Implications of ηη′ mixing for the decay η → 3π

Taken by itself, the interference with the η′ appears to strongly affect the amplitude of the transition η → 3π. I point out that this effect is fictitious and also occurs in the mass spectrum of the pseudoscalars. Chiral symmetry implies that the same combination of effective coupling constants which determines the small deviations from the Gell-Mann-Okubo formula also specifies the symmetry breaking effects in the decay amplitude and thus ensures that these are small.