M ay 2 00 3 η → π 0 γγ decay within a chiral unitary approach

We improve the calculations of the η → π 0 γγ decay within the context of me-son chiral lagrangians. We use a chiral unitary approach for the meson-meson interaction, thus generating the a 0 (980) resonance and fixing the longstanding sign ambiguity on its contribution. This also allows us to calculate the loops with one vector meson exchange, thus removing a former source of uncertainty. In addition we ensure the consistency of the approach with other processes. First, by using vector meson dominance couplings normalized to agree with radiative vector meson decays. And, second, by checking the consistency of the calculations with the related γγ → π 0 η reaction. We find an η → π 0 γγ decay width of 0.47 ± 0.10 eV, in clear disagreement with published data but in remarkable agreement with the most recent measurement.