Abstract
The decay Bs to mu+ mu- is one of the milestones of the flavour program at
the LHC. We reappraise its Standard Model prediction. First, by analyzing the
theoretical rate in the light of its main parametric dependence, we highlight
the importance of a complete evaluation of higher-order electroweak
corrections, at present known only in the large-mt limit, and leaving sizable
dependence on the definition of electroweak parameters. Using insights from a
complete calculation of such corrections for K to pi barnu nu decays, we find
a scheme in which NLO electroweak corrections are likely to be negligible.
Second, we address the issue of the correspondence between the initial and the
final state detected by the experiments, and those used in the theoretical
prediction. Particular attention is devoted to the effect of the soft
radiation, that has not been discussed for this mode in the previous
literature, and that can lead to O(10\%) corrections to the decay rate. The
"non-radiative" branching ratio (that is equivalent to the branching ratio
fully inclusive of bremsstrahlung radiation) is estimated to be (3.23 +/- 0.27)
x 10^-9 for the flavour eigenstate, with the main uncertainty resulting from
the value of f\_Bs, followed by the uncertainty due to higher order
electroweak corrections. Applying the same strategy to Bd to mu+ mu-, we find
for its non-radiative branching ratio (1.07 +/- 0.10) x 10^-10.
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