Search for the decay ${\pi }^0\to e^{+}{e}^{-}$

The reaction ${\pi }^{-}p\to {\pi }^{0}n$ at rest was used as a source of tagged monoenergetic ${\pi }^0\text{'}\mathrm{s}$ in a search for the decay ${\pi }^0\to e^{+}{e}^{-}$ with the SINDRUM I spectrometer. The measurement resulted in $B_{{\pi }^0\to e^{+}{e}^{-}}\equiv \frac{{\Gamma }_{{\pi }^0\to e^{+}{e}^{-}}}{{\Gamma }_{{\pi }^0\to 2\gamma }}<1.3\mathrm{}\times {10}^{-7\mathrm{}}$ (90% confidence level), consistent with the QED prediction $B_{{\pi }^0\to e^{+}{e}^{-}}=(6.5\mathrm{}\pm 0.5)\times {10}^{-8\mathrm{}}$. The combined result of two previous measurements, $B_{{\pi }^0\to e^{+}{e}^{-}}=\left({1.8}_{-0.6\mathrm{}}^{+0.7\mathrm{}}\right)\times {10}^{-7\mathrm{}}$, had suggested sizable additional contributions to the decay amplitude. This possibility now seems unlikely.