Measurement of the $B^0_stomu^+mu^-$ branching fraction and effective lifetime and search for $B^0tomu^+mu^-$ decays

A search for the rare decays $B^0_stomu^+mu^-$ and $B^0tomu^+mu^-$ is performed at the LHCb experiment using data collected in $pp$ collisions corresponding to a total integrated luminosity of 4.4 fb$^{-1}$. An excess of $B^0_stomu^+mu^-$ decays is observed with a significance of 7.8 standard deviations, representing the first observation of this decay in a single experiment. The branching fraction is measured to be ${cal B}(B^0_stomu^+mu^-)=left(3.0pm 0.6^{+0.3}_{-0.2}right)times 10^{-9}$, where the first uncertainty is statistical and the second systematic. The first measurement of the $B^0_stomu^+mu^-$ effective lifetime, $tau(B^0_stomu^+mu^-)=2.04pm 0.44pm 0.05$ ps, is reported. No significant excess of $B^0tomu^+mu^-$ decays is found and a 95 % confidence level upper limit, ${cal B}(B^0tomu^+mu^-)<3.4times 10^{-10}$, is determined. All results are in agreement with the Standard Model expectations.

Search for the lepton number violating decays $B^{+}to pi^- mu^+ mu^+$ and $B^{+}to K^- mu^+ mu^+$

A search is performed for the lepton number violating decay $B^{+}to h^- mu^+ mu^+$, where $h^-$ represents a $K^-$ or a $pi^-$, using data from the LHCb detector corresponding to an integrated luminosity of $36pb^{-1}$. The decay is forbidden in the Standard Model but allowed in models with a Majorana neutrino. No signal is observed in either channel and limits of $B(B^{+} to K^- mu^+ mu^+) < 5.4times 10^{-8}$ and $B(B^{+} to pi^- mu^+ mu^+) < 5.8times 10^{-8}$ are set at the 95% confidence level. These improve the previous best limits by factors of 40 and 30, respectively.

Search for the rare decay $B^{+} rightarrow {mu}^{+}{mu}^{-}{mu}^{+}{ u}_{{mu}}$

A search for the rare leptonic decay $B^{+} rightarrow {mu}^{+}{mu}^{-}{mu}^{+}{ u}_{{mu}}$ is performed using proton-proton collision data corresponding to an integrated luminosity of $4.7$ fb$^{-1}$ collected by the LHCb experiment. The search is carried out in the region where the lowest of the two ${mu}^{+}{mu}^{-}$ mass combinations is below $980$MeV/c$^{2}$. The data are consistent with the background-only hypothesis and an upper limit of $1.6 times 10^{-8}$ at 95% confidence level is set on the branching fraction in the stated kinematic region.

Measurement of the $B^0_s to mu^+ mu^-$ branching fraction and search for $B^0 to mu^+ mu^-$ decays at the LHCb experiment

A search for the rare decays $B^0_s tomu^+mu^-$ and $B^0 tomu^+mu^-$ is performed at the LHCb experiment. The data analysed correspond to an integrated luminosity of 1 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 7 TeV and 2 fb$^{-1}$ at 8 TeV. An excess of $B^0_s tomu^+mu^-$ signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of ${cal B}(B^0_s tomu^+mu^-) = (2.9^{+1.1}_{-1.0})times 10^{-9}$ is obtained and an upper limit of ${cal B}(B^0 tomu^+mu^-) < 7.4times 10^{-10}$ at 95% confidence level is set. These results are consistent with the Standard Model expectations.

b -> s mu+ mu- and b -> mu+ mu- At the LHC

With their 2010-2011 data set, the LHC experiments have started their quest to observe the rare decays B0_{s/d} -> mu+ mu-. This study will provide very sensitive probes of New Physics (NP) effects. NP discovery potential lies as well in the study of the decay B0_d -> K*0 mu+ mu-. Results and perspectives are presented for studies at the LHC of rare B decays involving flavor changing neutral currents.