There seems to be some information missing in this article. The photon collisions have enough energy to produce muons but muons have charges so the collision has to produce a muon (negative) and anti-muon (positive) pair to maintain charge conservation.
Because of this charge the muons would have different trajectories in the vicinity of a positively charged ion nucleus. The article did not explain how this angular deviation was accounted for to calculate the residual angular deviation that was reported as novel. This is always a hazard when scientific papers are simplified to make them readable to a general audience.
Could be my excerpting.