Testing solar neutrino MSW oscillations at low delta m^2 through time variations of event rates in GNO and BOREXINO

TL;DR

This paper investigates low delta m^2 MSW neutrino oscillations by analyzing seasonal variations in GNO and day-night effects in BOREXINO, providing detailed calculations and analytical expressions for expected neutrino rate differences.

Contribution

It demonstrates that seasonal GNO rate differences and day-night BOREXINO variations can serve as signatures of low delta m^2 MSW oscillations, with detailed quantitative predictions.

Findings

Winter-summer GNO rate difference of 4-6 SNU for low delta m^2

Significant Earth regeneration effects observable in seasonal and day-night variations

Analytical expressions for solar exposure functions at Gran Sasso

Abstract

The Mikheyev-Smirnov-Wolfenstein (MSW) explanation of the solar neutrino problem is currently compatible with three distinct regions of the two-neutrino oscillation parameter space (delta m^2,sin^2 2theta). We focus on the region with the lowest value of delta m^2 (~10^{-7} eV^2), which implies significant Earth regeneration effects for low-energy solar neutrinos. We point out that such effects are not only observable as day-night variations of neutrino event rates in the real-time BOREXINO experiment, but also as seasonal variations in the radiochemical Gallium Neutrino Observatory (GNO) at Gran Sasso. We present detailed calculations of the difference between winter and summer rates in GNO (six months averages) in excess of the trivial seasonal variation due to the Earth orbital eccentricity. We show that, within the low-delta m^2 MSW solution, the net winter-summer GNO rate difference amounts to 4-6 SNU, with a dominant contribution from pp neutrinos. We also give analytical expressions for the winter and summer solar exposure functions at the Gran Sasso site.