Day-Night and Energy Dependence of MSW Solar Neutrinos for Maximal Mixing

TL;DR

This paper investigates how maximal mixing angles in solar neutrinos can still produce observable day-night effects and energy dependence due to earth regeneration, challenging previous assumptions of no such effects.

Contribution

It demonstrates that even with maximal mixing, earth regeneration effects cause energy-dependent day-night asymmetries, providing a new way to distinguish neutrino oscillation parameters.

Findings

Earth regeneration leads to day-night effects at maximal mixing.

Energy dependence of asymmetry varies with  m^2.

Potential to differentiate large mixing angle solutions.

Abstract

It has been stated in the literature that the case of maximal mixing angle for \nu_e leads to no day-night effect for solar neutrinos and an energy independent flux suppression of 1/2. While the case of maximal mixing angle and \Delta m^2 in the MSW range does lead to suppression of the electron neutrinos reaching the earth from the sun by P_S=1/2, the situation is different for neutrinos that have passed through the earth. We make the pedagogical point that, just as with smaller mixing angles, the earth regenerates the |\nu_1> state from the predominantly |\nu_2 > state reaching the earth, leading to coherent interference effects. This regeneration can lead to a day-night effect and an energy dependence of the suppression of solar electron neutrinos, even for the case of maximal mixing. For large mixing angles, the energy dependence of the day-night asymmetry depends heavily on Delta m^2. With a sufficiently sensitive measurement of the day-night effect, this energy dependence could be used to distinguish among the large mixing angle solutions of the solar neutrino problem.