NOvA's Current and Future Sterile Neutrino Searches

The NOvA experiment’s most recent search for eV-scale sterile neutrinos under a 3+1 model simultaneously analyses muon neutrino and neutral current datasets from the NuMI beam at its Near ($\sim$\qty{1}{km} baseline) and Far (\qty{810}{km} baseline) detectors to look for oscillations consistent with a sterile neutrino. The analysis is systematically limited in the region of parameter space where $Δm^2_{41} \gtrsim 1~\mathrm{eV}^2$. This region of parameter space is preferred by sterile neutrino interpretations of current experimental anomalies and so improving sensitivity here is high-priority. These proceedings present our current search strategy, and discusses future plans to include data from a second beamline, the Booster Neutrino Beam, to improve our sensitivity in systematics-dominated regions of parameter space.

💡 Research Summary

The NOvA experiment has performed its most recent sterile‑neutrino search within the 3 + 1 framework, simultaneously analysing muon‑neutrino charged‑current (νμ CC) and neutral‑current (NC) samples from both the Near Detector (≈ 1 km) and the Far Detector (≈ 810 km) of the NuMI beam. By fitting four samples (ND νμ CC, ND NC, FD νμ CC, FD NC) together, the analysis extracts constraints on the new mixing angles θ₁₄, θ₂₄, θ₃₄ and the effective mixing sin²θ_{μτ}=sin²2θ₂₄·sin²θ₃₄ across a wide mass‑splitting range (10⁻³ – 10² eV²). The resulting 90 % confidence limits on sin²θ₂₄, sin²θ₃₄ and sin²θ_{μτ} are world‑leading, especially in the region Δm²₄₁ ≳ 1 eV² where the measurement is dominated by systematic uncertainties rather than statistics.

In the high‑Δm² regime the Near Detector contains hundreds of thousands to millions of events, but the sensitivity is limited by flux, cross‑section and detector‑response systematics. To break this limitation NOvA proposes to incorporate data from the Booster Neutrino Beam (BNB), which reaches the Near Detector off‑axis at an angle of about 160 mrad (≈ 9.2°) and a baseline of 770 m. The BNB flux, supplied by MiniBooNE simulations, exhibits two distinct energy peaks: a low‑energy π‑decay component around 200 MeV and a higher‑energy K‑decay component near 1.4 GeV. Although the L/E values of the BNB sample are comparable to those of the NuMI beam, the different neutrino energies provide an independent probe of the same oscillation phase, allowing systematic effects that are largely energy‑independent to be disentangled from genuine sterile‑neutrino oscillations.

A dedicated BNB trigger has been running since 2015, and a timing analysis confirms a clear BNB signal within the NOvA readout window. Simulations predict roughly 5 000 νμ CC events (≈ 95 % purity) for an exposure equivalent to 2.5 × 10²¹ POT, with about 35 % of them fully contained. Current reconstruction efficiency drops sharply below 500 MeV, but a re‑training of the convolutional neural network on low‑energy topologies is expected to raise the selection efficiency for the π‑decay peak.

Sensitivity studies using Asimov datasets and Wilks’ theorem show that the BNB sample alone matches the NuMI neutrino‑mode (FHC) and antineutrino‑mode (RHC) sensitivities for log₁₀Δm²₄₁ ≲ 0.5. When combined with the full NuMI dataset, the inclusion of BNB data improves the overall 90 % CL exclusion reach by roughly 30 % in certain Δm² regions, even before any optimisation of the BNB selection. The short 50 m BNB decay pipe also yields sharper oscillation features, further aiding discrimination of sterile‑induced effects from systematic variations.

Future work will double the NuMI neutrino‑mode exposure, add the antineutrino‑mode data, and incorporate a ν‑e scattering sample to provide an in‑situ flux constraint. Parallel improvements to low‑energy reconstruction and BNB‑specific event selection are planned to boost statistics and reduce systematic uncertainties. By leveraging the complementary energy spectra of NuMI and BNB, NOvA aims to overcome the current systematic ceiling, maintain its leadership in sterile‑neutrino searches, and contribute valuable, independent information to the global effort to resolve the existing experimental tensions.