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

Abstract

We present a detailed estimate of the transverse muon polarization (P_T) due to electromagnetic final-state interactions in the decay $K^+ \to μ^+ νγ$. 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 $α/4π^2$. 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.