Chiral-loop and vector-meson contributions to η → ππγγ decays

The process η → π0π0γγ is discussed in Chiral Perturbation Theory (ChPT). Special attention is deveted to one-loop corrections, η-η′ mixing effects and vector-meson dominance of ChPT counter-terms. The less interesting η → π+π−γγ transition is briefly discussed too.