Enhancing sensitivity to leptonic CP violation using complementarity among DUNE, T2HK, and T2HKK

After the landmark discovery of non-zero $$\theta _{13}$$ θ 13 by the modern reactor experiments, unprecedented precision on neutrino mass-mixing parameters has been achieved over the past decade. This has set the stage for the discovery of leptonic CP violation (LCPV) at high confidence level in the next-generation long-baseline neutrino oscillation experiments. In this work, we explore in detail the possible complementarity among the on-axis DUNE and off-axis T2HK experiments to enhance the sensitivity to LCPV suppressing the $$\theta _{23}-\delta _{\textrm{CP}}$$ θ 23 - δ CP degeneracy. We find that none of these experiments individually can achieve the milestone of 3 $$\sigma $$ σ LCPV for at least 75% choices of $$\delta _{\textrm{CP}}$$ δ CP in its entire range of $$[-180^{\circ }, 180^{\circ }]$$ [ - 180 ∘ , 180 ∘ ] , with their nominal exposures and systematic uncertainties. However, their combination can attain the same for all values of $$\theta _{23}$$ θ 23 with only half of their nominal exposures. We observe that the proposed T2HKK setup in combination with DUNE can further increase the CP coverage to more than 80% with only half of their nominal exposures. We study in detail how the coverage in $$\delta _{\textrm{CP}}$$ δ CP for $$\ge $$ ≥ 3 $$\sigma $$ σ LCPV depends on the choice of $$\theta _{23}$$ θ 23 , exposure, optimal runtime in neutrino and antineutrino modes, and systematic uncertainties in these experiments in isolation and combination. We find that with an improved systematic uncertainty of 2.7% in appearance mode, the standalone T2HK setup can provide a CP coverage of around 75% for all values of $$\theta _{23}$$ θ 23 . We also discuss the pivotal role of intrinsic, extrinsic, and total CP asymmetries in the appearance channel and extrinsic CP asymmetries in the disappearance channel while analyzing our results.