T-odd correlation in the K_(l 3 γ)-decay

The dependence of the $K^+ \to \pi^0 l^+ \nu_l \gamma$ decay width on the $T$-odd kinematical variable, $ \xi = {\vec q \cdot [\vec p_l \times \vec p_\pi]} / {M^3}$, is studied at the tree and one-loop levels of Standard Model. It is shown that at the tree level this decay width is the even function of $\xi$, while the odd contribution arises due to the electromagnetic final state interaction. This contribution is determined by imaginary parts of one-loop diagrams. The calculations performed show that the $\xi$-odd contribution to the $K^+ \to \pi^0 e^+ \nu_e \gamma$ and $K^+ \to \pi^0 \mu^+ \nu_\mu \gamma$ decay widths is four orders of magnitude smaller than even contribution coming from the tree level of SM.