No abstract

We argue that the anomalous magnetic moment of the electron (a e ) can be used to probe new physics. We show that the present bound on new-physics contributions to a e is 8 × 10 −13 , but the sensitivity can be improved by about an order of magnitude with new measurements of a e and more refined determinations of α in atomic-physics experiments. Tests on new-physics effects in a e can play a crucial role in the interpretation of the observed discrepancy in the anomalous magnetic moment of the muon (a µ ). In a large class of models, new contributions to magnetic moments scale with the square of lepton masses and thus the anomaly in a µ suggests a new-physics effect in a e of (0.7±0.2)×10 −13 . We also present examples of new-physics theories in which this scaling is violated and larger effects in a e are expected. In such models the value of a e is correlated with specific predictions for processes with violation of lepton number or lepton universality, and with the electric dipole moment of the electron.

We propose a new experiment to measure the running of the electromagnetic coupling constant in the spacelike region by scattering high-energy muons on atomic electrons of a low-Z target through the elastic process μ e → μ e. The differential cross section of this process, measured as a function of the squared momentum transfer t = q 2 < 0, provides direct sensitivity to the leading-order hadronic contribution to the muon anomaly a HLO μ . By using a muon beam of 150 GeV, with an average rate of ∼1.3 ×10 7 muon/s, currently available at the CERN North Area, a statistical uncertainty of ∼0.3% can be achieved on a HLO μ after two years of data taking. The direct measurement of a HLO μ via μe scattering will provide an independent determination, competitive with the time-like dispersive approach, and consolidate the theoretical prediction for the muon g-2 in the Standard Model. It will allow therefore a firmer interpretation of the measurements of the future muon g-2 experiments at Fermilab and J-PARC. a