ s NN = 0.2 TeV 0 10 20 30 40 50 60 70 centrality, % 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 √s NN TeV = 2.76 25.06.2012
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2 –particle correlations: Centrality dependence
1 0 -1 0 7 ´ 10 -3 6 5
Pb-Pb @ s NN = 2.76 TeV
(+-) (++) (--) 4 3 2 10 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 (++) 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 1 0 -1 0 4 3 2 7 ´ 10 -3
same opp.
6 ALICE Pb-Pb @ s NN = 2.76 TeV 5 STAR Au-Au @ s NN = 0.2 TeV 10 20 30 40 50 60 70 centrality, % 25.06.2012
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Decomposition
0.003
Pb-Pb @ same opp.
s NN = 2.76 TeV
é é cos( sin( D D f f a ) cos( D f b ) é a ) sin( D f b ) é 0.002
0.001
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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 [email protected] - P and CP odd effects in hot and dense matter, BNL 12
Background effects: flow fluctuations
The orientation angle of the dipole asymmetry shows a preference out-of plane.
o 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?
D. Teaney and L. Yan, arXiv:1010.1876v1 [nucl-th] 25.06.2012
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Background effects: initial state fluctuations (cont.)
0.6
´ 10 -3 Charge independent correlator 0.4
é cos( f a + f b - 2 f c ) é HIJING / v 2 {2} CME expectation (Toneev
et al.
) same+opp. mean 0.2
0 -0.2
-0.4
-0.6
same opp.
ALICE Pb-Pb @ s NN = 2.76 TeV STAR Au-Au @ s NN = 0.2 TeV 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) 25.06.2012
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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 √s NN = 0.2 TeV o Hydro calculations indicate that the dipole asymmetry ’s preferential out-of-plane orientation might result into a v 1 asymmetry is not explained.
contribution out-of-plane, but the charge 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 25.06.2012
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BACKUP
25.06.2012
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