Observation of a rare beta decay of the charmed baryon with a Graph Neural Network

The beta decay of the lightest charmed baryon ${\mathrm{\Lambda }}_c^{+}$ 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 ${\mathrm{\Lambda }}_c^{+}\to n{e}^{+}{\nu }_e$, utilizing $4.5{\mathrm{f}\mathrm{b}}^{-1}$ of electron-positron annihilation data collected with the BESIII detector. A novel Graph Neural Network based technique effectively separates signals from dominant backgrounds, notably ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{e}^{+}{\nu }_e$, achieving a statistical significance exceeding $10\sigma$. The absolute branching fraction is measured to be $(3.57\pm {0.34}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.14}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-3}$. For the first time, the CKM matrix element $\mid V_{cd}\mid$ is extracted via a charmed baryon decay as $0.208\pm {0.011}_{\mathrm{e}\mathrm{x}\mathrm{p}\mathrm{.}}\pm {0.007}_{\mathrm{L}\mathrm{Q}\mathrm{C}\mathrm{D}}\pm {0.001}_{{\tau }_{{\mathrm{\Lambda }}_c^{+}}}$. 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.