Update on non-unitary mixing in the recent NO$ν$A and T2K data

TL;DR

This work tests the non-unitary mixing hypothesis using the latest NOνA and T2K data, both individually and in combination, by varying one non-unitary parameter at a time and examining its impact on the appearance probabilities $P_{\mu e}$ and $P_{\bar{\mu} \bar{e}}$. The framework $N = N_{NP} U_{PMNS}$ with $α_{ii}$ real and $α_{ij} = |α_{ij}| e^{i φ_{ij}}$ is employed, focusing on $α_{00}$, $α_{10}$, and $α_{11}$ as the dominant sources of non-unitarity. The results show a preference for unitary mixing for $α_{00}$ and $α_{11}$ with improved but global-weak limits, while a non-zero $|α_{10}|$ around 0.06 can partially reconcile NOνA and T2K tensions in normal ordering, though this value is disfavored by global fits. Future measurements including DUNE significantly enhance sensitivity, potentially tightening constraints on all three parameters and aiding hierarchy resolution in the presence of non-unitarity.

Abstract

In this paper, we have tested the non-unitary mixing hypothesis with the latest data from NO$ν$A and T2K experiments. We have also analysed their combined data. We have provided the best-fit values of the standard and non standard parameters after the analysis. $90\%$ limits on the non-unitary mixing parameters have also been provided. The constraints on unitary violation is stronger, compared to the constraints obtained from previous data from NO$ν$A and T2K. The tension between NO$ν$A and T2K at the $1\,σ$ for normal mass hierarchy can be reduced for non-unitary mixing due to $α_{10}$, albeit for a value of $|α_{10}|$ larger than the present global $90\%$ limit. Additionally a study of the future sensitivity of NO$ν$A, T2K and DUNE has been provided.

Figures (7)

• Figure 1: Allowed regions in the $\sin^2{\theta_{23}}-\delta_{\mathrm{CP}}$ plane for NO$\nu$A and T2K after analysing the data with non-unitary mixing with $\alpha_{00}$ ($\alpha_{10}$) in the upper (lower) panel. The left (right) panel is for NH (IH). The red (blue) line indicates NO$\nu$A (T2K), and the black line indicates the combined data. The solid (dotted) lines indicate the boundaries of the $1\,\sigma$ ($3\,\sigma$) allowed regions.
• Figure 2: $\Delta\chi^2$ as a function of individual non-unitary parameters for 2024 long baseline data.
• Figure 3: Sensitivity of $|\alpha_{10}|$, $\alpha_{00}$ and $\alpha_{11}$ assuming $\alpha_{10}$ as the true parameter for future combination of NO$\nu$A and T2K, and DUNE.
• Figure 4: Allowed regions in the $\sin^2{\theta_{23}}-\delta_{\mathrm{CP}}$ plane for NO$\nu$A and T2K after analysing the data with standard unitary mixing. The left (right) panel is for NH (IH). The red (blue) line indicates NO$\nu$A (T2K), and the black line indicates the combined data. The solid (dotted) lines indicate the boundaries of the $1\,\sigma$ ($3\,\sigma$) allowed regions.
• Figure 5: $\nu_\mu \to \nu_e$ (left panel) and $\bar{\nu}_\mu \to \bar{\nu}_e$ (right panel) oscillation probability as a function of energy with different hierarchy-$\delta_{\mathrm{CP}}$ combinations for standard oscillation and non-unitary mixing due to $\alpha_{10}$ for the NO$\nu$A experiment. The oscillation parameter values including $|\alpha_{10}|$ are fixed to the combined best-fit values of NO$\nu$A and T2K. For NH (IH), $\phi_{10}=120^\circ$ ($60^\circ$). The left (right) panels are for neutrino (anti-neutrino) probabilities, and the top (bottom) panels are for ${\theta_{23}}$ in HO (LO). For HO (LO), we have used $\sin^2{\theta_{23}}=0.57$ (0.43).