After the landmark discovery of non-zero $$\theta _{13}$$θ13 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 $$\theta _{23}-\delta _{\textrm{CP}}$$θ23-δCP degeneracy. We find that none of these experiments individually can achieve the milestone of 3 $$\sigma $$σ LCPV for at least 75% choices of $$\delta _{\textrm{CP}}$$δCP 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 $$\theta _{23}$$θ23 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 $$\delta _{\textrm{CP}}$$δCP for $$\ge $$≥ 3 $$\sigma $$σ LCPV depends on the choice of $$\theta _{23}$$θ23 , 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 $$\theta _{23}$$θ23 . 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.

Discovering new neutrino interactions would represent evidence of physics beyond the Standard Model. We focus on new flavor-dependent long-range neutrino interactions mediated by ultra-light mediators, with masses below 10⁻¹⁰ eV, introduced by new lepton-number gauge symmetries L_e – L_μ, L_e – L_τ, and L_μ – L_τ. Because the interaction range is ultra-long, nearby and distant matter — primarily electrons and neutrons — in the Earth, Moon, Sun, Milky Way, and the local Universe, may source a large matter potential that modifies neutrino oscillation probabilities. The upcoming Deep Underground Neutrino Experiment (DUNE) and the Tokai-to-Hyper-Kamiokande (T2HK) long-baseline neutrino experiments will provide an opportunity to search for these interactions, thanks to their high event rates and well-characterized neutrino beams. We forecast their probing power. Our results reveal novel perspectives. Alone, DUNE and T2HK may strongly constrain long-range interactions, setting new limits on their coupling strength for mediators lighter than 10⁻¹⁸ eV. However, if the new interactions are subdominant, then both DUNE and T2HK, together, will be needed to discover them, since their combination lifts parameter degeneracies that weaken their individual sensitivity. DUNE and T2HK, especially when combined, provide a valuable opportunity to explore physics beyond the Standard Model.

Recent global fit analyses of 3ν oscillation data show a preference for normal mass ordering (NMO) at 2.5σ and provide 1.6σ indications for lower θ₂₃ octant (sin²θ₂₃< 0.5) and leptonic CP violation (sin δ_CP< 0). In this work, we study in detail the capabilities of DUNE to establish the deviation from maximal θ₂₃ and to resolve its octant in light of the current data. Introducing for the first time, a bi-events plot in the plane of total ν and $$ \overline{\nu} $$ν ¯ disappearance events, we discuss the impact of sin²θ₂₃ – ∆ $$ {m}_{31}^2 $$m 31 2 degeneracy in establishing non-maximal θ₂₃ and show how this degeneracy can be resolved with the help of spectral analysis. A 3σ (5σ) determination of non-maximal θ₂₃ is possible in DUNE with an exposure of 336 kt·MW·years if the true value of sin²θ₂₃ ≲ 0.465 (0.450) or sin²θ₂₃ ≳ 0.554 (0.572). We study the role of appearance and disappearance channels, systematic uncertainties, marginalization over oscillation parameters, and the importance of spectral analysis in establishing non-maximal θ₂₃. We observe that both ν and $$ \overline{\nu} $$ν ¯ data are essential to settle the θ₂₃ octant at a high confidence level. DUNE can resolve the octant of θ₂₃ at 4.2σ (5σ) using 336 (480) kt·MW·years of exposure for the present best-fit values of oscillation parameters. DUNE can improve the current relative 1σ precision on sin²θ₂₃ (∆ $$ {m}_{31}^2 $$m 31 2 ) by a factor of 4.4 (2.8) using 336 kt·MW·years of exposure.

Upcoming neutrino experiments will soon search for new neutrino interactions more thoroughly than ever before, boosting the prospects of extending the Standard Model. In anticipation of this, we forecast the capability of two of the leading long-baseline neutrino oscillation experiments, DUNE and T2HK, to look for new flavor-dependent neutrino interactions with electrons, protons, and neutrons that could affect the transitions between different flavors. We interpret their sensitivity in the context of long-range neutrino interactions, mediated by a new neutral boson lighter than 10⁻¹⁰ eV, and sourced by the vast amount of nearby and distant matter in the Earth, Moon, Sun, Milky Way, and beyond. For the first time, we explore the sensitivity of DUNE and T2HK to a wide variety of U(1) ^′ symmetries, built from combinations of lepton and baryon numbers, each of which induces new interactions that affect oscillations differently. We find ample sensitivity: in all cases, DUNE and T2HK may constrain the existence of the new interaction even if it is supremely feeble, may discover it, and, in some cases, may identify the symmetry responsible for it.

The Earth's matter effect is going to play a crucial role in measuring the unknown three-flavor neutrino oscillation parameters at high confidence level in future high-precision long-baseline experiments. We observe that owing to the new degeneracies among the most uncertain oscillation parameters $\left(\delta_{\mathrm{CP}},~\theta_{23}\right)$ and the average Earth's matter density $\left(\rho_{\mathrm{avg}}\right)$ for the 1300 km baseline, the sensitivity of the upcoming Deep Underground Neutrino Experiment (DUNE) to establish Earth's matter effect reaches only about 2$\sigma$ C.L. for all possible choices of oscillation parameters. We notice that the current uncertainty in $\delta_{\mathrm{CP}}$ degrades the measurement of $\rho_{\mathrm{avg}}$ more as compared to $\theta_{23}$. To lift these degeneracies, we explore the possible complementarity between DUNE and Tokai to Hyper-Kamiokande (T2HK/JD) facility with a second detector in Korea, popularly known as T2HKK or JD+KD setup. While DUNE uses wide-band beam with on-axis detector, T2HKK setup plans to use narrow-band beam with two off-axis detectors: one in Japan and other in Korea. We exhibit how the high-precision measurement of $\delta_{\mathrm{CP}}$ in JD+KD setup and the information on $\rho_{\mathrm{avg}}$ coming from DUNE can reduce the impact of these degeneracies in both $(\rho_{\mathrm{avg}}-\delta_{\mathrm{CP}})$ and $(\rho_{\mathrm{avg}}-\theta_{23})$ planes. We show that the combined data from DUNE and JD+KD setups can establish Earth’s matter effect at more than 6$\sigma$ C.L. irrespective of both the choices of mass hierarchy: normal (NH) and inverted (IH), $\delta_{\mathrm{CP}}$, and $\theta_{23}$. With the help of this combined data set, we can measure the average matter density $\left(\rho_{\mathrm{avg}}\right)$ with a relative 1$\sigma$ precision of around 11.2\% (9.4\%) assuming true NH (IH) and $\delta_{\mathrm{CP}} = -90^{\circ}/90^{\circ}$.

A high-precision measurement of $$ \Delta {m}_{31}^2 $$∆ m 31 2 and θ₂₃ is inevitable to estimate the Earth’s matter effect in long-baseline experiments which in turn plays an important role in addressing the issue of neutrino mass ordering and to measure the value of CP phase in 3ν framework. After reviewing the results from the past and present experiments, and discussing the near-future sensitivities from the IceCube Upgrade and KM3NeT/ORCA, we study the expected improvements in the precision of 2–3 oscillation parameters that the next-generation long-baseline experiments, DUNE and T2HK, can bring either in isolation or combination. We highlight the relevance of the possible complementarities between these two experiments in obtaining the improved sensitivities in determining the deviation from maximal mixing of θ₂₃, excluding the wrong-octant solution of θ₂₃, and obtaining high precision on 2–3 oscillation parameters, as compared to their individual performances. We observe that for the current best-fit values of the oscillation parameters and assuming normal mass ordering (NMO), DUNE + T2HK can establish the non-maximal θ₂₃ and exclude the wrong octant solution of θ₂₃ at around 7σ C.L. with their nominal exposures. We find that DUNE + T2HK can improve the current relative 1σ precision on sin²θ₂₃ $$ \left(\Delta {m}_{31}^2\right) $$∆ m 31 2 by a factor of 7 (5) assuming NMO. Also, we notice that with less than half of their nominal exposures, the combination of DUNE and T2HK can achieve the sensitivities that are expected from these individual experiments using their full exposures. We also portray how the synergy between DUNE and T2HK can provide better constraints on (sin²θ₂₃–δ_CP) plane as compared to their individual reach.