https://phys.org/news/2018-07-theorists-publish-highest-precision-muon-magnetic.html

"If there's another particle that pops into existence and interacts with the muon before it interacts with the magnetic field, that could explain the difference between the experimental measurement and our theoretical prediction," said Christoph Lehner, one of the Brookhaven Lab theorists involved in the latest calculations. "That could be a particle we've never seen before, one not included in the Standard Model."

Finding new particles beyond those already cataloged by the Standard Model has long been a quest for particle physicists. Spotting signs of a new particle affecting the behavior of muons could guide the design of experiments to search for direct evidence of such particles, said Taku Izubuchi, another leader of Brookhaven's theoretical physics team.