a Measurement of Ξ0 Beta Decay by NA48/1 and the Extraction of VUS

A large sample of Ξ⁰ decays was collected by the NA48/1 collaboration at the CERN SPS accelerator during a high-intensity K_S and hyperon run in 2002. Clean signals for the semileptonic decays Ξ ⁰ -> Σ ⁺ e^- bar {ν }_e and Ξ ⁰ -> Σ ⁺ μ ^- bar {ν }_μ have been obtained. The measurement of the Ξ ⁰ -> Σ ⁺ e^- bar {ν }_e branching ratio has been used to extract V_us. This measurement is of special interest because recent results on V_us obtained mostly from "Ke3" decays of neutral and charged kaons (K → πeν), in combination with measured values for V_ud seemed to be in conflict with the unitarity condition of the CKM matrix.