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

TL;DR

This study searches for a rare beta decay of 8B to the ground state of 8Be, setting an upper limit on its branching ratio, which has implications for solar neutrino spectra and neutrino physics.

Contribution

It provides the first experimental upper limit on the second forbidden beta decay of 8B to 8Be ground state and offers a theoretical calculation for this branching ratio.

Findings

Upper limit of 7.3 x 10^{-5} for the branching ratio

No significant observation of the decay was made

Implications for solar neutrino spectrum modeling

Abstract

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 alpha particles from the decay of 8B, with the goal of observing the two 46-keV alpha 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 alpha-particles from the 8Be ground state. From this measurement we find an upper limit (at 90% confidence level) of 7.3 x 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.