The p-bar p --> pi_0 pi_0 Puzzle

Abstract

According to conventional theory, the annihilation reaction p-bar p --> pi_0 pi_0 cannot occur from a p-bar p atomic S state. However, this reaction occurs so readily for antiprotons stopping in liquid hydrogen, that it would require 30% P-wave annihilations. Experimental results from other capture and p-bar p annihilation channels show that the fraction of P-wave annihilations is less than 6% in agreement with theoretical expectations. An experimental test to determine whether this reaction can occur from an atomic S state is suggested. If indeed this reaction is occurring from an atomic S state, then certain neutral vector mesons should exhibit a pi_0 pi_0 decay mode, and this can also be tested experimentally.