The Standard Model prediction for R_{e/mu}^{(pi,K)}

TL;DR

This paper calculates precise Standard Model predictions for the ratios of pion and kaon decay rates into electrons versus muons using Chiral Perturbation Theory, including two-loop effects and counterterm matching.

Contribution

It provides the first detailed two-loop order calculations of these decay ratios with improved theoretical precision.

Findings

R_{e/mu}^{(pi)} = (1.2352 ± 0.0001)×10^{-4}

R_{e/mu}^{(K)} = (2.477 ± 0.001)×10^{-5}

Enhanced understanding of lepton flavor universality in meson decays.

Abstract

We study the ratios R_{e/mu}^{(P)} = Gamma(P -> e nu [gamma])/Gamma(P -> mu nu [gamma]) (P=pi,K) in Chiral Perturbation Theory to order e^2 p^4. We complement the two-loop effective theory results with a matching calculation of the counterterm, finding R_{e/mu}^{(pi)} = (1.2352 \pm 0.0001)*10^(-4) and R_{e/mu}^{(K)} = (2.477 \pm 0.001)*10^(-5).