Model-independent constraints on leptoquarks from rareμandτlepton processes

Abstract

We perform a model-independent analysis so as to constrain the leptoquark (LQ) models from negative searches for $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e\ensuremath{\gamma},$ $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}3e$ decays (and analogous processes in the $\ensuremath{\tau}$ sector), and coherent $\ensuremath{\mu}\ensuremath{-}e$ conversion in nuclei. We considerably improve some constraints obtained by analyses known in the literature, analyses which we show have by far underestimated the LQ contributions to the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}3e.$ In particular we find that the coherent $\ensuremath{\mu}\ensuremath{-}e$ conversion in nuclei mediated by the photon-conversion mechanism and the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}3e$ decay are golden plates where the flavor-changing leptoquark couplings, involving the second and third quark generations, can be strongly constrained. This is due to the fact that these processes get the enhancements by large $\mathrm{log}{(m}_{q}^{2}{/m}_{\mathrm{LQ}}^{2})$ terms which are induced by the so-called ``photon-penguin'' diagrams. These enhancements, which produce a mild Glashow-Iliopoulos-Maiani (GIM) suppression in the amplitudes, have not been taken into account in the previous analyses. We show that the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e\ensuremath{\gamma}$ decay can set weaker constraints on the LQ models and this is because its amplitude is strongly GIM suppressed by the terms of order ${O(m}_{q}^{2}{/m}_{\mathrm{LQ}}^{2}).$ We also present the results for the corresponding constraints in the \ensuremath{\tau} sector. Finally, the prospects of the future muon experiments for the improvement of the present bounds are analyzed and discussed.