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

The beta decay of the lightest charmed baryon Λ 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 Λ c + → n e + ν 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 Λ c + → Λ e + ν e , achieving a statistical significance exceeding 10 σ . 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 V c d is extracted via a charmed baryon decay as 0.208 ± 0.01 1 exp. ± 0.00 7 LQCD ± 0.00 1 τ Λ 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. The semileptonic decay channels of the Λ c baryon can give important insights into weak interaction, but decay into a neutron, positron and electron neutrino has not been reported so far, due to difficulties in the final products’ identification. Here, the BESIII Collaboration reports its observation in e+e- collision data, exploiting machine-learning-based identification techniques.