Evidence for the semileptonic decays $\Lambda_c^{+} \to \Sigma^{\pm} \pi^{\mp} e^+ \nu_e$

Abstract

Using $4.5\, fb^{-1}$ of $e^+e^-$ collision data collected by the BESIII detector at center-of-mass energies between $4.600$ and $4.699\,GeV$, we search for the semileptonic decays $\Lambda_c^{+} \to \Sigma^{+} \pi^{-} e^+ \nu_e$ and $\Lambda_c^{+} \to \Sigma^{-} \pi^{+} e^+ \nu_e$ for the first time. Assuming their branching fractions are equal under isospin symmetry, evidence for $\Lambda_c^{+} \to \Sigma^{\pm} \pi^{\mp} e^+ \nu_e$ is reported with a significance of $3.6\sigma$. The corresponding branching fraction is measured to be $\mathcal{B}(\Lambda_c^{+} \to \Sigma^\pm\pi^\mp e^+\nu_e) = (7.7^{+2.5}_{-2.3_{\rm stat.}}\pm1.3_{\rm syst.})\times 10^{-4}$, which is consistent with quark model predictions within two standard deviations.