Clues for flavor from rare lepton and quark decays

Abstract

Flavor symmetries successfully explain lepton and quark masses and mixings yet it is usually hard to distinguish different models that predict the same mixing angles. Further experimental input could be available, if the agents of flavor breaking are sufficiently low in mass and detectable or if new physics with non-trivial flavor charges is sufficiently low in mass and detectable. The recent hint for lepton-nonuniversality in the ratio of branching fractions $B\to K\mu \mu$ over $B\to Kee$, $R_K$, suggests the latter, at least for indirect detection via rare decays. We demonstrate the discriminating power of the rare decay data on flavor model building taking into account viable leptonic mixings and show how correlations with other observables exist in leptoquark models. We give expectations for branching ratios $B\to K\ell {\ell }^{\prime },B_{(s)}\to \ell {\ell }^{\prime }$ and $\ell \to {\ell }^{\prime }\gamma$, and Higgs decays $h\to \ell {\ell }^{\prime }$.

23 pages plus references and appendices, 1 figure; v2: comment on mu to e conversion and references added, conclusions unchanged; v3: footnote 3 added, typos fixed

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