The beta decay of the lightest charmed baryon $\Lambda_c^+$ provides unique insights into the fundamental mechanism of strong and electro-weak interactions, serving as a testbed for investigating non-perturbative quantum chromodynamics and constraining the Cabibbo-Kobayashi-Maskawa (CKM) matrix parameters. This article presents the first observation of the Cabibbo-suppressed decay $\Lambda_c^+ \rightarrow n e^+ \nu_{ e}$, utilizing $4.5~\mathrm{ fb}^{ -1}$ of electron-positron annihilation data collected with the BESIII detector. A novel Graph Neural Network based technique effectively separates signals from dominant backgrounds, notably $\Lambda_c^+ \rightarrow \Lambda e^+ \nu_{ e}$, achieving a statistical significance exceeding $10\sigma$. The absolute branching fraction is measured to be $(3.57\pm0.34_{ \mathrm{ stat.}}\pm0.14_{ \mathrm{ syst.}})\times 10^{ -3}$. For the first time, the CKM matrix element $\left|V_{ cd}\right|$ is extracted via a charmed baryon decay as $0.208\pm0.011_{ \rm exp.}\pm0.007_{ \rm LQCD}\pm0.001_{ \tau_{ \Lambda_c^+}}$. This work highlights a new approach to further understand fundamental interactions in the charmed baryon sector, and showcases the power of modern machine learning techniques in experimental high-energy physics.
Categories: hep-ex hep-lat hep-ph nucl-ex