Cusp effects in meson decays

Abstract

The pion mass difference generates a pronounced cusp in the pi0 pi0 invariant mass distribution of K+ --> pi0 pi0 pi+ decays. As originally pointed out by Cabibbo, an accurate measurement of the cusp may allow one to pin down the S-wave pion-pion scattering lengths to high precision. We present the non-relativistic effective field theory framework that permits to determine the structure of this cusp in a straightforward manner, including the effects of radiative corrections. Applications of the same formalism to other decay channels, in particular eta and eta' decays, are also discussed.

12 pages, 14 figures; plenary talk at the 19th International IUPAP Conference on Few-Body Problems in Physics, Bonn, Germany, August 31 - September 5, 2009