Charged Lepton G-2 and Constraints on New Physics

Abstract

A review of the theoretical and experimental values for the charged lepton (electron and muon) anomalous magnetic moment $a_l= (g_l-2)/2$ is presented. Employing the most accurate value for the fine structure constant $\alpha^{-1}= 137.03599993(52) (0.0038 ppm)$ obtained \cite {Kin196} from the electron $(g-2)$ we find the new complete standard model prediction for the anomalous magnetic moment of the muon $a^{th}_{\mu}= 116591595(67)\times 10^{-11}$. The comparison of this theoretical value and the precise experimental result \cite {Sch} yields the estimation for the difference $\Delta a_{\mu}= a^{exp}_{\mu}- a^{th}_{\mu}$ at the 95% confidence level: $-95 \times 10^{-10} \leq \Delta a_{\mu}\leq 236 \times 10^{-10}$. The implication of the expected a factor of about 20 increase of accuracy in the forthcoming Brookhaven National Laboratory measurement of $a_{\mu}$ implies $-47 \times 10^{-11} \leq \Delta a_{\mu}\leq 118 \times 10^{-11}$, ($95 % C.L.$). This interval is used to get constraints on the "new physics". The value of the one-loop contributions $a^{B_i}_l$ of different bosons predicted within extension of the standard model and coupled to a charged lepton are discussed. The dependence of $a^{B_i}_l$ on the masses of the bosons and leptons of the vacuum polarization loops are investigated. The constraints on "new physics" by requiring that the new contributions $a^{B_i}_{\mu}$ to the muon anomalous magnetic moment lie within the latter interval $\Delta a_{\mu}$ are obtained.