Stringent constraint on CPT violation with the synergy of T2K-II, NOνA extension, and JUNO

Abstract

Neutrino oscillation experiments have measured precisely at few percent levels the mass-squared differences ($\mathrm{\ensuremath{\Delta}}{m}_{21}^{2}$, $\mathrm{\ensuremath{\Delta}}{m}_{31}^{2}$) of three neutrino mass eigenstates, and the three leptonic mixing angles (${\ensuremath{\theta}}_{12}$, ${\ensuremath{\theta}}_{13}$, ${\ensuremath{\theta}}_{23}$) by utilizing both neutrino and antineutrino oscillations. The possible $CPT$ violation may manifest itself in the difference of neutrino and antineutrino oscillation parameters, making these experiments promising tools for testing $CPT$ invariance at unprecedented precision. We investigate empirically the sensitivity of the $CPT$ test via the difference in mass-squared splittings ($\mathrm{\ensuremath{\Delta}}{m}_{31}^{2}\ensuremath{-}\mathrm{\ensuremath{\Delta}}{\overline{m}}_{31}^{2}$) and in leptonic mixing angles (${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{23}\ensuremath{-}{\mathrm{sin}}^{2}{\overline{\ensuremath{\theta}}}_{23}$) with the synergy of T2K-II, $\mathrm{NO}\ensuremath{\nu}\mathrm{A}$ extension, and JUNO experiments. If the $CPT$ symmetry is found to be conserved, the joint analysis of the three experiments will be able to establish limits of $|\mathrm{\ensuremath{\Delta}}{m}_{31}^{2}\ensuremath{-}\mathrm{\ensuremath{\Delta}}{\overline{m}}_{31}^{2}|<5.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }{\mathrm{eV}}^{2}$ and $|{\mathrm{sin}}^{2}{\ensuremath{\theta}}_{23}\ensuremath{-}{\mathrm{sin}}^{2}{\overline{\ensuremath{\theta}}}_{23}|<0.10$ at $3\ensuremath{\sigma}$ confidence level (CL) on the possible $CPT$ violation, extending substantially the current bound of these parameters. We find that with ($\mathrm{\ensuremath{\Delta}}{m}_{31}^{2}\ensuremath{-}\mathrm{\ensuremath{\Delta}}{\overline{m}}_{31}^{2}$), the dependence of the statistical significance on the relevant parameters to exclude the $CPT$ conservation is marginal, and that, if the difference in the best-fit values of $\mathrm{\ensuremath{\Delta}}{m}_{31}^{2}$ and $\mathrm{\ensuremath{\Delta}}{\overline{m}}_{31}^{2}$ measured by $\mathrm{MINOS}(+)$ and $\mathrm{NO}\ensuremath{\nu}\mathrm{A}$ persists as the true, the combined analysis will rule out the $CPT$ conservation at $4\ensuremath{\sigma}$ CL. With the (${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{23}\ensuremath{-}{\mathrm{sin}}^{2}{\overline{\ensuremath{\theta}}}_{23}$), the statistical significance to exclude $CPT$ invariance depends strongly on the true value of ${\ensuremath{\theta}}_{23}({\overline{\ensuremath{\theta}}}_{23})$ mixing angle. In the case of maximal mixing of ${\ensuremath{\theta}}_{23}$, as indicated by the current T2K and $\mathrm{NO}\ensuremath{\nu}\mathrm{A}$ measurements, the $CPT$ conservation will be excluded at $3\ensuremath{\sigma}$ CL or higher if the difference in the best-fit values of ${\ensuremath{\theta}}_{23}$ and ${\overline{\ensuremath{\theta}}}_{23}$ remains as the true.