Observation of the charge symmetry breaking d+d → 4He+π0 reaction near threshold -: art. no. 142302

We report the first observation of the charge symmetry breaking d+d--> He-4 +pi(0) reaction near threshold. Measurements using a magnetic channel (gated by two photons) of the He-4 scattering angle and momentum (from time of flight) permitted reconstruction of the pi(0) "missing mass," the quantity used to separate He-4 +pi(0) events from the continuum of double radiative capture 4He +gamma+gamma events. We measured total cross sections for neutral pion production of 12.7+/-2.2 pb at 228.5 MeV and 15.1+/-3.1 pb at 231.8 MeV. The uncertainty is dominated by statistical errors. These cross sections arise fundamentally from the down-up quark mass difference and quark electromagnetic effects that contribute in part through meson mixing (e.g., pi(0)-eta) mechanisms.