Probing the chiral magnetic wave in pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV using charge-dependent azimuthal anisotropies

TL;DR

This study measures charge-dependent azimuthal anisotropies in pPb and PbPb collisions at 5.02 TeV, finding linear relationships that challenge the chiral magnetic wave explanation for charge asymmetry effects.

Contribution

It provides the first detailed comparison of charge-dependent anisotropies in pPb and PbPb collisions, testing the chiral magnetic wave hypothesis against local charge conservation effects.

Findings

Linear dependence of $v_2$ charge asymmetry on event charge asymmetry in both collision types

Similar slopes for $v_2$ and $v_3$ in PbPb collisions

Results challenge the chiral magnetic wave explanation for charge asymmetry effects

Abstract

Charge-dependent anisotropy Fourier coefficients ($v_n$) of particle azimuthal distributions are measured in pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV with the CMS detector at the LHC. The normalized difference in the second-order anisotropy coefficients ($v_2$) between positively and negatively charged particles is found to depend linearly on the observed event charge asymmetry with comparable slopes for both pPb and PbPb collisions over a wide range of charged particle multiplicity. In PbPb, the third-order anisotropy coefficient, $v_3$, shows a similar linear dependence with the same slope as seen for $v_2$. The observed similarities between the $v_2$ slopes for pPb and PbPb, as well as the similar slopes for $v_2$ and $v_3$ in PbPb, are compatible with expectations based on local charge conservation in the decay of clusters or resonances, and constitute a challenge to the hypothesis that, at LHC energies, the observed charge asymmetry dependence of $v_2$ in heavy ion collisions arises from a chiral magnetic wave.