Improved measurement of $^8$B solar neutrinos with 1.5 kt y of Borexino exposure

TL;DR

This paper reports an improved measurement of the $^8$B solar neutrino flux using Borexino with enhanced background reduction and analysis techniques, providing more precise flux data and setting new limits on $hep$ neutrino interactions.

Contribution

The study introduces a more accurate measurement of $^8$B solar neutrinos with reduced background and novel analysis, improving upon previous Borexino results.

Findings

Measured $^8$B neutrino flux with high precision.

Set a new limit on $hep$ neutrino interaction rate.

Utilized almost the entire detector volume for the first time.

Abstract

We report on an improved measurement of the $^8$B solar neutrino interaction rate with the Borexino experiment at the Laboratori Nazionali del Gran Sasso. Neutrinos are detected via their elastic scattering on electrons in a large volume of liquid scintillator. The measured rate of scattered electrons above 3 MeV of energy is $0.223\substack{+0.015 \\ -0.016}\,(stat)\,\substack{+0.006 \\ -0.006}\,(syst)$ cpd/100 t, which corresponds to an observed solar neutrino flux assuming no neutrino flavor conversion of $\Phi\substack{\rm ES \\ ^8\rm B}=2.57\substack{+0.17 \\ -0.18}(stat)\substack{+0.07\\ -0.07}(syst)\times$10$^6$ cm$^{-2}\,$s$^{-1}$. This measurement exploits the active volume of the detector in almost its entirety for the first time, and takes advantage of a reduced radioactive background following the 2011 scintillator purification campaign and of novel analysis tools providing a more precise modeling of the background. Additionally, we set a new limit on the interaction rate of solar $hep$ neutrinos, searched via their elastic scattering on electrons as well as their neutral current-mediated inelastic scattering on carbon, $^{12}$C($\nu,\nu'$)$^{12}$C* ($E_{\gamma}$= 15.1 MeV).