Disentangling genuine from matter-induced CP violation in neutrino oscillations

TL;DR

This paper demonstrates that neutrino oscillation CP asymmetries in matter can be separated into matter-induced and genuine CP violation components, enabling clearer analysis of CP violation and matter effects in long-baseline experiments.

Contribution

It provides a theoretical framework to disentangle matter effects from genuine CP violation in neutrino oscillations, identifying observable signatures and a magic energy for optimal measurement.

Findings

Two distinct CP asymmetry components identified: CPT-odd T-invariant and T-odd CPT-invariant.

A magic energy at 0.91 GeV at 1300 km baseline where matter effects vanish and genuine CP violation peaks.

Energy-dependent signatures allow discrimination of neutrino mass hierarchy.

Abstract

We prove that, in any flavor transition, neutrino oscillation CP violating asymmetries in matter have two disentangled components: i) a CPT-odd T-invariant term, non-vanishing iff there are interactions with matter; ii) a T-odd CPT-invariant term, non-vanishing iff there is genuine CP violation. As function of the baseline, these two terms are distinct L-even and L-odd observables to separately test (i) matter effects sensitive to the neutrino hierarchy and (ii) genuine CP violation in the neutrino sector. For the golden $\nu_\mu \to \nu_e$ channel, the different energy distributions of the two components provide a signature of their separation. At long baselines, they show oscillations in the low energy region, with zeros at different positions and peculiar behavior around the zeros. We discover a magic energy $E = (0.91 \pm 0.01)$ GeV at L = 1300 km with vanishing CPT-odd term and maximal genuine CP asymmetry proportional to $\sin\delta$, with $\delta$ the weak CP phase. For energies above 1.5 GeV, the sign of the CP asymmetry discriminates the neutrino hierarchy.