When CPT Violation Hides in Plain Sight: How CP Measurements Are Compromised and How to Fix Them

Abstract

The extraction of the leptonic charge-parity (CP)-violating phase $δ_{\rm CP}$ from long-baseline neutrino oscillation experiments rests on the assumption of charge-parity-time (CPT) conservation. We show that CPT violation, parametrized as an asymmetry $δΔm^2_{31} \equiv Δ\bar{m}^2_{31} - Δm^2_{31}$ between neutrino and antineutrino mass splittings, induces an effective, energy-dependent phase shift $ϕ_{\rm eff}(E)$ that is functionally degenerate with $δ_{\rm CP}$ in the appearance asymmetry $\langleΔP\rangle$. This has a profound implication for long-baseline experiments, where the tension between T2K and NO$ν$A CPT-conserving best-fit $δ_{\rm CP}$ values can be significantly alleviated by a CPT-violating truth; and a CPT-conserving fit can miss the true CP phase entirely for $|δΔm^2_{31}|\gtrsim 0.3\times10^{-3}~\text{eV}^2$ for DUNE. We then demonstrate that atmospheric neutrino telescopes provide the natural tool to resolve this degeneracy: using existing data from IceCube-DeepCore (7.74 yr) and KM3NeT/ORCA-6 (433 kt-yr), we derive a world-leading constraint on CPT-violation at $| δΔm^2_{31}|\leq 0.57\times10^{-3}~\text{eV}^2$ at 90% CL. With the IceCube Upgrade and full ORCA detector, we can reach a $1σ$ constraint at $10^{-4}~\text{eV}^2$ within a decade, providing the independent CPT constraint needed to ensure that DUNE's $δ_{\rm CP}$ measurement is unambiguous.