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A search for non-resonant D+(s) to pi+mu+mu- and D+(s) to pi-mu+mu+ decays is performed using proton-proton collision data, corresponding to an integrated luminosity of 1.0 fb-1, at sqrt(s) = 7 TeV recorded by the LHCb experiment in 2011. No signals are observed and the 90% (95%) confidence level (CL) limits on the branching fractions are B(D+ to pi+mu+mu-) < 7.3 (8.3) x 10-8, B(Ds+ to pi+mu+mu-) < 4.1 (4.8) x 10-7, B(D+ to pi-mu+mu+) < 2.2 (2.5) x 10-8, B(Ds+ to pi-mu+mu+) < 1.2 (1.4) x 10-7. These limits are the most stringent to date.
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The first observation of the $D^0 to pi^+pi^-mu^+mu^-$ and $D^0 to K^+K^-mu^+mu^-$ decays is reported using a sample of proton-proton collisions collected by LHCb at a center-of-mass energy of 8$,$TeV, and corresponding to 2$,$fb$^{-1}$ of integrated luminosity. The corresponding branching fractions are measured using as normalization the decay $D^0 to K^- pi^+[mu^+mu^-]_{rho^0/omega}$, where the two muons are consistent with coming from the decay of a $rho^0$ or $omega$ meson. The results are $mathcal{B}(D^0 to pi^+pi^-mu^+mu^-)=(9.64pm0.48pm0.51pm0.97)times10^{-7}$ and $mathcal{B}(D^0 to K^+K^-mu^+mu^-)=( 1.54pm0.27pm0.09pm0.16)times10^{-7}$, where the uncertainties are statistical, systematic, and due to the limited knowledge of the normalization branching fraction. The dependence of the branching fraction on the dimuon mass is also investigated.
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.
A search for the D0 to pi+ pi- mu+ mu- decay, where the muon pair does not originate from a resonance, is performed using proton-proton collision data corresponding to an integrated luminosity of 1.0 fb-1 recorded by the LHCb experiment at a centre-of-mass energy of 7 TeV. No signal is observed and an upper limit on the relative branching fraction with respect to the resonant decay mode D0 to pi+ pi- phi(to mu+ mu-), under the assumption of a phase-space model, is found to be B(D0 to pi+ pi- mu+ mu-)/B(D0 to pi+ pi- phi(to mu+ mu-)) < 0.96 at the 90% confidence level. The upper limit on the absolute branching fraction is evaluated to be B(D0 to pi+ pi- mu+ mu-) < 5.5 x 10-7 at 90% confidence level. This is the most stringent to date.
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 heavy Majorana neutrinos produced in the $B^- to pi^+mu^-mu^-$ decay mode is performed using 3 fb$^{-1}$ of integrated luminosity collected with the LHCb detector in $pp$ collisions at center-of-mass energies of 7 TeV and 8 TeV at the LHC. Neutrinos with masses in the range 250-5000 MeV and lifetimes from zero to 1000 ps are probed. In the absence of a signal, upper limits are set on the branching fraction ${cal{B}}(B^- to pi^+mu^-mu^-)$ as functions of neutrino mass and lifetime. These limits are on the order of $10^{-9}$ for short neutrino lifetimes of 1 ps or less. Limits are also set on the coupling between the muon and a possible fourth-generation neutrino.
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