Search for the baryon- and lepton-number violating decays $B^0\to p{\mu }^{-}$ and $B_s^0\to p{\mu }^{-}$

A search for the baryon- and lepton-number violating decays $B^0\to p{\mu }^{-}$ and $B_s^0\to p{\mu }^{-}$ is performed at the LHCb experiment using data collected in proton-proton collisions at $\sqrt{s}=7$, 8 and 13 TeV, corresponding to integrated luminosities of 1, 2, and $6{\mathrm{fb}}^{-1}$, respectively. No significant signal for $B^0\to p{\mu }^{-}$ and $B_s^0\to p{\mu }^{-}$ decays is found and the upper limits on the branching fractions are determined to be $\mathcal{B}(B^0\to p{\mu }^{-})<2.6(3.1)\times {10}^{-9}$ and $\mathcal{B}(B_s^0\to p{\mu }^{-})<12.1(14.0)\times {10}^{-9}$, respectively, at 90% (95%) confidence level. These are the first limits on these decays to date.

Figure 1

Hypothetical Feynman diagrams of $B_{(s)}^0\to p{\ell }^{-}$ and $\overline{p}{\ell }^{+}$ mediated by a hypothetical $Y$ boson.

Figure 2

Comparison of the distributions of the $B^0$ vertex fit ${\chi }^2$ [${\chi }_{\mathrm{fit}}^2(B^0)$] and the minimum ${\chi }_{\mathrm{IP}}^2$ of the two tracks in the final state [$\mathrm{Min}({\chi }_{\mathrm{IP}}^2)$], for the same-sign sample and data sidebands in run 2.

Figure 3

Expected distribution of the MLP response for $B_{(s)}^0\to p{\mu }^{-}$ decays as obtained from the $B^0\to K^{+}{\pi }^{-}$ control channel, for (left) run 1 and (right) run 2 data. The total (statistical and systematic combined) uncertainty is shown as a light gray band, and the thickness of each horizontal line at the center indicates the statistical uncertainty. Each interval is normalized to its width.

Figure 4

Mass distribution of $B^0\to K^{+}{\pi }^{-}$ candidates in data for (left) run 1 and (right) run 2. The results of the invariant mass fits are superimposed.

Figure 5

Mass distribution of $B^{+}\to J/\psi K^{+}$ candidates in data for (left) run 1 and (right) run 2. The results of the invariant mass fits are superimposed.

Figure 6

Mass distribution of signal candidates (black dots) for run 1 samples in regions of MLP. The results of the fit described in the text are also shown. The contributions from the two signal channels, $B^0\to p{\mu }^{-}$ (red dashed lines) and $B_s^0\to p{\mu }^{-}$ (green dashed lines), are scaled by a factor of 10 for better visibility.

Figure 7

Mass distribution of signal candidates (black dots) for run 2 samples in regions of MLP. The results of the fit described in the text are also shown. The contributions from the two signal channels, $B^0\to p{\mu }^{-}$ (red dashed lines) and $B_s^0\to p{\mu }^{-}$ (green dashed lines), are scaled by a factor of 10 for better visibility.

Figure 8

Results from the CLs scan used to obtain the limit on (left) $\mathcal{B}(B^0\to p{\mu }^{-})$ and (right) $\mathcal{B}(B_s^0\to p{\mu }^{-})$. The background-only expectation is shown by the red line and the $1\sigma$ and $2\sigma$ bands are shown as light blue and blue bands, respectively. The observation is shown as the solid black line. The two dashed lines intersecting with the observation indicate the limits at 90% and 95% CL for the upper and lower line, respectively.