# Perturbing neutrino oscillations around the solar resonance

## TL;DR

This paper develops a perturbative approach to analyze low-energy atmospheric neutrino oscillations around the solar resonance, revealing that CP violation effects are significantly enhanced in this regime.

## Contribution

It introduces a novel perturbation theory framework that isolates solar-scale effects and shows that CP violation is much larger near the solar resonance than at atmospheric scales.

## Key findings

- CP violation in appearance probability is about 10 times larger near the solar resonance.
- The perturbation theory effectively separates solar and atmospheric scale effects.
- Higher order corrections decrease as the atmospheric mass-squared difference increases.

## Abstract

Atmospheric neutrinos at low energies, $E \lsim 500$ MeV, is known to be a rich source of information of lepton mixing parameters. We formulate a simple perturbative framework to elucidate the characteristic features of neutrino oscillation at around the solar-scale enhancement due to the matter effect. The clearest message we could extract from our perturbation theory is that CP violation in the appearance oscillation probability is large, a factor of $\sim 10$ times larger than CP violation at around the atmospheric-scale oscillation maximum. Underlying mechanism for it is that one of the suppression factors on the CP phase dependent terms due to smallness of $\Delta m^2_{21} / \Delta m^2_{31}$ are dynamically lifted by the solar-scale enhancement. Our framework has a unique feature as a perturbation theory in which large $\Delta m^2_{31}$ term outside the key 1-2 sector for the solar-scale resonance does not yield sizeable corrections. On the contrary, the larger the $\Delta m^2_{31}$, the smaller the higher order corrections.