On the Pais--Treiman method to measure $\pi \pi$ phase shifts in $K_{e4}$ decays

G. Colangelo, M. Knecht, J. Stern

We evaluate theoretical uncertainties to the method of Pais and Treiman to measure the $\pi \pi$ phase shifts in $K_{e4}$ decays. We find that they are very small, below $1\%$.