DUNE-PRISM: Precision Neutrino Measurement

• DUNE-PRISM is a methodology using a movable near detector to capture off-axis neutrino energy spectra for precision measurements.
• It enables model-independent cross-section extractions by constructing virtual fluxes, significantly reducing systematic uncertainties in neutrino oscillation analyses.
• The design integrates segmented LArTPCs and high-pressure gaseous argon detectors, supporting diverse studies from standard oscillations to new physics searches.

DUNE-PRISM (Deep Underground Neutrino Experiment—Precision Reaction Independent Spectrum Measurement) is an experimental methodology and near-detector platform integral to the DUNE long-baseline neutrino program. It centers on the lateral translation of the near detector complex to sample a family of off-axis neutrino energy spectra, enabling model-independent constraints on flux and interaction systematics. This paradigm exploits beam geometry and decay kinematics to address severe limitations in the precision of neutrino oscillation, cross-section, and new physics measurements.

1. Conceptual Foundations and Detector Architecture

DUNE-PRISM employs a modular near detector system—including a segmented liquid argon time projection chamber (ND-LAr), high-pressure gaseous argon TPC (ND-GAr), and an on-axis beam monitor (SAND)—mounted on a laterally movable platform (Abud et al., 2021). By translating the ND (typical range: several meters up to ~30 m transverse to the beam), the experiment samples a series of neutrino fluxes, each characterized by a distinct mean energy and spectral width determined by the decay kinematics of parent mesons.

A simplified spectral shift relation for pion decay is

$$E_\nu(\theta) \approx \frac{0.43 E_\pi}{1 + (\gamma \theta)^2}$$

where $E_\pi$ is the parent pion energy, $\gamma$ is the Lorentz boost, and $\theta$ is the off-axis angle (Gil-Botella, 19 Dec 2024).

Segmented designs (ArgonCube for ND-LAr) provide high pile-up resilience and enable robust translation of the ND. Flexible utilities (“energy chains”) maintain uninterrupted operation. ND-GAr modules add momentum and charge identification for muons (Abud et al., 2021). SAND supplies continuous on-axis monitoring to correct for time-dependent beam variations.

2. Off-Axis Spectral Sampling: Principles and Data Utilization

Off-axis ND positions access different effective neutrino energy spectra, narrowing and lowering the mean energy with increasing displacement. Each position $\alpha$ yields a flux $\Phi_\alpha(E_\nu)$, allowing the construction of “virtual” or “custom” fluxes via linear combinations: $$\tilde{\Phi}(E_\nu) = \sum_\alpha c_\alpha \Phi_\alpha(E_\nu)$$ Coefficients $c_\alpha$ are determined by fitting this sum to a target spectrum (e.g., narrow Gaussian or quasi-monoenergetic flux), typically using least-squares with regularization to suppress statistical variance (Tikhonov) (Collaboration et al., 9 Sep 2025).

This enables near model-independent measurement of observables conditional on the true neutrino energy, with the integrated cross section given by: $$\langle \sigma \rangle = \frac{\tilde{N}}{\mathcal{E} \cdot \epsilon \cdot N_\text{targets} \cdot \int \tilde{\Phi}(E_\nu) d E_\nu}$$ where $\tilde{N}$ is the weighted sum of events, $\mathcal{E}$ the exposure, $\epsilon$ the selection efficiency, and $N_\text{targets}$ the number of targets (Collaboration et al., 9 Sep 2025).

Access to different spectral shapes enables powerful deconvolution of energy-dependent interaction effects. DUNE-PRISM allows for virtual spectra as narrow as $\sigma \lesssim 70 \ \text{MeV}$, facilitating quasi-monoenergetic cross-section extraction and model validation (Collaboration et al., 9 Sep 2025).

3. Systematic Uncertainty Reduction in Oscillation Analysis

Standard neutrino oscillation experiments face degeneracies and systematic biases originating from:

• Uncertainties in the initial neutrino flux shape,
• Model-dependent mapping from observable detector energy to true neutrino energy (i.e., nuclear effects, cross-section systematics),
• Detector efficiency and energy migration.

DUNE-PRISM mitigates these by performing energy spectrum “scans.” Multiple off-axis samples constrain the flux and cross-section uncertainties, as well as detector response, by allowing flexible linear combinations of ND measurements matched to FD target spectra (Hasnip, 17 Jan 2025). The methodology produces FD event predictions using only ND data and simulated fluxes, independent of interaction modeling: $$F_j^{(\text{LC})} = \sum_i N_{ij}^{(\text{data})} c_i$$ where $N_{ij}^{(\text{data})}$ is the corrected event rate at ND off-axis position $i$ in bin $j$, and $c_i$ are coefficients derived from neutrino flux simulation (Hasnip, 17 Jan 2025).

This decoupling substantially limits propagation of biases from mis-modeled neutrino-nucleus interactions to key oscillation observables.

4. Cross-Section and Nuclear Physics Applications

Traditional neutrino cross-section measurements, typically performed in wide-band beams, suffer from flux averaging, which obscures spectral features and amplifies model dependence. DUNE-PRISM reconstructs integrated and differential cross sections over narrow, virtual fluxes: $\sigma(E_\nu) \approx \langle \sigma \rangle$ for a sufficiently narrow $\tilde{\Phi}(E_\nu)$ centered at $E_\nu$, with extension to differential observables via reweighting (Collaboration et al., 9 Sep 2025): $$\frac{d \langle \sigma \rangle}{d\omega_\text{reco}} = \frac{d\tilde{N}/d\omega_\text{reco}}{\mathcal{E} \cdot \epsilon \cdot N_\text{targets} \cdot \int\tilde{\Phi}(E_\nu)dE_\nu}$$ where $\omega_\text{reco}$ is the reconstructed energy transfer.

Such measurements afford model-independent access to nuclear responses (e.g., energy transfer distributions, quasi-elastic peaks) as seen in electron scattering, crucial for oscillation analysis. Statistical uncertainties scale with the sum of squared coefficients: $${(\Delta \tilde{N})}/{\tilde{N}} \approx \sqrt{\sum_\alpha c_\alpha^2 N_\alpha}/|\sum_\alpha c_\alpha N_\alpha|$$ requiring high statistics and optimal regularization.

5. New Physics Searches and Additional Capabilities

The DUNE-PRISM movable ND dramatically enhances sensitivity to new physics:

• Dark Matter: Off-axis measurements suppress highly-boosted neutrino backgrounds, allowing the isolation of signals such as electron recoil from sub-GeV dark matter produced via unfocused neutral meson decay (Romeri et al., 2019, Breitbach et al., 2021). Signal-to-background ratios improve at large off-axis, especially for hadrophilic dark matter scenarios.
• Heavy Neutral Leptons (HNLs): Off-axis positioning maintains competitive sensitivity across production and decay channels, as the isotropic nature of sterile neutrino signals parallels the broader angular acceptance (Breitbach et al., 2021).
• Electroweak Precision: Off-axis data alter beam flavor composition, increasing $\nu_e$ fraction and enabling stringent constraints on the weak mixing angle via neutrino-electron scattering (Gouvêa et al., 2019).

The configurational flexibility supports further searches in electromagnetic properties, sterile neutrino oscillations, and rare event signatures.

6. Integration with Detector Technologies and the Broader DUNE Physics Program

DUNE-PRISM mandates ND systems with robust mechanical and tracking capabilities. Segmented LArTPCs (ArgonCube), high-pressure gaseous argon TPCs, and precision muon spectrometers collectively deliver high event rates, low energy thresholds, and accurate PID and calorimetry (Abi et al., 2020, Abud et al., 2021). In Phase II, upgrades—including increased beam power, augmented ND complexity, and expanded FD mass—broaden the accessible parameter space and statistical reach (Collaboration et al., 2022, Collaboration et al., 30 Mar 2025).

A consistent LArTPC technology in both ND and FD supports systematic cancellation. Off-axis scanning and spectrum matching underpin not only oscillation measurements but also nucleon decay searches and supernova neutrino burst detection, by improving background modeling and transient event identification (Abi et al., 2020).

7. Limitations, Challenges, and Future Directions

While DUNE-PRISM substantially reduces interaction model dependence, several limitations persist:

• Statistical uncertainties can be amplified by the subtraction of large, oppositely weighted off-axis contributions, especially for narrow target fluxes and differential measurements (Collaboration et al., 9 Sep 2025).
• Systematic uncertainties in flux simulation and detector calibration remain a concern, necessitating careful cross-checks and regularization strategies.
• Large exposures and multi-year datasets are required to achieve few-percent precision in key cross-section bins.
• Extension of the method to other long-baseline experiments and further integration with improved interaction models is anticipated (Hasnip, 17 Jan 2025).

Future work may broaden the reach to inelastic channels, additional BSM physics analyses, and synergistic studies with other experiments.

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In summary, DUNE-PRISM is a foundational methodology for precision neutrino physics, providing extensive reduction of flux and interaction systematics through spectrum scanning and linear combination analysis at a movable near detector. Its integration enables model-independent oscillation and cross-section measurements, strengthens sensitivity to new physics, and underpins the overall experimental capabilities of DUNE in pursuit of ambitious discovery goals (Hasnip, 17 Jan 2025, Collaboration et al., 9 Sep 2025, Abud et al., 2021, Romeri et al., 2019, Collaboration et al., 2022, Gil-Botella, 19 Dec 2024, Collaboration et al., 30 Mar 2025).