The muon g-2 discrepancy: errors or new physics?

Understand

After a brief review of the muon g-2 status, we discuss hypothetical errors in the Standard Model prediction that could explain the present discrepancy with the experimental value.

• None of them looks likely.
• In particular, an hypothetical increase of the hadroproduction cross section in low-energy e^+e^- collisions could bridge the muon g-2 discrepancy, but is shown to be unlikely in view of current experimental error estimates.
• If, nonetheless, this turns out to be the explanation of the discrepancy, then the 95% CL upper bound on the Higgs boson mass is reduced to about 130 GeV which, in conjunction with the experimental 114.4 GeV 95% CL lower bound, leaves a narrow window for the mass of this fundamental particle.