One-loop analysis of β decays in SMEFT
arxiv(2024)
Abstract
We perform a loop-level analysis of charged-current (CC) processes involving
light leptons and quarks within the Standard Model Effective Field Theory
(SMEFT). This work is motivated by the high precision reached in experiment and
Standard Model calculations for CC decays of mesons, neutron, and nuclei, and
by a lingering tension in the Cabibbo universality test. We identify the SMEFT
operators that induce the largest loop-level contributions to CC processes.
These include four-quark and four-fermion semileptonic operators involving two
third-generation quarks. We discuss the available constraints on the relevant
effective couplings and along the way we derive new loop-level bounds from K
→πνν̅ on four-quark operators involving two top quarks. We find
that low-energy CC processes are quite competitive with other probes, set
constraints that do not depend on flavor-symmetry assumptions, and probe
operators involving third-generation quarks up to effective scales of
Λ≃ 8 TeV. Finally, we briefly discuss single-field ultraviolet
completions that could induce the relevant operators.
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