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Non-standard neutrino interactions as a solution to the NO$ν$A and T2K discrepancy

The latest data of the two long-baseline accelerator experiments NO$ν$A and T2K, interpreted in the standard 3-flavor scenario, display a discrepancy. A mismatch in the determination of the standard CP-phase $δ_{\mathrm {CP}}$ extracted by the two experiments is evident in the normal neutrino mass ordering. While NO$ν$A prefers values close to $δ_{\mathrm {CP}} \sim 0.8 π$, T2K identifies values of $δ_{\mathrm {CP}} \sim 1.4 π$. Such two estimates are in disagreement at more than 90$\%$ C.L. for 2 degrees of freedom. We show that such a tension can be resolved if one hypothesizes the existence of complex neutral-current non-standard interactions (NSI) of the flavor changing type involving the $e-μ$ or the $e-τ$ sectors with couplings $|\varepsilon_{eμ}| \sim |\varepsilon_{eτ}|\sim 0.2$. Remarkably, in the presence of such NSI, both experiments point towards the same common value of the standard CP-phase $δ_{\mathrm {CP}} \sim 3π/2$. Our analysis also highlights an intriguing preference for maximal CP-violation in the non-standard sector with the NSI CP-phases having best fit close to $ϕ_{eμ} \sim ϕ_{eτ}\sim 3π/2$, hence pointing towards imaginary NSI couplings.