Using muons to probe for new physics

Searches for new physics using muons are reviewed. Particular attention is paid to muon number non-conserving processes, like the decay μ → eγ and muon–electron conversion in muonic atoms. Also, experimental determinations and theoretical predictions for the muon anomalous magnetic moment are reviewed. Muons were first observed in cosmic ray detectors in 1936 [1, 2]. However, for several following years they were being confused with pions, strongly interacting mesons predicted by Yukawa. It was not until 1947 that the identity of the muon as a weekly interacting particle was demonstrated [3]. Evidence for the existence of two kinds of charged particles with similar masses was provided by a cosmic ray experiment by Powell and collaborators [4] (for the early history of the muon physics see [5, 6]). Muons have proved to be extremely useful both in fundamental studies and as a tool in applied science. They are relatively long-living (lifetime 2.2 μs) and can be produced in abundance so that intense muon beams can be obtained. In the latter aspect we expect future improvements by several orders of magnitude which may lead to construction of muon colliders, a new generation of particle accelerators. After 50 years of rich and varied muon physics programs it is appropriate to review what has been achieved so far and what are the future prospects. This talk is but a modest contribution towards this goal. I will focus on two aspects of muon physics, the anomalous magnetic moment measurements and muon number non-conservation searches. Plenary talk at the 5th International Conference Beyond the Standard Model, Balholm, Norway, Apr. 29th May 4th, 1997.