Search for the second forbidden β decay of 8 B to the ground state of 8 Be

A significant decay branch of 8B to the ground state of 8Be would extend the solar neutrino spectrum to higher energies than anticipated in the standard solar models. These high-energy neutrinos would affect current neutrino oscillation results and also would be a background to measurements of the hep process. We have measured the delayed α particles from the decay of 8B, with the goal of observing the two 46-keV α particles arising from the ground-state decay. The 8B was produced using an in-flight radioactive beam technique. It was implanted in a silicon PIN-diode detector that was capable of identifying the α particles from the 8Be ground state. From this measurement we find an upper limit (at 90% confidence level) of 7.3 × 10−5 for the branching ratio to the ground state. In addition to describing this measurement, we present a theoretical calculation for this branching ratio.