First measurement of ${\mathrm{\Sigma }}^{+}n\to \mathrm{\Lambda }p$ and ${\mathrm{\Sigma }}^{+}n\to {\mathrm{\Sigma }}^{0}p$ cross sections via ${\mathrm{\Sigma }}^{+}$-nucleus scattering at an electron-positron collider

Using $(1.0087\pm 0.0044)\times {10}^{10}$ $J\mathrm{/}\psi$ events collected with the BESIII detector at the BEPCII storage ring, the reactions ${\mathrm{\Sigma }}^{+}n\to \mathrm{\Lambda }p$ and ${\mathrm{\Sigma }}^{+}n\to {\mathrm{\Sigma }}^{0}p$ are studied, where the ${\mathrm{\Sigma }}^{+}$ baryon is produced in the process $J\mathrm{/}\psi \to {\mathrm{\Sigma }}^{+}{\bar{\mathrm{\Sigma }}}^{-}$ and the neutron is a component of the ${}^9\mathrm{B}\mathrm{e}$, ${}^{12}\mathrm{C}$ and ${}^{197}\mathrm{A}\mathrm{u}$ nuclei in the beam pipe. Clear signals of these two reactions are observed for the first time. Their cross sections are measured to be $\sigma ({\mathrm{\Sigma }}^{+}+{}^9\mathrm{B}\mathrm{e}\to \mathrm{\Lambda }+p+X)=(45.2\pm {12.1}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {7.2}_{\mathrm{s}\mathrm{y}\mathrm{s}})$ mb and $\sigma ({\mathrm{\Sigma }}^{+}+{}^9\mathrm{B}\mathrm{e}\to {\mathrm{\Sigma }}^0+p+X)=(29.8\pm {9.7}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {6.9}_{\mathrm{s}\mathrm{y}\mathrm{s}})$ mb for a ${\mathrm{\Sigma }}^{+}$ average momentum of $0.992$ GeV/$c$, within a range of $\pm 0.015$ GeV/$c$, where $X$ represents the residual nucleus. This is the first study of ${\mathrm{\Sigma }}^{+}$-nucleon scattering at an electron-positron collider.