Speaker
Description
Solar $hep$ neutrinos, produced in the rare $\rm ^3He +p \to ^4He + e^+ + \nu_e$ fusion, are the only Standard Solar Model (SSM) neutrino component not yet experimentally established. Their observation is challenging due to the extremely small expected rate and the presence of large $\rm ^8B$ background; sensitivity is concentrated at the highest energies of the solar neutrino spectrum up to ~19MeV, where the $hep$ spectrum extends beyond the $\rm ^8B$ 16MeV endpoint.
We present an optimized $hep$ search with the full Super-Kamiokande-IV dataset based on a global 2D template fit in recoil electron energy and solar angle. A key feature of the analysis is that $\rm ^8B$ and $hep$ are modeled with independent 2D signal PDFs, avoiding the conventional treatment in which hep is absorbed into a single mixed solar angle template. Neutrino oscillation effects are incorporated directly at the PDF-construction level, and the fit simultaneously constrains the unoscillated $\rm ^8B$ and $hep$ flux. We report the hep best-fit result and 90% C.L. upper limit, which is more stringent compared to the previous limit reported with SK-I, due to increased statistics, improved calibration and new analysis method. We also discuss the dominant limitations and pathways to further improve $hep$ sensitivity at SK.