2019/11/12 by Andrzej Czarnecki, Czarnecki, Andrzej, William J. Marciano +3
Physics and Astronomy · #FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #High Energy Physics - Lattice (hep-lat) #High Energy Physics - Phenomenology (hep-ph) #Neutrino Physics Research #Nuclear Theory (nucl-th) #Particle physics theoretical and experimental studies #Quantum Chromodynamics and Particle Interactions
paper · pdf · doi:10.48550/arxiv.1911.04685
openalex publication_date 2019/11/12 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Pion beta decay, π+→ π0 e+ν(γ), provides a theoretically clean ± 0.3% determination of the CKM matrix element Vud. That value falls short of super-allowed nuclear beta decays where an order of magnitude better precision already exists. We advocate a new strategy for utilizing pion beta decay, based on its role in determining Vus / Vud via the ratio RV=Γ(K→ πl ν(γ))/Γ(π+→ π0 e+ν(γ)). RV measures f+K(0) |Vus| / f+π(0) |Vud| and is insensitive to the Fermi constant and some radiative corrections. Its dependence on the ratio of form factors may also prove useful for lattice gauge theory calculations. Employing a lattice based value f+K(0) / f+π(0) = 0.970(2), we find Vus/Vud=0.22918(88) compared to Vus/Vud=0.23131(45) obtained from RA=Γ(K→ μν(γ))/Γ(π→ μν(γ)). Those vector and axial-vector based Vus/Vud values exhibit a 2.2σ discrepancy. That tension may be relieved by a shift in the lattice f+K(0) / f+π(0) towards the consistency range 0.961(4), changes in experimental input or new physics. Other implications of RV and RA are also discussed.