More Lepton Flavor Violating Observables for LHCb's Run 2

Abstract

The R_K measurement by LHCb suggests non-standard lepton non-universality (LNU) to occur in b -> s l+ l- transitions, with effects in muons rather than electrons. A number of other measurements of b -> s l+ l- transitions by LHCb and B-factories display disagreement with the SM predictions and, remarkably, these discrepancies are consistent in magnitude and sign with the R_K effect. Non-standard LNU suggests non-standard lepton flavor violation (LFV) as well, for example in B -> K l l' and B_s -> l l'. There are good reasons to expect that the new effects may be larger for generations closer to the third one. In this case, the B_s -> mu e decay may be the most difficult to reach experimentally. We propose and study in detail the radiative counterpart of this decay, namely B_s -> mu e gamma, whereby the chiral-suppression factor is replaced by a factor of order alpha/pi. A measurement of this mode would be sensitive to the same physics as the purely leptonic LFV decay and, depending on experimental efficiencies, it may be more accessible. A realistic expectation is a factor of two improvement in statistics for either of the B_{d,s} modes.

14 pages, 2 figures. v4: some typos corrected. Such corrections are not implemented in the journal version

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