Probing the chiral magnetic wave in pPb and PbPb collisions at sNN=5.02TeV using charge-dependent azimuthal anisotropies

Abstract

Charge-dependent anisotropy Fourier coefficients (v n) of particle azimuthal distributions are measured in pPb and PbPb collisions at √ s 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.

Anomalous chiral transports and spin polarization in heavy-ion collisions

“…Such a charge dependence of v 2 was measured by STAR Collaboration [97] at RHIC and by ALICE Collaboration [98] and CMS Collaboration [99] at LHC. The data shows indeed an elliptic-flow difference ∆v 2 linear in A ch with a positive slope whose centrality dependence is consistent with the expectation of the CMW.…”

Applying deep learning technique to chiral magnetic wave search*

“…However, further comparisons with experimental observables are necessary before constructing a measurement based on the model. Figure 3 shows that logit( ) is correlated with f, which in turn has a positive correlation with the slope r. In addition to this slope, the following covariance between and , which is essentially a three-particle correlator, constitutes another noteworthy observable [21],…”

“…Owing to event-by-event fluctuation of charges, some events could have net negative charges in the fireball, thereby leading to . This characteristic feature of CMW provides a method to detect it in heavy-ion collisions, and a series of experiments have found signals of charged pion elliptic flow consistent with CMW expectations [20][21][22][23][24]. However, similar to CME, CMW in heavy-ion collisions faces strong background noise [25][26][27][28][29][30][31], which significantly obscures the observables designed for CMW detection.…”

Causality and stability in relativistic viscous non-resistive magneto-fluid dynamics

“…However, it is still a challenge to extract the CME signals from the huge backgrounds caused by the collective flows refs. [66][67][68]. Therefore, it requires the systematic and quantitative studies of the evolution of the QGP coupled with the electromagnetic fields for the discovery of CME.…”