Observation of the Charge Symmetry Breaking $d+d\to {}^4\mathrm{H}\mathrm{e}+{\pi }^0$ Reaction Near Threshold

We report the first observation of the charge symmetry breaking $d+d\to {}^4\mathrm{H}\mathrm{e}+{\pi }^0$ reaction near threshold. Measurements using a magnetic channel (gated by two photons) of the ${}^4\mathrm{H}\mathrm{e}$ scattering angle and momentum (from time of flight) permitted reconstruction of the ${\pi }^0$ “missing mass,” the quantity used to separate ${}^4\mathrm{H}\mathrm{e}+{\pi }^0$ events from the continuum of double radiative capture ${}^4\mathrm{H}\mathrm{e}+\gamma +\gamma$ events. We measured total cross sections for neutral pion production of $12.7\pm 2.2\mathrm{p}\mathrm{b}$ at 228.5 MeV and $15.1\pm 3.1\mathrm{p}\mathrm{b}$ 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.

Figure 1

Layout of the experimental setup showing the target, approximate locations of the Pb glass arrays, and the magnetic channel in relation to a segment of the electron-cooled storage ring. Quadrupole magnets (Q1, Q2, and Q3), wire chambers (WC1, WC2, and WC3), and scintillation trigger ($\Delta \mathrm{E}1$, $\Delta \mathrm{E}2$, and E) and veto (V1 and V2) detectors are shown. The luminosity detectors are small and consequently are omitted here.

Figure 2

Histograms of the candidate events at the two deuteron bombarding energies as a function of their missing mass value. The smooth curves show the reproduction of these histograms with a Gaussian peak and a continuum.