Charge correlations and collective dynamics in Pb-Pb collisions at sqrt(sNN)=2.76 TeV

Abstract

The theory of the strong force is called Quantum Chromodynamics (QCD). Within experimental limits it is observed that QCD respects CP-symmetry, although there is no confirmed fundamental explanation for why this is the case. This is known as the strong CP problem. Despite the observed symmetry conservation, it is expected that fluctuations in a larger system may locally result in a violation of CP-symmetry. It was argued that local parity violation, which is driven by the nontrivial topological structure of the QCD vacuum, coupled with a strong magnetic field may result in a separation of charges along the direction of the magnetic field. This phenomenon is called the chiral magnetic effect (CME). Relativistic heavy ion collisions such as at the LHC could meet the conditions for a measurement of the CME. For this, knowledge of the orientation of the magnetic field, which is perpendicular to the reaction plane, is needed. Collision symmetry planes can be estimated with Q-vectors that quantify the preference in the azimuthal direction of measured particles. To correct for detector imperfections in the measurement of the Q-vector, a ROOT-based correction framework was developed, building on corrections proposed by Selyuzhenkov and Voloshin. The functionality is increasingly used as a common tool in the ALICE collaboration. Due to its general applicability to data from heavy ion experiments, the package is publicly available and has already gained interest from researchers in CBM and NA61. With understanding of the orientation of the magnetic field, the CME can be searched for with parity-even two- and multi-particle correlations, such as the charge dependence of two-particle correlations with respect to the reaction plane. However, these observables are also sensitive to the presence of background correlations, notably arising from the anisotropic flow modulation of locally created opposite charged particle pairs (local charge conservation), which obstructed a definite conclusion on the origins of the observed charge dependence. This work presents the measurement of an extended set of mixed harmonic correlations based on 〈cos(φ_α + φ_β − 2Ψ_RP)〉, in Pb-Pb collisions at sqrt(sNN)=2.76 TeV with ALICE, that characterize the facets of the shape of the signal. Additionally, the mentioned correlation is measured with one identified hadron (pion, kaon, or proton), reported as a function of the identified hadron transverse momentum. The measurements of the correlators with different sensitivity to the CME and background effects show varying degrees of charge dependence. The 〈cos(φ_α + φ_β − 2Ψ_RP)〉 correlation, which has maximum sensitivity to the CME but also background effects, shows a charge dependence increasing towards peripheral collisions, as is qualitatively in line with expectations. The second harmonic of this correlator is not sensitive to the charge separation perpendicular to the reaction plane, as associated to the CME, but can be used to quantify correlations from background sources, for example clusters of particles exhibiting charge conservation with angular correlations modulated by anisotropic flow, and is found to be consistent with zero within the systematic uncertainty. The measurement of the CME correlator with pions, kaons and protons reveals a particle type dependence in the charge dependence. More data for higher precision and model comparisons are required to make a better defined and quantitative conclusion about the contributions from background sources in the observation of the charge separation in the various correlations