On the $K^+ \to \pi^0\pi^0\pi^0 e^+ \nu$ decay

Abstract

The OKA Collaboration has obtained a new upper limit on the $K^+ \to \pi^0\pi^0\pi^0 e^+ \nu$ decay probability which is 65 times lower than the one currently listed by PDG. However it is still about $10^{4}$ times worse than the theoretical prediction. It was suggested that production of pionium $A_{2\pi}$ in the final state of the semileptonic $K^+$ decay with subsequent $A_{2\pi} \to \pi^0 \pi^0$ decay will increase the value of theoretical branching ratio due to the absence of five-particle phase space suppression. The estimates done in our paper show that despite the gain in the phase space the decay $K^+ \to A_{2\pi}\pi^0 e^+ \nu$ appeares to be strongly suppressed because of the smallness of the pionium wave function at zero $\pi$ mesons separation.