Search for the lepton flavour violating decay $tau^-to mu^-mu^+mu^-$

A search for the lepton flavour violating decay $tau^-to mu^-mu^+mu^-$ is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of $1.0mathrm{,fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $7mathrm{,Tekern -0.1em V}$ and $2.0mathrm{,fb}^{-1}$ at $8mathrm{,Tekern -0.1em V}$. No evidence is found for a signal, and a limit is set at $90%$ confidence level on the branching fraction, $mathcal{B}(tau^-to mu^-mu^+mu^-) < 4.6 times 10^{-8}$.

Search for the lepton-flavour violating decays $B^0_s rightarrow e^{pm}mu^{mp}$ and $B^0 rightarrow e^{pm} mu^{mp}$

A search for the lepton-flavour violating decays $B^0_s to e^{pm} mu^{mp}$ and $B^0 to e^{pm} mu^{mp}$ is performed with a data sample, corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of $pp$ collisions at $sqrt{s} = 7$ TeV, collected by the LHCb experiment. The observed number of $B^0_s to e^{pm} mu^{mp}$ and $B^0 to e^{pm} mu^{mp}$ candidates is consistent with background expectations. Upper limits on the branching fractions of both decays are determined to be $BR(B^0_s to e^{pm} mu^{mp}) < 1.1 ,(1.4) times 10^{-8}$ and $BR (B^0 to e^{pm} mu^{mp}) < 2.8 ,(3.7) times 10^{-9}$ at 90% (95%) confidence level (C.L.). These limits are a factor of twenty lower than those set by previous experiments. Lower bounds on the Pati-Salam leptoquark masses are also calculated, $M_{rm LQ} (B^0_s to e^{pm} mu^{mp}) > 107$ TeV/c$^2$ and $M_{rm LQ} (B^0 to e^{pm} mu^{mp}) > 126$ TeV/c$^2$ at 95% C.L., and are a factor of two higher than the previous bounds.

Search for the lepton-flavour violating decays $B^0_{(s)} rightarrow e^pm mu^mp$

A search for the lepton-flavour violating decays $B^0_s rightarrow e^pm mu^mp$ and $B^0 rightarrow e^pm mu^mp$ is performed based on a sample of proton-proton collision data corresponding to an integrated luminosity of 3 fb$^{-1}$, collected with the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The observed yields are consistent with the background-only hypothesis. Upper limits on the branching fractions are determined to be $mathcal{B}(B^0_s rightarrow e^pm mu^mp) < 5.4,(6.3) times 10^{-9}$ and $mathcal{B}(B^0 rightarrow e^pm mu^mp) < 1.0,(1.3) times 10^{-9}$ at $90%,(95%)$ confidence level, which are the strongest limits on these decays to date.

Search for the lepton-flavour violating decays $B^+ to K^+ {mu}^{pm} e^{mp}$

A search for the lepton-flavour violating decays $B^+ to K^+ {mu}^{pm} e^{mp}$ is performed using a sample of proton-proton collision data, collected with the LHCb experiment at centre-of-mass energies of $7$ and $8~{rm TeV}$ and corresponding to an integrated luminosity of 3$rm~fb^{-1}$. No significant signal is observed, and upper limits on the branching fractions are set as $mathcal{B}(B^+ to K^+ {mu}^- e^+) < 7.0~(9.5) times 10^{-9}$ and $mathcal{B}(B^+ to K^+ {mu}^+ e^-) < 6.4~(8.8) times 10^{-9}$ at 90 (95) % confidence level. The results improve the current best limits on these decays by more than one order of magnitude.

Search for the lepton-flavour-violating decays $B^{0}_{s}totau^{pm}mu^{mp}$ and $B^{0}totau^{pm}mu^{mp}$

A search for $B^{0}_{s}totau^{pm}mu^{mp}$ and $B^{0}totau^{pm}mu^{mp}$ decays is performed using data corresponding to an integrated luminosity of 3 fb$^{-1}$ of proton-proton collisions, recorded with the LHCb detector in 2011 and 2012. For this search, the $tau$ lepton is reconstructed in the $tau^{-}topi^{-}pi^{+}pi^{-} u_{tau}$ channel. No significant signal is observed. Assuming no contribution from $B^{0}totau^{pm}mu^{mp}$ decays, an upper limit is set on the $B^{0}_{s}totau^{pm}mu^{mp}$ branching fraction of $mathcal{B}left( B^{0}_{s}totau^{pm}mu^{mp}right) < 4.2times 10^{-5}$ at $95%$ confidence level. If instead no contribution from $B^{0}_{s}totau^{pm}mu^{mp}$ decays is assumed, a limit of $mathcal{B}left( B^{0}totau^{pm}mu^{mp}right) < 1.4times 10^{-5}$ is obtained at $95%$ confidence level. These are the first limit on $mathcal{B}left( B^{0}_{s}totau^{pm}mu^{mp}right)$ and the worlds best limit on $mathcal{B}left( B^{0}totau^{pm}mu^{mp}right)$.