The cusp effect in eta' --> eta pi pi decays

Understand

Strong final-state interactions create a pronounced cusp in eta' --> eta pi0 pi0 decays.

• We adapt and generalize the non-relativistic effective field theory framework developed for the extraction of pi pi scattering lengths from K --> 3 pi decays to this case.
• The cusp effect is predicted to have an effect of more than 8% on the decay spectrum below the pi+ pi- threshold.