Fermi Constant from Muon Decay Versus Electroweak Fits and Cabibbo-Kobayashi-Maskawa Unitarity

Area of Science:

• Particle Physics
• Standard Model Physics
• Beyond Standard Model Physics

Background:

• The Fermi constant (G_{F}) is a fundamental parameter of the Standard Model (SM), precisely determined from muon lifetime measurements.
• Discrepancies exist between G_{F} derived from muon decay and alternative methods like electroweak fits or superallowed beta decays.
• These tensions may indicate new physics beyond the Standard Model (BSM) or issues with fundamental symmetries.

Purpose of the Study:

• To investigate potential BSM physics that could resolve the tensions between different determinations of the Fermi constant (G_{F}).
• To analyze how BSM physics can reconcile G_{F} values from muon decay, electroweak fits, and beta decays.
• To assess the implications for precision tests of the Standard Model and search for new physics.

Main Methods:

• Utilizing the Standard Model effective field theory (SMEFT) framework to parameterize potential BSM effects.
• Comparing G_{F} values obtained from muon lifetime, global electroweak fits, and superallowed beta decays.
• Analyzing the role of the Cabibbo angle, measured in kaon, tau, and D decays, in the G_{F} determinations.

Main Results:

• The study identifies BSM scenarios that could bring the three independent G_{F} measurements into agreement.
• It highlights how tensions within the electroweak fit and potential violations of Cabibbo-Kobayashi-Maskawa unitarity are linked to G_{F} discrepancies.
• The analysis provides insights into the interplay between different precision measurements in the search for new physics.

Conclusions:

• BSM physics offers a potential resolution to the observed tensions in Fermi constant determinations.
• Reconciling these measurements is crucial for advancing precision tests of the Standard Model.
• Future experimental and theoretical advancements are needed to further probe these discrepancies and their BSM implications.