The CP-conserving contribution to the transverse muon polarization in K+ -> mu+ nu gamma

TL;DR

This paper estimates the CP-conserving electromagnetic contribution to the transverse muon polarization in K+ decay within the Standard Model, finding it to be very small (~10^{-4}), which informs the sensitivity needed to detect new physics.

Contribution

It provides a detailed calculation of the CP-conserving contribution to P_T in K+ decay, highlighting its small magnitude and implications for new physics searches.

Findings

CP-conserving P_T is of order 10^{-4}

Electromagnetic final-state interactions dominate within the Standard Model

Sensitivity to new sources of T-violation is necessary for detecting beyond Standard Model effects

Abstract

We present a detailed estimate of the transverse muon polarization (P_T) due to electromagnetic final-state interactions in the decay $K^+ \to \mu^+ \nu \gamma$. This CP-conserving effect represents the dominant contribution to P_T within the Standard Model. As a result of an explicit calculation, we find that the CP-conserving contribution to P_T is quite small, typically of $\cO(10^{-4})$, essentially due to the suppression factor $\alpha/4\pi^2$. This enforces the sensitivity of P_T in probing extensions of the Standard Model with new sources of time-reversal violations. A brief discussion about possible CP-violating contributions to P_T in the framework of supersymmetric models with unbroken R-parity is also presented.