Constraining the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Pb-Pb collisions at $\sqrt{\it{s}_{\mathrm{NN}}}$ = 2.76 and 5.02 TeV

Abstract

Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at $\sqrt{\it{s}_\mathrm{NN}} = $ 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity ($η$) and transverse momentum ($p_{\rm{T}}$) ranges $\left|η\right| < 0.8$ and $0.2 < p_{\mathrm{T}} < 5$ GeV/$c$. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators $γ_{1,1} = \langle \cos (φ_α +φ_β - 2Ψ_{2}) \rangle$ and $γ_{1,-3} = \langle \cos (φ_α -3φ_β + 2Ψ_{2}) \rangle$. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured $γ_{1,1}$. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form $γ_{1,2} = \langle \cos (φ_α +2φ_β - 3Ψ_{3}) \rangle$, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of $γ_{1,1}$.