μ-e conversion in nuclei versus μ-->e γ: an effective field theory point of view

Abstract

Using an effective lagrangian description we analyze possible new physics contributions to the most relevant muon number violating processes: $\hat{I}¼\to e{\hat{I}}^3$ and $\hat{I}¼$--$e$ conversion in nuclei. We identify a general class of models in which those processes are generated at one loop level and in which $\hat{I}¼$--$e$ conversion is enhanced with respect to $\hat{I}¼\to e{\hat{I}}^3$ by a large $\ln (m_{\hat{I}}^2¼/\hat{I}{›}^2),$ where $\hat{I}›$ is the scale responsible for the new physics. For this wide class of models bounds on $\hat{I}¼$--$e$ conversion constrain the scale of new physics more stringently than $\hat{I}¼\to e{\hat{I}}^3$ already at present and, with the expected improvements in $\hat{I}¼$--$e$ 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.

A few references added, to appear in PLB