Abstract
In ultrarelativistic heavy-ion collisions, the event-by-event variation of the elliptic flow v2 reflects fluctuations in the shape of the initial state of the system. This allows to select events with the same centrality but different initial geometry. This selection technique, Event Shape Engineering, has been used in the analysis of charge-dependent two- and three-particle correlations in Pb–Pb collisions at sNN=2.76 TeV. The two-particle correlator 〈cos(φα−φβ)〉, calculated for different combinations of charges α and β, is almost independent of v2 (for a given centrality), while the three-particle correlator 〈cos(φα+φβ−2Ψ2)〉 scales almost linearly both with the event v2 and charged-particle pseudorapidity density. The charge dependence of the three-particle correlator is often interpreted as evidence for the Chiral Magnetic Effect (CME), a parity violating effect of the strong interaction. However, its measured dependence on v2 points to a large non-CME contribution to the correlator. Comparing the results with Monte Carlo calculations including a magnetic field due to the spectators, the upper limit of the CME signal contribution to the three-particle correlator in the 10–50% centrality interval is found to be 26–33% at 95% confidence level.
Highlights
In ultrarelativistic heavy-ion collisions, the event-by-event variation of the elliptic flow v2 reflects fluctuations in the shape of the initial state of the system
The charge dependence of the three-particle correlator is often interpreted as evidence for the Chiral Magnetic Effect (CME), a parity violating effect of the strong interaction
A local imbalance of chirality, coupled with the strong magnetic field produced in heavy-ion collisions (B ∼ 1015 T) [6,7,8], would lead to charge separation along the direction of the magnetic field, which is on average perpendicular to the reaction plane, a phenomenon called Chiral Magnetic Effect (CME) [9,10,11,12]
Summary
Constraining the magnitude Shape Engineering in Pb–Pb of the Chiral collisions at. Article history: Received 27 September 2017 Received in revised form 21 November 2017 Accepted 8 December 2017 Available online 12 December 2017 Editor: L. The Event Shape Engineering (ESE) technique was proposed to disentangle background contributions from the potential CME signal [29] This method makes it possible to select events with eccentricity values significantly larger or smaller than the average in a given centrality class [30,31] since v2 scales approximately linearly with eccentricity [32]. The magnitude of the correlator in p–Pb and Pb–Pb collisions is comparable for similar final-state charged-particle multiplicities This measurement indicates that the contribution of the CME to this observable in this multiplicity range is small. The reconstruction efficiency of primary particles defined in [46], which may bias the determination of the pT averaged charge-dependent correlations and flow, increases from 70% at pT = 0.2 GeV/c to 85% at pT ∼ 1.5 GeV/c where it has a maximum. The event plane resolution is calculated from correlations between the event planes determined in the TPC and
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