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

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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.

Figures and captions

s-channel diagram for $ B ^+ \rightarrow K ^- \mu ^+ \mu ^+ $ ( $ B ^+ \rightarrow \pi ^- \mu ^+ \mu ^+ $ ) where the decay is mediated by an on-shell 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 ^+ $ (light-coloured line) and $ B ^+ \rightarrow \pi ^- \mu ^+ \mu ^+ $ (dark-coloured 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.


Tables and captions

Sources of systematic error and their fractional uncertainty on the relative efficiency.

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tex code

Created on 20 February 2021.