Decay of theηmeson intoπμ+μ−

We calculate the unitarity bound for the decay of $\eta \to {\pi }^{0}l\overline{l}(l=e or \mu )$. The input parameters are the two transition form factors in the radiative decay of the $\eta \to {\pi }^0\gamma \gamma$. They are calculated using the vector-meson-dominance model plus the pseudoscalar $a_0$ exchange. This yields a width which is lower than the measured rate of $\eta \to {\pi }^0\gamma \gamma$ by a factor of 2. In view of the uncertainties in the measurement this is not unsatisfactory. A conservative calculation gives a lower bound for the width of $\eta \to {\pi }^0\mu \overline{\mu }$ of 0.9 μeV.