Leptonic Pion Decay And Physics Beyond The Electroweak Standard Model

TL;DR

The paper investigates how the ratio of leptonic pion decay rates can reveal new physics beyond the Standard Model, providing precise calculations and limits for various theoretical extensions.

Contribution

It offers comprehensive calculations of the pion decay ratio including QCD corrections for multiple beyond-Standard-Model theories, establishing benchmarks for future experiments.

Findings

Sets limits on new physics models based on current $R_{ ext{pi}}$ measurements.

Provides state-of-the-art theoretical predictions for leptonic pion decay ratios.

Identifies the sensitivity of $R_{ ext{pi}}$ to various beyond-Standard-Model interactions.

Abstract

The ratio of branching ratios in leptonic pion decay $R_{\pi} \equiv (\Gamma(\pi^- \to e \nu_e))/(\Gamma(\pi^- \to \mu \nu_\mu))$ is a powerfully sensitive probe of new interactions beyond the electroweak standard model. This is due to the chirality suppression of the standard model amplitude for the decay, which results in a precise prediction for the ratio, and suppressed amplitudes for new contributions to interfere with. We calculate, including QCD corrections, the contributions to $R_{\pi}$ arising from a broad selection of standard model extensions to which it is sensitive, including: R-parity violating interactions in supersymmetric theories, theories with light (electroweak scale) leptoquark degrees of freedom, non-minimal models of extra doublet Higgs bosons, models in which the quarks and leptons are composite both with and without supersymmetry, and models with strong TeV scale gravitational interactions. Comparing with existing measurements of $R_{\pi}$ we provide limits on each of these classes of models; our calculations also represent state of the art theoretical benchmarks against which the results from the upcoming round of leptonic pion decay experiments may be compared.