Evidence for leptonic CP phase from NOνA, T2K and ICAL

Abstract

The phenomenon of neutrino oscillation is now well understood from the solar, atmospheric, reactor and accelerator neutrino experiments. This oscillation is characterized by a unitary PMNS matrix which is parametrized by three mixing angles (θ₁₂, θ₂₃ and θ₁₃) and one phase (δ_CP) known as the leptonic CP phase. Neutrino oscillation also involves two mass squared differences: the solar mass square difference (Δ₂₁ = m²₂ − m²₁ and the atmospheric mass square difference (Δ₃₁ = |m²₃ − m²₁|). Though there is already significant amount of information about the three mixing angles, the CP phase is still unknown. Apart from the CP phase, one should also know what is the true nature of the neutrino mass hierarchy, i.e., normal (m₃ > m₁: NH) or inverted (m₁ > m₃: IH) and what is the true octant of θ₂₃, i.e., lower (θ₂₃ < 45∘: LO) or higher (θ₂₃ > 45^∘: HO). The long-baseline experiments (LBL) have CP sensitivity coming from the appearance channel (ν_μ → ν_e). On the other hand, atmospheric neutrinos are known to have negligible CP sensitivity. In this work, we study the synergy between the LBL experiment NOνA, T2K and the atmospheric neutrino experiment ICAL@INO for obtaining the first hint of CP violation in the lepton sector. We find that due to the lack of knowledge of hierarchy and octant, CP sensitivity of NOνA/T2K is poorer for some parameter ranges. Addition of ICAL data to T2K and NOνA can exclude these spurious wrong-hierarchy and /or wrong-octant solutions and cause a significant increase in the range of δ_CP values for which a hint of CP violation can be achieved. Similarly, the precision with which δ_CP can be measured also improves with the inclusion of ICAL data.