Probing Quasi-Dirac Neutrino Oscillations at Long Baseline Experiments
The Dirac or Majorana nature of neutrinos remains one of the most fundamental open problems in particle physics. A natural intermediate scenario arises when small lepton-number-violating Majorana mass terms break the exact Dirac symmetry. These cause mass eigenstates to form nearly degenerate pairs, each separated by a small mass splitting. We study this quasi-Dirac scenario within a five-neutrino framework, extending the Standard Model by two right-handed neutrinos, which form a nearly degenerate sterile pair alongside the three active states. This extended mixing structure introduces additional phenomenological parameters, including new mixing angles, mass splittings, and CP phases. We use appearance and disappearance data from NO$\nu$A and T2K to constrain the active-sterile mixing angles $\theta_{sa}$ and the mass splitting within the sterile pair $\Delta m_{54}^2$ and forecast the sensitivity achievable at DUNE. We analyse how these parameters modify CP-violating observables relative to the standard three-neutrino case, comparing predicted event rates across both mass orderings and as a function of $\delta_{CP}$.