On Resolving the Dark LMA Solution at Neutrino Oscillation Experiments

TL;DR

This paper proposes a method to distinguish the standard LMA and Dark LMA neutrino solutions by combining data from long-baseline and reactor experiments, especially if the neutrino mass hierarchy is known from non-oscillation experiments.

Contribution

It demonstrates that the degeneracy between LMA and DLMA solutions can be broken using combined measurements of oscillation probabilities and mass hierarchy determination from non-oscillation experiments.

Findings

Combined measurements can resolve the DLMA solution within a few years.

The $P_{ee}$ channel at reactor experiments is crucial for distinguishing solutions.

Mass hierarchy determination enhances the ability to resolve the degeneracy.

Abstract

In presence of non standard interactions, the solar neutrino data is consistent with two solutions, one close to the standard LMA solution with $\sin^2\theta_{12} \simeq 0.3$ and another with $\sin^2\theta_{12}^D \simeq 0.7(=\cos^2\theta_{12})$. The latter has been called the Dark LMA (DLMA) solution in the literature. This issue is hard to resolve via oscillations because of the existence of the so-called "generalised mass hierarchy" degeneracy of the neutrino mass matrix in presence of NSI. However, if the mass hierarchy is independently determined in a non-oscillation experiment such as neutrino-less double beta decay, the invariance of neutrino oscillation probabilities under $\sin^2\theta_{12} \leftrightarrow \cos^2\theta_{12}$ is lost and the possibility of resolving the LMA vs DLMA opens up. We point out that the $P_{\mu\mu}$ channel can distinguish $\theta_{12}$ from $\theta_{12}^D$ and study the corresponding difference in long-baseline experiments. We show that a key ingredient required is the input from the $P_{ee}$ channel measured at a reactor experiment. We find that if the mass hierarchy is determined by neutrino-less double beta decay, then a combined measurement of the effective mass squared differences in long-baseline experiments such as T2HK and DUNE and reactor experiment such as JUNO can resolve the DLMA conundrum to better than $3\sigma$ within 1 year for T2HK and little more than 3 years for DUNE.