The Sensitivity of DUNE in Presence of Off-Diagonal Scalar NSI Parameters

Scalar non-standard interactions (NSI) presents an exciting pathway for probing potential new physics that extends beyond the Standard Model (BSM). The scalar coupling of neutrinos with matter can appear as a sub-dominant effect that can impact the neutrino oscillation probabilities. The uniqueness of these interactions is that it can directly affect the neutrino mass matrix. This makes oscillations sensitive to the absolute neutrino mass. The effects of scalar NSI scales linearly with matter density which motivates its exploration in long-baseline sector. The presence of scalar NSI can influence the key measurements in the field of neutrino physics, including the precise determination of the leptonic CP phase ($δ_{CP}$), neutrino mass ordering and the octant of $θ_{23}$. The precise determination of $δ_{CP}$ is one of the major goals of DUNE, which is an upcoming long-baseline experiment. A better understanding of the impact of scalar NSI on CP measurement sensitivities is crucial for accurate interpretation of $δ_{CP}$ phase. In this work, we have explored the impact of the complex off-diagonal scalar NSI elements $η_{αβ}$ and their associated phases $ϕ_{αβ}$ on the CP-measurement sensitivities at DUNE. We have explored the impact of the neutrino mass scale on these sensitivities. We look for constraining these off-diagonal elements for different neutrino mass scales. We also explore their correlation with $δ_{CP}$, investigating potential degeneracies that can arise due to additional phases. We also perform a correlation study among different scalar NSI elements. We show that the inclusion of the complex scalar NSI elements can significantly modify the CP phase measurements.

💡 Research Summary

The paper investigates how off‑diagonal scalar non‑standard interactions (NSI) affect the CP‑phase measurement capabilities of the Deep Underground Neutrino Experiment (DUNE). Scalar NSI differ from the more widely studied vector NSI because they modify the neutrino mass matrix directly, introducing a dependence on the absolute neutrino mass scale and scaling linearly with matter density. The authors parameterize the off‑diagonal scalar NSI as ηαβ = |ηαβ| e⁻ⁱϕαβ, where |ηαβ| are dimensionless strengths and ϕαβ are new CP‑violating phases. They fix the overall scaling factor Sm to |Δm²₃₁|, which is typical for atmospheric mass‑splitting, and explore a range of absolute mass scales (from minimal to ~0.1 eV).

Using the GLoBES framework, the study simulates DUNE with its planned 1300 km baseline, 40 kt liquid‑argon far detector, 1.1×10²¹ protons‑on‑target per year, and equal running time in neutrino and antineutrino modes (3.5 yr each). Systematic uncertainties are incorporated via the pull method (2 % signal, 5 % background). Event rates for νe appearance and νμ disappearance (and their antineutrino counterparts) are calculated, and χ² analyses are performed to compare the standard interaction hypothesis with scenarios that include scalar NSI.

Key findings include:

1. Probability‑level impact: Even modest values of |ηαβ| ≈ 10⁻³ produce noticeable distortions (≈5 % or more) in the νe appearance probability, especially when the associated phase ϕαβ interferes with the intrinsic Dirac CP phase δCP. This leads to a “δCP–ϕ” degeneracy, where different combinations of (δCP, ϕαβ) yield nearly identical oscillation spectra.

2. CP‑violation sensitivity: In the standard three‑flavor scenario, DUNE can discover CP violation at 5σ for roughly 50 % of possible δCP values. Introducing off‑diagonal scalar NSI can shrink this coverage to below 30 % for unfavorable phase choices, but can also expand it up to ~60 % if the new phases align constructively. Thus, scalar NSI can both degrade and, in special cases, enhance CP sensitivity.

3. Dependence on absolute mass scale: When the lightest neutrino mass is increased (e.g., m₁ ≈ 0.1 eV), the scalar‑NSI contribution scales up, tightening the 3σ bounds on |ηαβ| to the order of 10⁻³. For minimal mass scenarios the bounds are slightly weaker. This demonstrates that DUNE could indirectly probe the absolute neutrino mass through scalar NSI effects.

4. Correlations among η parameters: The analysis reveals significant correlations between ηeμ and ημτ, as well as between ηeτ and ημτ. These correlations generate additional degeneracies that can be partially lifted by combining appearance and disappearance channels and by using both neutrino and antineutrino data. The authors suggest that synergy with other long‑baseline experiments (e.g., T2HK, P2SO) would further improve constraints.

5. Implications for BSM physics: The presence of complex off‑diagonal scalar NSI introduces new sources of CP violation beyond the Dirac phase, complicating the interpretation of any observed CP‑violating signal. However, the distinct density‑dependent signature of scalar NSI offers a pathway to disentangle it from vector NSI or other new‑physics scenarios, especially when combined with cosmological or astrophysical observations.

In conclusion, the paper demonstrates that off‑diagonal scalar NSI constitute a non‑negligible systematic for DUNE’s primary goal of measuring δCP. While they can obscure the intrinsic CP signal, careful experimental design—leveraging multiple channels, balanced neutrino/antineutrino running, and external data—can mitigate degeneracies and even turn scalar NSI into a probe of absolute neutrino mass and new CP‑violating physics.