Implications of p+Pb measurements on the chiral magnetic effect in heavy ion collisions

R. Belmont and J. L. Nagle
Phys. Rev. C 96, 024901 – Published 7 August 2017

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 [V. Khachatryan et al., Phys. Rev. Lett. 118, 122301 (2017)] 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 (i) that the magnetic field in p+Pb collisions is smaller than that in Pb+Pb collisions and (ii) 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 collisions is not an indication of the CME. Similar measurements in d+Au over a range of energies accessible at the BNL Relativistic Heavy Ion Collider 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.

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  • Received 21 November 2016
  • Revised 21 April 2017

DOI:https://doi.org/10.1103/PhysRevC.96.024901

©2017 American Physical Society

Physics Subject Headings (PhySH)

Nuclear PhysicsParticles & Fields

Authors & Affiliations

R. Belmont and J. L. Nagle

  • Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, USA

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Vol. 96, Iss. 2 — August 2017

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