charge dependent azimuthal correlations in pb–pb collisions at √s nn = 2.76tev

Charge dependent azimuthal correlations in Pb–Pb collisions at √sNN = 2.76TeV

Panos Christakoglou1,2, for the ALICE Collaboration

1NIKHEF2Utrecht University

27.05.2011 [email protected] - Quark Matter 2011, Annecy-France

Motivation

Suggestions that heavy-ion collisions may form domains where the parity symmetry 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 factorso 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). The existence of the CME, is directly related to the Chiral Symmetry

restoration and to weird B field effects

27.05.2011 [email protected] - Quark Matter 2011, Annecy-France 2

• 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).

See also Dima’s talk on Monday at this session

Proposed tools: correlation analysis

Affected by the signal and the correlations both in and out-of-plane.

Sensitive also to detector effects


2–particle correlator 3–particle correlator

Measuring both correlators allows us to get an idea about the potential parity signal but also about the background contributionso Correlations in and out of plane

Background: Difference between the correlations projected onto an axis in the reaction plane and the ones projected onto an axis perpendicular to the reaction plane

Studies in ALICE: Analysis details

Analysis of the Pb-Pb events recorded in November/December 2010 during the first LHC heavy-ion runo Event sample split in two sets having

different magnetic field polarities (results used for the systematic uncertainties)

The trigger consists of the following criteria (at least two out of three):o two pixel chips hit in the outer layer of the

SPD,o signal in VZERO-A detector,o signal in VZERO-C detector.


For a description of the experimental setup, check Jurgen Schukraft‘s talk

The centrality is selected using the VZERO magnitude as the default estimatoro Centrality bins: 0-5%, 5-10%, 10-20%,

…,70-80%o Different centrality estimators (TPC tracks,

SPD clusters) investigates 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 correctionso Disadvantage: contamination from

secondaries Investigated by varying the cut on the

distance of closest approach (results used for the systematic uncertainty).

For a description of the centrality determination, check Alberica Toia’s talk

ALICE 2–particle correlations: Centrality dependence

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

3-particle correlations


Three-particle correlators: Differential analysis in Δη


Charge separation starts to develop when moving away from the most central bins

Correlations between opposite charges are smaller than those between same charges

Correlation width Δη = |ηα - ηβ|~1

Three-particle correlators: Differential analysis in ΔpT


Correlations not localized in small values of ΔpT

o Contribution from short range correlations of same/opposite charges limited?

Three-particle correlators: Differential analysis in sum pT


Correlations of same charges have larger signal with increasing transverse momentum of the pair contrary to the expectation from theory (i.e. signal localized at the low pT region)

D. Kharzeev et al., Nucl. Phys. A803, 227 (2008)

Centrality dependence: Charge combinations

Clear charge separation observed Results for (++) and (--) consistent (combined in the next plots into one “Same

charge” point) The magnitude of the correlations between the same charged pairs is larger

than the one of the opposite charges (excluding the last bin)


Different methods: event plane estimation


TPC

VZERO

ZDC

Investigation with four independent methods

For further details on the ZDC and the VZERO check the talk of Ilya Selyuzhenkov

Centrality dependence: Comparison of methods


Very good agreement between the four methods

Integrated 3-particle correlator: LHC vs RHIC


Stat. error: error barsSyst. error: shaded area

Magnitude of the effect seems to be similar to what is reported by STAR. Most theories predict a much lower effect at LHC energies.

o Signal and background should both scale with the square of the multiplicity

The effect can be similar depending on the t0 of the magnetic field (D. Kharzeev et al., Nucl. Phys. A803, (227) 2008)

STAR Collaboration: Phys. Rev. Lett. 81, 251601 (2009) STAR Collaboration: Phys. Rev. C81, 054908 (2010)

Comparison of 2– and 3–particle correlators


STAR Collaboration: Phys. Rev. C81, 054908 (2010)

STAR’s 2-particle correlations for same charged pairs have the same magnitude as the points coming from the 3-particle correlation analysis.o Larger magnitude of the correlations in than out of plane?

ALICE data demonstrate a larger magnitude of the 2-particle correlations but also a change in signo Differences in the correlations vs reaction plane between energies?o Larger magnitude of the correlations out of than in plane?

Integrated 3-particle correlator: Comparison with models


HIJING points consistent with the (+-) data pointso HIJING w/o flow consistent with each other

HIJING points scaled with the square of the multiplicity, consistent with the idea of having the correlations originating from emerging clusters (jets, resonances)

V.D. Toneev and V. Voronyuk, arXiv:1012.1508v1 [nucl-th]

S. A. Voloshin, Phys. Rev. C 70, 057901 (2004).

The only published prediction for LHC energies (@4.5 TeV)

According to the authors the magnitude should roughly scale with 1/√so Applied in the figure to convert the prediction to √sNN

= 2.76 TeV

Summary and outlook

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 STARo Change in the correlation pattern vs reaction plane?

The results of the 3-particle correlator indicate that:o the signal has a hadronic width of one unit in η,o doesn’t have any obvious contribution from short range correlations (i.e.

HBT),o increases with increasing pair pt.

The centrality dependence of the integrated 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

The majority of models predict a smaller signal at LHC energies.


Theory is challenged by the latest findings; looking forward to the feedback from the theory community!!!

BACKUP


What we know so far…(STAR)


Correlations between opposite charges are positive and large

Correlations of same charged pairs are negative and have a significantly smaller magnitude

STAR Collaboration: Phys. Rev. C81, 054908 (2010)

Differential analysis of the 3-particle correlator indicates that the signal:o increases with increasing pt of the pair,o has a hadronic width of one unit in η,o demonstrates a weak dependence on the pair’s pt difference

Differences in the correlations of same and opposite charged pairs.o The sign follows the one of the 2-particle correlatoro The magnitude of the correlations of same charged pairs is

larger than the one of opposite charged particleso None of the studied models describe the data

STAR Collaboration: Phys. Rev. Lett. 81, 251601 (2009)

ALICE 2–particle correlations: Differential analysis

Correlations have the same behavior regardless of the charge combination.

Change of sign @ ~2 GeV/c Change of physics @ ~5

GeV/c in ΔpT


Correlations localized in η Different charge

combinations have the same correlations in sign but not in magnitude.

Summary and outlook

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:o there is a change in physics at Δpt~5GeV/c, which is already seen by

other analyses (flow, RAA),

o the sign of the correlations is the same regardless of the charge combination, contrary to what was observed in STAR

Change in the correlation pattern vs reaction plane? The results of the 3-particle correlator indicate that:

o the signal has a hadronic width of one unit in η,o doesn’t have any obvious contribution from short range correlations

(i.e. HBT),o increases with increasing pair pt.

The centrality dependence of the integrated 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

The majority of models predict a smaller signal at LHC energies.


Theory is challenged by the latest findings; looking forward to the feedback from the theory community!!!

charge dependent azimuthal correlations in pb–pb collisions at √s nn = 2.76tev

Documents

quark matter

potential signal

potential parity signal

separation of charge

reaction plane

annecyfranceproposed

vzero magnitude

topological charge