Leading hadronic contribution to the muon magnetic moment from lattice QCD

• Laurent Lellouch (CPT Marseille)

Twenty years ago in an experiment at Brookhaven National Laboratory, physicists measured the muon's anomalous magnetic moment, $a_\mu=(g_\mu-2)/2$, with a remarkable precision of 0.54 parts per million. Ever since that time, the standard model (SM) prediction has exhibited an increasing discrepancy with experiment of over 3 standard deviations. On April 7, a new Fermilab experiment presented its first results, brilliantly confirming Brookhaven's measurement and bringing the discrepancy with the SM to the near discovery level of 4.2 sigmas. To fully leverage this and future measurements, and possibly claim the presence of new fundamental physics, it is imperative to check the SM prediction with independent methods and to reduce its uncertainties. After an introduction and a discussion of the current experimental and theoretical status of $a_\mu$, I will present a precise lattice QCD calculation, by the BMW collaboration, of the contribution to this quantity that most limits the precision of its SM prediction. The result of this calculation significantly reduces the gap between the SM and experiment and thus suggests that new physics is not needed to explain the current, experimental world average of $a_\mu$.