The strong interaction that binds together quarks in a nucleus appears to respect the symmetry of parity. However, physicists have hypothesized that smashing heavy ions against each other in a particle collider can cause the parity symmetry to be violated locally. The presence of a magnetic field then can cause charge separation, the so-called chiral magnetic effect (CME). Khachatryan et al. (CMS Collaboration) compared charge-dependent correlations among particles produced by proton-lead ion collisions with those produced by the collisions of two lead ions. In the former case, the CME would be expected to be negligible—but the researchers found the two signals to be strikingly similar. The findings challenge the interpretation of earlier heavy ion collision experiments in terms of the CME.
Phys. Rev. Lett. 118, 122301 (2017).