Muon anomalous magnetic moment: A harbinger for “new physics”

Standard model loop contributions to the muon anomalous magnetic moment, $a_{\mu }\equiv {(g}_{\mu }-2)/2,$ and their theoretical uncertainties are scrutinized. The status and implications of the recently reported 2.6 sigma experiment versus theory deviation $a_{\mu }^{\mathrm{expt}}-a_{\mu }^{\mathrm{SM}}=426\mathrm{}\left(165\right)\mathrm{}\times {10}^{-11}$ are discussed. Possible explanations due to supersymmetric loop effects with $m_{\mathrm{SUSY}}\simeq {55}_{-8}^{+16\mathrm{}}\sqrt{\tan \beta }\hspace{0.5em}\mathrm{GeV},$ radiative mass mechanisms at the 1–2 TeV scale and other “new physics” scenarios are examined.