A search for hep solar neutrinos at the Sudbury Neutrino Observatory

Thesis (Ph.D.)--University of Washington, 2016-08

Solar neutrinos from the fusion hep reaction, (helium-3 fusing with a proton to become helium-4, releasing a positron and neutrino), have previously remained undetected due to their flux being about one one-thousandth that of boron-8 neutrinos. These neutrinos are interesting theoretically because they are less dependent on solar composition than other solar neutrinos, and therefore provide a somewhat independent test of the Standard Solar Model. In this analysis, we develop a new event fitter for existing data from the Sudbury Neutrino Observatory. We also use the fitter to remove backgrounds that previously limited the fiducial volume, which we increase by 30%. We use a modified Wald-Wolfowitz test to increase the amount of live time by 200 days (18%) and show that this data is consistent with the previously-used data. Finally, we develop a Bayesian analysis technique to make full use of the posterior distributions of energy returned by the event fitter. In the first significant detection of hep neutrinos, we find that the most-probable rate of hep events is 3.5 x 10^4 /cm^2/s, which is significantly higher than the theoretical prediction. We find that the 95% credible region extends from 1.0 to 7.2 x 10^4 /cm^2/s, and that we can therefore exclude a rate of 0 hep events at greater than 95% probability.