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

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 \({\Lambda }_{c}^{+}\to n{e}^{+}{\nu }_{e}\), utilizing 4.5 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 \({\Lambda }_{c}^{+}\to \Lambda {e}^{+}{\nu }_{e}\), achieving a statistical significance exceeding 10σ. The absolute branching fraction is measured to be (3.57 ± 0.34_stat. ± 0.14_syst.) × 10⁻³. For the first time, the CKM matrix element \(\left\vert {V}_{cd}\right\vert\) is extracted via a charmed baryon decay as \(0.208\pm 0.01{1}_{{{{\rm{exp.}}}}}\pm 0.00{7}_{{{{\rm{LQCD}}}}}\pm 0.00{1}_{{\tau }_{{\Lambda }_{c}^{+}}}\). 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.