Evidence for a Second Neutral Pion

Evidence suggests the existence of a second neutral pion based on: (1) the anomalous branching ratios in the reactions p ¯ p→ππ and p ¯ d→ππN , and (2) the 1960s findings of Tsai-Chü et al. regarding antinucleon annihilation stars in emulsions. The anomaly in (1) disappears if the two neutral pions in the reactions p ¯ p→ π 0 π 0 and p ¯ d→ π 0 π 0 n are not identical. Tsai-Chü et al. observed a second neutral pion that “decays more rapidly into electron pairs with larger opening angles and more frequently into double pairs.” One antineutron annihilation event produced three neutral particles, each with a mass of 135 ± 14 MeV, decaying into four electrons with significantly wider opening angles than those of the internal conversion electrons observed in π 0 decays. The larger opening angles and higher frequency of double pair production could be caused by neutral pions with such a short lifetime that they decay into photon pairs before they can leave the annihilation nucleus (e.g., Ag) of the emulsion. We discuss several methods for searching for a second neutral pion.