Charge dependent azimuthal correlations with the ALICE detector at the LHC

Panos Christakoglou 1 , for the ALICE Collaboration 1 Nikhef 25.06.2012

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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

25.06.2012

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

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