The CP-conserving contribution to the transverse muon polarization in K+→μ+νγ

We present an estimate of the transverse muon polarization (PT) due to electromagnetic final-state interactions in the decay K+→μ+νγ. This CP-conserving effect represents the dominant contribution to PT within the Standard Model. As a result of an explicit calculation, we find that the CP-conserving contribution to PT is quite small, typically of O(10−4), essentially due to the suppression factor α/4π2. This enforces the sensitivity of PT in probing extensions of the Standard Model with new sources of time-reversal violations. A brief discussion about possible CP-violating contributions to PT in the framework of supersymmetric models with unbroken R-parity is also presented.