MU-E CONVERSION IN NUCLEI VERSUS MU-]E-GAMMA - AN EFFECTIVE-FIELD THEORY POINT-OF-VIEW

Using an effective lagrangian description we analyze possible new phys ics contributions to the most relevant muon number violating processes : mu-->e gamma and mu-e conversion in nuclei. We identify a general cl ass of models in which those processes are generated at one loop level and in which mu-e conversion is enhanced with respect to mu-->e gamma by a large ln(m(mu)(2)/Lambda(2)), where Lambda is the scale responsi ble for the new physics. For this wide class of models bounds on mu-e conversion constrain the scale of new physics more stringently than mu -->e gamma already at present and, with the expected improvements in m u-e conversion experiments, will push it upwards by about one order of magnitude more. To illustrate this general result we give an explicit model containing a doubly charged scalar and derive new bounds on its couplings to the leptons. (C) 1998 Elsevier Science B.V.