Investigation of $\Lambda_c \to (\Lambda,n)\ell^+ \nu_\ell $ Decays in Standard Model and Beyond

Abstract

In this work, we study the decays $\Lambda_c \to (\Lambda, n) \ell^+ \nu_\ell$ ($\ell = \mu, e$) within a model-independent framework. We calculate the helicity amplitudes for all possible four-fermion operators, including the interactions between different new physics (NP) operators. The form factors for the $\Lambda_c \to (\Lambda, n)$ transitions are taken from lattice QCD calculations. We present detailed results for the branching fractions and other key observables. Although our results are generally consistent with previous studies, we find that the predicted central values for the branching fractions are approximately $10\%$ larger than the experimental measurements. Additionally, we explore the potential impacts of NP, focusing particularly on the scenario in which NP particles couple exclusively to the muon. Using Wilson coefficients fitted from $D$ and $D_s$ meson decays, we examine NP effects in the $\Lambda_c \to (\Lambda, n) \mu^+ \nu_\mu$ decay channels. It is found that although most potential contributions of NP are obscured by the uncertainties inherent in both theory and experiment, the right-handed scalar operator could suppress the branching fraction of $\Lambda_c \to (\Lambda, n) \mu^+ \nu_\mu$ by up to $10\%$. We also highlight that the ratio of the forward-backward asymmetry in the $\Lambda_c \to (\Lambda, n) \mu^+ \nu_\mu$ decay to that in the $\Lambda_c \to (\Lambda, n) e^+ \nu_e$ decay provides a novel probe for NP, as it is less sensitive to hadronic uncertainties and is largely unaffected by current NP operators. All of our results can be tested in ongoing experiments such as BESIII, Belle-II, and LHCb, as well as in future high-energy facilities like the Super Tau-Charm Factory (STCF) and the Circular Electron Positron Collider (CEPC).