Absence of day-night asymmetry of 862 keV Be-7 solar neutrino rate in Borexino and MSW oscillation parameters

TL;DR

This study measures the Be-7 solar neutrino rate during day and night with Borexino, finding no significant asymmetry, which strongly supports the LMA-MSW neutrino oscillation solution over alternative models.

Contribution

It provides the first precise measurement of day-night asymmetry for Be-7 neutrinos, strongly constraining neutrino oscillation parameters without assuming CPT symmetry.

Findings

No significant day-night asymmetry detected (Adn≈0).

Results exclude the LOW oscillation solution at over 8.5 sigma.

Data favor the LMA-MSW solution at high confidence.

Abstract

We report on a search for the day-night asymmetry of the Be-7 solar neutrino rate measured by Borexino at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. The measured value, Adn=0.001 +- 0.012 (stat) +- 0.007 (syst), shows the absence of a significant asymmetry. This result alone rejects the so-called LOW solution at more than 8.5 sigma. Combined with the other solar neutrino data, it isolates the Large Mixing Angle (LMA) -- MSW solution at DeltaChi2 > 190 without relying on the assumption of CPT symmetry in the neutrino sector. We also show that including the day-night asymmetry, data from Borexino alone restricts the MSW neutrino oscillations to the LMA solution at 90% confidence level.