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A search for the decay KS -> mu+ mu- is performed, based on a data sample of 1.0 fb^-1 of pp collisions at sqrt{s}=7 TeV collected by the LHCb experiment at the Large Hadron Collider. The observed number of candidates is consistent with the background-only hypothesis, yielding an upper limit of BR(KS -> mu+ mu-) < 11 (9) x 10^-9 at 95 (90)% confidence level. This limit is a factor of thirty below the previous measurement.
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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.
A search for the decays Bs -> mu+ mu- and B0 -> mu+ mu- is performed with 0.37 fb^-1 of pp collisions at sqrt{s} = 7 TeV collected by the LHCb experiment in 2011. The upper limits on the branching fractions are BR (Bs -> mu+ mu-) < 1.6 x 10^-8 and BR(B0 -> mu+ mu-) < 3.6 x 10^-9 at 95% confidence level. A combination of these results with the LHCb limits obtained with the 2010 dataset leads to BR (Bs -> mu+ mu-) < 1.4 x 10^-8 and BR (B0 -> mu+ mu-) < 3.2 x 10^-9 at 95% confidence level.
oindent A search for the decays $B^0_{s}rightarrow mu^+ mu^- mu^+ mu^-$ and $B^0 rightarrow mu^+ mu^- mu^+ mu^-$ is performed using data, corresponding to an integrated luminosity of 1.0ensuremath{{,fb}^{-1}}xspace, collected with the LHCb detector in 2011. The number of candidates observed is consistent with the expected background and, assuming phase-space models of the decays, limits on the branching fractions are set: {${ensuremath{cal B}xspace}(B^0_{s}rightarrow mu^+ mu^- mu^+ mu^-) < 1.6 (1.2) times 10^{-8}$} and {${ensuremath{cal B}xspace}(B^0 rightarrow mu^+ mu^- mu^+ mu^-)< 6.6 (5.3) times 10^{-9}$} at 95,% (90,%) confidence level. In addition, limits are set in the context of a supersymmetric model which allows for the $B^0_{(s)}$ meson to decay into a scalar ($S$) and pseudoscalar particle ($P$), where $S$ and $P$ have masses of 2.5 GeV and 214.3 MeV, respectively, both resonances decay into $mu^+mu^-$. The branching fraction limits for these decays are {${ensuremath{cal B}xspace}(ensuremath{B^0_{s}rightarrow SP}xspace) < 1.6 (1.2) times 10^{-8}$} and {${ensuremath{cal B}xspace}(ensuremath{B^0rightarrow SP}xspace)< 6.3 (5.1) times 10^{-9}$} at 95% (90%) confidence level.
A search for the flavor-changing neutral-current decay $Lambda_{c}^{+} to pmu^+mu^-$ is reported using a data set corresponding to an integrated luminosity of $3.0rm fb^{-1}$ collected by the LHCb collaboration. No significant signal is observed outside of the dimuon mass regions around the $phi$ and $omega$ resonances and an upper limit is placed on the branching fraction of $mathcal{B} (Lambda_{c}^{+} to pmu^+mu^-) < 7.7~(9.6)times 10^{-8}~{rm at}~90%~(95%)$ confidence level. A significant signal is observed in the $omega$ dimuon mass region for the first time.
A search for the rare decay $Sigma^+ to p mu^+ mu^-$ is performed using $pp$ collision data recorded by the LHCb experiment at centre-of-mass energies $sqrt{s} = 7$ and $8$ TeV, corresponding to an integrated luminosity of $3 fb^{-1}$. An excess of events is observed with respect to the background expectation, with a signal significance of 4.1 standard deviations. No significant structure is observed in the dimuon invariant mass distribution, in contrast with a previous result from the HyperCP experiment. The measured $Sigma^+ to p mu^+ mu^-$ branching fraction is $(2.2,^{+,1.8}_{-,1.3})times 10^{-8}$, where statistical and systematic uncertainties are included, which is consistent with the Standard Model prediction.
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