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 $B(B^{+} \to K^ \mu^+ \mu^+) < 5.4\times 10^{8}$ and $B(B^{+} \to \pi^ \mu^+ \mu^+) < 5.8\times 10^{8}$ are set at the 95% confidence level. These improve the previous best limits by factors of 40 and 30, respectively.
schannel diagram for $ B ^+ \!\rightarrow K ^ \mu ^+ \mu ^+ $ ( $ B ^+ \!\rightarrow \pi ^ \mu ^+ \mu ^+ $ ) where the decay is mediated by an onshell Majorana neutrino. 
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Invariant mass distribution of $ K ^+ \mu ^+ \mu ^ $ events after the application of the selection criteria. In (a) requiring the muon pair to be compatible with coming from a $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ decay and in (b) excluding invariant mass windows around the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ for the muon pair. The curve is the fit to data as described in the text. 
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Relative efficiency between the $ B ^+ \!\rightarrow K ^ \mu ^+ \mu ^+ $ signal and the $ B ^+ \!\rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K ^+ $ normalisation channel. The plot has been symmetrised over the diagonal. 
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The 95 % CL branching fraction limits for $ B ^+ \!\rightarrow K ^ \mu ^+ \mu ^+ $ (lightcoloured line) and $ B ^+ \!\rightarrow \pi ^ \mu ^+ \mu ^+ $ (darkcoloured line) as a function of the Majorana neutrino mass $m_\nu=m_{h\mu}$. 
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Animated gif made out of all figures. 
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Sources of systematic error and their fractional uncertainty on the relative efficiency. 
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Created on 22 February 2019.Citation count from INSPIRE on 22 February 2019.