To CME or not to CME? Implications of p+Pb measurements of the chiral magnetic effect in heavy ion collisions

TL;DR

This paper critically examines p+Pb collision measurements related to the Chiral Magnetic Effect, challenging previous interpretations by analyzing magnetic field assumptions and their correlation with flow angles, ultimately questioning the CME signal's origin.

Contribution

It tests key assumptions about magnetic fields in p+Pb collisions and demonstrates that the observed correlations are unlikely due to the CME, highlighting the importance of magnetic field inhomogeneities.

Findings

Magnetic fields in p+Pb are not significantly smaller than in Pb+Pb.

Magnetic field direction is uncorrelated with flow angle in p+Pb.

Inhomogeneities on the scale of topological domains are present in Pb+Pb collisions.

Abstract

The Chiral Magnetic Effect (CME) is a fundamental prediction of QCD, and various observables have been proposed in heavy ion collisions to access this physics. Recently the CMS Collaboration \cite{Khachatryan:2016got} has reported results from p+Pb collisions at 5.02 TeV on one such observable, the three-point correlator. The results are strikingly similar to those measured at the same particle multiplicity in Pb+Pb collisions, which have been attributed to the CME. This similarity, combined with two key assumptions about the magnetic field in p+Pb collisions, presents a major challenge to the CME picture. These two assumptions as stated in the CMS paper are (1) that the magnetic field in p+Pb collisions is smaller than that in Pb+Pb collisions and (2) that the magnetic field direction is uncorrelated with the flow angle. We test these two postulates in the Monte Carlo Glauber framework and find that the magnetic fields are not significantly smaller in central p+Pb collisions, however the magnetic field direction and the flow angle are indeed uncorrelated. The second finding alone gives strong evidence that the three-point correlator signal in Pb+Pb and p+Pb is not an indication of the CME. Similar measurements in d+Au over a range of energies accessible at RHIC would be elucidating. In the same calculational framework, we find that even in Pb+Pb collisions, where the magnetic field direction and the flow angle are correlated, there exist large inhomogeneities that are on the size scale of topological domains. These inhomogeneities need to be incorporated in any detailed CME calculation.