The BESIII Collaboration has reported the first measurement of the semileptonic decay Ξ⁻ → Λ e⁻ νbare at an electron-positron collider, providing the first determination of its absolute branching fraction, as well as the first update of the Ξ⁻ axial charge in more than four decades. The result has been published in Physical Review Letters [Phys. Rev. Lett. 137, 091801 (2026)].
The axial charge of a hyperon is a fundamental observable in hadronic physics. It plays an important role in testing SU(3)-flavor symmetry breaking and in determining the quark spin content of the proton, providing a valuable supplement to polarized deep-inelastic scattering experiments.
Using a data sample of (10087 ± 44) × 10⁶ J/ψ events collected with the BESIII detector, the collaboration combined a double-tag method with an 11-dimensional angular analysis based on the advanced joint angular-distribution fit method. The absolute branching fraction was determined for the first time to be (3.60 ± 0.40 ± 0.10) × 10⁻⁴, which is lower than the world average value by 3.9 standard deviations. The axial-vector to vector coupling was measured to be 0.18 ± 0.07 ± 0.01. Within the SU(3)-flavor symmetry limit, the axial charge of the Ξ⁻ was determined to be 0.22 ± 0.08 ± 0.02.
This is the first exploitation of polarization and quantum entanglement in cascade baryon semileptonic decays, and it delivers comparable precision on the axial-vector to vector coupling using only one-twentieth of the statistics employed by previous experiments. This methodology is portable and can be readily extended to other decays at BESIII and to future facilities, including PANDA and proposed Super Tau Charm Factories.
