  Results are presented from a search for the rare decays B(s) to mu+ mu- and
B0 to mu+ mu- in pp collisions at sqrt(s) = 7 and 8 TeV, with data samples
corresponding to integrated luminosities of 5 and 20 inverse femtobarns,
respectively, collected by the CMS experiment at the LHC. An unbinned
maximum-likelihood fit to the dimuon invariant mass distribution gives a
branching fraction B(B(s) to mu+ mu-) = (3 +1.0/-0.9)E-9, where the uncertainty
includes both statistical and systematic contributions. An excess of B(s) to
mu+ mu- events with respect to background is observed with a significance of
4.3 standard deviations. For the decay B0 to mu+ mu- an upper limit of B(B0 to
mu+ mu-) < 1.1E-9 at the 95% confidence level is determined. Both results are
in agreement with the expectations from the standard model.
