Quenching of the weak axial coupling derived from the scattering of stopped-pion neutrinos on 127I

The need for quenching of the weak axial-vector coupling in calculations involving weak processes in nuclei is a well-known and long-standing problem arising from several contributing factors. Theoretical studies in momentum exchanges beyond that of typical β decays have been hindered in the past by lack of experimental results. To this end, the recent measurement of inelastic scattering cross section of electron neutrinos from the stopped-pion neutrino source at Oak Ridge National Laboratory (ORNL), impinging on 127 I, by the COHERENT collaboration [P. An et al., Phys. Rev. Lett. 131, 221801 (2023)] offers a rare opportunity for such a study. We compute this cross section by employing the microscopic quasiparticle-phonon model, based on nuclear excitations produced by the quasiparticle random-phase approximation (QRPA) and the related microscopic effective Hamiltonian based on a G-matrix approach. In the computations we include the meson-exchange two-body currents at the level of effective one-body currents. Comparison of the measured and computed cross sections allows us for the first time to go beyond the β -decay studies of P. Gysbers et al. [Nat. Phys. 15, 428 (2019)] in separating the contributions coming from the two-body currents and the nuclear many-body approach to the quenching of the weak axial-vector coupling g A for a medium-mass open-shell nucleus in a momentum-exchange region relevant for, e.g., the neutrinoless double β decay.