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 $K^{+}\to {\mu }^{+}\nu \gamma$. 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 $\text{cO}(10^{-4})$, 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.

Comments: 10 pages, 3 figures