  The Standard Model predicts a branching ratio for the decay mode Bs -> mu mu
of (3.4 +/- 0.5)10e-9, while some SUSY models predict enhancements up to 2
orders of magnitude. It is expected that at the end of its life Tevatron will
set and exclusion limit for this branching ratio of the order of 10e-8, leaving
one order of magnitude to explore. The excellent vertex reconstruction,
invariant mass resolution and muon identification of the LHCb detector make it
well suited to observe a branching ratio in this range in the first years of
running of LHC. In this article an overview of the analysis that has been
developed for the measurement of this branching ratio is presented. The event
selection and the statistical tools used for the extraction of the branching
ratio are discussed. A special stress is put on the use of control channels for
calibration and normalization in order to make the analysis as independent of
simulation as possible. Finally, the expected performance in terms of exclusion
and observation significance are given for a set of values of integrated
luminosities.
