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

TL;DR

This paper investigates $ ext{Lambda}_c$ decays to $ ext{Lambda}$ or neutron with leptons within and beyond the Standard Model, calculating form factors, branching ratios, and exploring potential new physics effects, especially on muon channels.

Contribution

It provides a model-independent analysis of $ ext{Lambda}_c$ decays using lattice QCD form factors and examines potential new physics impacts on branching ratios and asymmetries.

Findings

Predicted branching fractions are about 10% higher than experimental data.

Right-handed scalar operators can reduce muon decay branching by up to 10%.

The ratio of forward-backward asymmetries offers a NP probe less affected by uncertainties.

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).