Ab initio calculations of nuclear structure corrections of super-allowed Fermi transitions

CKM unitarity is a sensitive probe of the Standard Model. The top row unitarity is dominated by the V_ud matrix element which is most cleanly extracted from nuclear Fermi β decays. The most recent data-driven analyses of measurements suggest a 2σ−3σ discrepancy [1-2]. Several theoretical corrections are required to extract the V_ud and, in fact, the current uncertainty in V_ud extracted from Fermi transitions is dominated by theory. We perform ab initio calculations of nuclear structure corrections δ_NS and δ_C for the ¹⁰C→¹⁰B Fermi transition that suffer from substantial uncertainties. We apply the no-core shell model (NCSM) [3] to calculate the δ_NS with the nuclear Green’s function obtained using the continued fraction Lanczos algorithm [4] and NCSM with continuum (NCSMC) [5] to calculate the δ_C using the recently introduced NCSMC β-decay formalism [6]. We will report preliminary results for the δ_NS and report our progress towards the δ_C calculation. Our approach is also applicable to ¹⁴O→¹⁴N Fermi transition as well as to transitions in light sd shell nuclei.

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