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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 powerfullysensitive probe of new interactions beyond the electroweak standard model. Thisis due to the chirality suppression of the standard model amplitude for thedecay, which results in a precise prediction for the ratio, and suppressedamplitudes for new contributions to interfere with. We calculate, including QCDcorrections, the contributions to $R_{\pi}$ arising from a broad selection ofstandard model extensions to which it is sensitive, including: R-parityviolating interactions in supersymmetric theories, theories with light(electroweak scale) leptoquark degrees of freedom, non-minimal models of extradoublet Higgs bosons, models in which the quarks and leptons are composite bothwith and without supersymmetry, and models with strong TeV scale gravitationalinteractions. Comparing with existing measurements of $R_{\pi}$ we providelimits on each of these classes of models; our calculations also representstate of the art theoretical benchmarks against which the results from theupcoming round of leptonic pion decay experiments may be compared.

Leptonic Pion Decay And Physics Beyond The Electroweak Standard Model