A LENS on DUNE-PRISM: Characterizing a Neutrino Beam with Off-Axis Measurements

TL;DR

The paper addresses how large systematic uncertainties in neutrino flux and cross sections limit precision in long-baseline oscillation experiments. It introduces LENS (Lateral Extraction of Neutrino Spectra) as a near-detector method to constrain the flux model using off-axis measurements, integrated with the PRISM framework to improve far-detector predictions. By decomposing the flux into five parent-meson components and fitting near-detector data across multiple off-axis angles, LENS can constrain flux normalizations to the percent level, reducing biases in oscillation fits and enhancing CP-violation sensitivity. The approach is data-driven, largely robust to cross-section systematics, and emphasizes the pivotal role of advanced near detectors for future neutrino oscillation programs, with potential gains from complementary external measurements and tagged-beam concepts.

Abstract

Upcoming precision long-baseline neutrino oscillation experiments will be severely limited by the large systematic uncertainties associated with neutrino flux predictions and neutrino--nucleus cross sections. A promising remedy is the PRISM (Precision Reaction Independent Spectrum Measurement) technique, whereby the near detector measures the neutrino spectrum at different angles with respect to the beam axis. These measurements are then linearly combined into a prediction of the oscillated neutrino flux at the far detector. This prediction is data-driven, but still dependent on some theoretical knowledge about the neutrino flux. In this paper, we study to what extent off-axis measurements themselves can be used to directly constrain neutrino flux models. In particular, we use them to extract separately the fluxes and spectra of different meson species in the beam. We call this measurement LENS (Lateral Extraction of Neutrino Spectra). Second, we demonstrate how the thus improved flux model helps to further constrain the far detector flux prediction, thereby ultimately improving oscillation measurements.

Figures (6)

• Figure 1: Neutrino flux in the DUNE/LBNF neutrino-dominated beam at different off-axis angles (left). For the on-axis ($0^\circ$) and maximally off-axis ($3.59^\circ$) positions, we also show separately the different flux components corresponding to different parent particles (right).
• Figure 2: Projected constraints on the individual components of the DUNE neutrino flux from a fit to PRISM data in forward horn current (FHC) mode (orange) and reverse horn current (RHC) mode (blue), assuming half of the total 6.6e21pot are collected in the on-axis position, while the rest are equally split over six different off-axis positions (6.3m, 12.6m, 18.9m, 24.3m, 30.6m, 36.0m, corresponding to off-axis angles out to 0.063 radians).
• Figure 3: The correlation matrix obtained from the fit of our neutrino flux model to simulated PRISM data for the $50\%$ on-axis running strategy with a $\nu$-dominated beam.
• Figure 4: Impact of the flux uncertainty on the PRISM prediction of the DUNE far detector event spectrum with and without using the LENS procedure to constrain the flux model. All curves were obtained by applying \ref{['eq:PRISM-prediction']} to near detector data generated using the nominal flux model. The PRISM coefficients $c_j$ in \ref{['eq:PRISM-1', 'eq:PRISM-coefficients', 'eq:PRISM-prediction']}, on the other hand, were derived from biased flux models in which the relative importance of individual flux components (i.e., the $r_p$ coefficients in \ref{['eq:flux-superposition']}) were allowed to vary either by 5% (red curves) or within the tighter constraints imposed by our LENS fit to on-axis and off-axis near detector data, see \ref{['tab:uncertainties']} (2nd and 4th column). In each case, we show 100 random realizations. The black curve shows for comparison the far detector spectrum based on the nominal flux model. We see that the spread in possible outcomes is substantially reduced when the LENS constraint is included, demonstrating how LENS helps making DUNE oscillation results more fiducial.
• Figure 5: The "McDonald's plot" showing the impact of neutrino flux uncertainties on DUNE’s sensitivity to leptonic CP violation with 1.6e23pot at 40kt fiducial detector mass (7 years of running at 1.2MW beam power + 3 years at 2.4MW = 624kt MW yrs, equally split between FHC and RHC). For each value of the true $\delta_{CP}$, we show the significance at which the CP-conserving hypothesis ($\delta_{CP} \in \{ 0, \pi \}$) can be excluded. Similar to \ref{['fig:spectrum']}, all curves were obtained using PRISM to predict the oscillated far detector fluxes based on near detector data. While the near and far detector data were simulated using the nominal flux model, the PRISM coefficients $c_j$ were derived based on biased flux models (red: 5% variations, blue: variations allowed from \ref{['tab:uncertainties']}). Note that the systematic uncertainties assumed in the oscillation fit are the same for all curves, even though LENS would allow us to reduce them, boosting the sensitivity. The black dotted curve shows the hypothetical sensitivity without systematic uncertainties. In grey, we illustrate how the impact of systematic uncertainties grows with larger exposure (15 years, corresponding to 1104kt MW yrs).