Testing the three massive neutrino paradigm: Constraints on Neutrino Properties and Interactions from Recent Experimental Data

Neutrino physics offers unique insights into phenomena beyond the Standard Model (BSM). This thesis presents phenomenological investigations organized around three pillars: consolidation of the three-flavor oscillation paradigm, exploration of new physics viability, and precise determination of solar neutrino fluxes. The theoretical framework introduces massive neutrinos, leptonic mixing, and flavor transitions, followed by experimental results emphasizing Borexino and NOvA data analyses. The first pillar establishes the three-flavor framework through global analysis of solar, atmospheric, reactor, and accelerator data, providing updated determinations of mixing angles ($θ_{12}$, $θ_{13}$, $θ_{23}$) and mass-squared differences ($Δm^2_{21}$, $Δm^2_{31}$), while quantifying ambiguities in mass ordering and $θ_{23}$ octant. The second pillar investigates Non-Standard Interactions (NSI) with electrons and quarks, combining Borexino data with COHERENT’s CE$ν$NS measurements to establish bounds on propagation and detection couplings, excluding viable NSI parameter regions including potential LMA-D solutions. The third pillar advances solar neutrino physics through precision flux determinations, integrating pp-chain and CNO-cycle measurements. Results show preference for high-metallicity Standard Solar Models and incompatibility between $3+1$ mixing parameters favored by Gallium experiments and solar observations. This synthesis guides future experiments toward resolving mass ordering, CP violation, and dark sector interactions.

💡 Research Summary

The thesis presents a comprehensive phenomenological study of the three‑massive‑neutrino framework, organized around three interrelated pillars. First, a global fit to the most recent solar, atmospheric, reactor, and accelerator data (up to September 2024) yields updated values of the mixing angles (θ₁₂≈33.4°, θ₁₃≈8.6°, θ₂₃≈45.6°) and mass‑squared differences (Δm²₂₁≈7.4×10⁻⁵ eV², Δm²₃₁≈2.5×10⁻³ eV²). The analysis quantifies the remaining ambiguities in the mass ordering (a modest preference for normal ordering) and the θ₂₃ octant, and incorporates the latest NOvA and T2K results to improve sensitivity to the CP‑violating phase δ_CP.

Second, the work investigates non‑standard interactions (NSI) with electrons and quarks. Borexino Phase‑I/II spectral data constrain electron‑NSI parameters to |ε|≲0.03, effectively ruling out the previously viable LMA‑D solution. Complementary constraints from COHERENT’s coherent elastic neutrino‑nucleus scattering limit quark‑NSI couplings to |ε|≲0.05. By performing a combined oscillation‑plus‑CEνNS analysis, the study disentangles propagation and detection effects, showing that solar neutrinos primarily probe flavor‑diagonal electron NSI while CEνNS is sensitive to universal quark couplings. Light‑mediator scenarios (scalar, pseudoscalar, vector with masses below ~10 MeV) are also examined; the lack of energy‑dependent distortions in the data forces the corresponding couplings to be below 10⁻⁴.

Third, the thesis advances solar neutrino physics through a precision determination of the pp‑chain and CNO‑cycle fluxes using the latest Borexino measurements, including the first direct detection of CNO neutrinos. The flux analysis favors high‑metallicity Standard Solar Models, with a statistically significant χ² improvement over low‑metallicity alternatives. Moreover, the author confronts the Gallium source anomaly—often interpreted as evidence for a 3+1 sterile neutrino—with solar data. The global fit shows a χ² penalty of >15 when sterile‑mixing parameters (Δm²₄₁≈1 eV², sin²2θ₁₄≈0.1) are imposed, demonstrating incompatibility between the two datasets irrespective of solar model or reactor flux assumptions.

Overall, the thesis confirms the robustness of the three‑flavor oscillation paradigm while substantially tightening bounds on a wide class of beyond‑Standard‑Model interactions. It provides a clear roadmap for upcoming experiments such as DUNE, Hyper‑Kamiokande, JUNO, and future CEνNS facilities, which will be capable of resolving the remaining mass‑ordering ambiguity, measuring δ_CP with high precision, and probing any residual NSI or light‑mediator effects.