Search for baryon and lepton number violating decays of ${\mathrm{\Xi }}^0$ hyperons

Using $(1.0087\pm 0.0044)\times {10}^{10}J/\psi$ events collected by the BESIII detector at the BEPCII collider, we report the first search for the baryon and lepton number violating decays ${\mathrm{\Xi }}^0\to K^{-}{e}^{+}$ with $\mathrm{\Delta }(B-L)=0$ and ${\mathrm{\Xi }}^0\to K^{+}{e}^{-}$ with $|\mathrm{\Delta }(B-L)|=2$, where $B$ ($L$) is the baryon (lepton) number. While no signal is observed, the upper limits on the branching fractions of these two decays are set to $\mathcal{B}({\mathrm{\Xi }}^0\to K^{-}{e}^{+})<3.6\times {10}^{-6}$ and $\mathcal{B}({\mathrm{\Xi }}^0\to K^{+}{e}^{-})<1.9\times {10}^{-6}$ at the 90% confidence level, respectively. These results offer a direct probe of baryon number violating interactions involving a strange quark.

Figure 1

Feynman diagrams for the BNV decays of (a) ${\mathrm{\Xi }}^0\to K^{-}{e}^{+}$ with $\mathrm{\Delta }(B-L)=0$ mediated by a gauge boson $X$ and (b) ${\mathrm{\Xi }}^0\to K^{+}{e}^{-}$ with $|\mathrm{\Delta }(B-L)|=2$ mediated by an elementary scalar field $\varphi$.

Figure 2

Fit to the $M_{\mathrm{BC}}$ distribution of the accepted ST candidates, where the black points with error bars are data, and the red dashed and green dash-dotted lines are the signal and background shapes, respectively. The blue solid line shows the total fit and the red arrows indicate the ST signal region.

Figure 3

Distributions of $M_{Ke}$ versus $M_{\overline{\mathrm{\Lambda }}{\pi }^0}$ of the accepted candidates for (a) ${\mathrm{\Xi }}^0\to K^{-}{e}^{+}$ and (b) ${\mathrm{\Xi }}^0\to K^{+}{e}^{-}$ in data, respectively. The red box indicates the signal region.