Michael Gennari

Topic

Electroweak Radiative Corrections in Super-Allowed Beta Decays from Ab Initio Theory

Department of Physics and Astronomy

Date & location

• Friday, August 8, 2025
• 10:30 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Petr Navratil, Department of Physics and Astronomy, University of Victoria (Co-Supervisor)
• Dr. Robert Kowalewski, Department of Physics and Astronomy, UVic (Co-Supervisor)
• Dr. Pavel Kovtun, Department of Physics and Astronomy, UVic (Member)
• Dr. Irina Paci, Department of Chemistry, UVic (Outside Member)

External Examiner

• Dr. Calvin Johnson, Department of Physics, San Diego State University

Chair of Oral Examination

• Dr. David Berg, Department of Chemistry, UVic

Abstract

A systematically improvable, ab initio model is developed to compute nuclear-structure-dependent electroweak radiative corrections in superallowed Fermi 𝛽 decays. With inter-nucleon interactions derived from the low-energy symmetries of quantum chromo-dynamics via a prescription of effective field theory, the nuclear many-body configurations are obtained in the quasi-exact, no-core shell model. This approach rigorously treats all nucleons as active degrees of freedom in solution of the non-relativistic, many-body Schrödinger equation with Hamiltonian constructed from chiral effective field theory.

One of the two key nuclear-structure corrections to superallowed 𝛽 decays, known as 𝛿_NS, arises from modifications to the one-nucleon 𝛾𝑊 box diagram when immersed in the nuclear medium. It is computed for the two lightest superallowed transitions: the ¹⁰C→¹⁰B and ¹⁴O→¹⁴N transitions. The nuclear 𝛾𝑊 box amplitude is itself explicitly evaluated as the time-ordered product of the electromagnetic and charge-changing weak current operators, providing a transparent multipole decomposition of the currents.

The resulting complicated amplitude structure involves many-body resolvent operators which are treated with the Lanczos strengths method, the key tool of this dissertation. As much as is permitted by the Lanczos algorithm, this method incorporates quasi-exact information about the complete intermediate nuclear spectrum. For ¹⁰C→¹⁰B, we find the nuclear-structure-dependent radiative correction 𝛿_NS to be

𝛿_NS→[¹⁰C→¹⁰B]=−0.422(14)_PME(4)_Ω(9)_𝜒(24)_sh(12)_𝑛,el% ,

which represents a 1.6x reduction in the quoted uncertainty compared to prior literature estimates despite the accounting for additional uncertainties. Preliminary results for the ¹⁴O→¹⁴N transition indicate a markedly different distribution of the amplitude strengths, reflecting a strong Gamow-Teller suppression and highlighting the need for higher-multipole analysis before a final value is quoted.

These precision gains directly impact the determination of 𝑉_𝑢𝑑 and thus the top-row, Cabibbo-Kobayashi-Maskawa matrix unitarity test, motivating renewed experimental efforts – particularly a more precise measurement of the ¹⁰C branching ratio – and opening the way to analogous, precision ab initio studies for other electroweak processes in light nuclei.