MINER Reactor Neutrino Experiment

• MINER is a reactor-based experiment using cryogenic detectors to study neutrino interactions including CEνNS and sterile neutrino oscillations.
• It deploys germanium, silicon, and sapphire arrays with transition-edge sensors to achieve sub-keV thresholds and fine spatial resolution.
• The experiment also probes axion-like particles, quantum decoherence, and Lorentz violation, opening new avenues for beyond-Standard Model physics.

The Mitchell Institute Neutrino Experiment at Reactor (MINER) is a multidisciplinary reactor-based experimental program designed to probe fundamental neutrino properties and search for physics beyond the Standard Model. Leveraging advanced low-threshold detector technology and close reactor proximity, MINER targets phenomena such as coherent elastic neutrino–nucleus scattering (CEνNS), neutrino oscillations (including signatures of sterile neutrinos and possible Lorentz violation), quantum decoherence effects, and the existence of axion-like particles (ALPs) using specialized cryogenic detector arrays. MINER has deployed multiple detector platforms—including germanium, silicon, and sapphire (Al₂O₃) crystals equipped with transition-edge sensors—in both low-background and reactor-correlated environments, with plans for future enhancements at high-flux facilities.

1. Experimental Principles and Detector Technologies

MINER builds on several foundational principles: maximizing neutrino flux by situating detectors close to compact reactor cores and minimizing backgrounds through multilayer passive and active shielding. Early implementations utilized cryogenic germanium and silicon detectors (Collaboration et al., 2016), with expected CEνNS event rates of 5–20 events/kg/day for recoil energies in the range of 10–1000 eV_nr. Sapphire detectors form the basis for recent CEνNS and ALP searches (Mirzakhani et al., 29 Apr 2025, Mondal et al., 14 Oct 2025). These detectors are equipped with microfabricated superconducting transition-edge sensors (TES/QETs) that enable energy thresholds down to 100 eV, operated at ∼10 mK temperatures within a dilution refrigerator.

The underlying detection methodologies include:

• Phonon-based readout, where athermal phonons produced by nuclear or electronic recoils propagate to the TES/QET, yielding precise energy measurement.
• Rigorous event reconstruction through software-based triggering, optimal filtering, and multi-detector cuts—single-scatter event selection and veto-layer discrimination remove backgrounds associated with multiple scattering or muon events.

Energy calibration is achieved using $\prescript{55}{}{\mathrm{Fe}}$ X-ray sources (5.89, 6.49 keV), with linear conversion of pulse template amplitudes to recoil energies.

2. Reactor Oscillation Experiments and Sterile Neutrino Investigations

MINER advances the search for short-baseline oscillations of electron antineutrinos ($\overline{\nu}_e$) into sterile neutrinos by utilizing the spatial and energy resolution provided by its detectors in a high-flux reactor environment. The program is informed by previous work employing position-sensitive liquid scintillation detectors at compact research reactors (Derbin et al., 2012): the oscillation length relevant to sterile neutrino hypotheses, $$L \approx 2.5 \cdot E_\nu\,[\mathrm{MeV}] / \Delta m^2\,[\mathrm{eV^2}]$$, is directly comparable to detector dimensions and baseline (5–15 m).

Key features and impacts:

• Event rate modulations attributable to oscillation “waves” inside the detector, deviating from the simple $1/R^2$ flux scaling, can be detected with percent-level precision.
• Sensitivity to mixing parameters in the region $\Delta m^2 = 0.3$–$6\,\mathrm{eV^2}$, $\sin^2(2\theta) \geq 0.01$ is achieved through sub-meter spatial reconstruction and high-statistics counting.
• Parallel efforts using intense (anti)neutrino sources ($^{51}$Cr, $^{144}$Ce) deployed in or near large detectors (Lasserre, 2012) can probe the same parameter space and test anomalous deficits found in historical gallium and reactor experiments, via oscillation patterns in both spatial and energy event distributions.

3. CEνNS Measurements: Results and Background Control

MINER’s cryogenic sapphire detector search for CEνNS at the TRIGA reactor (Mondal et al., 14 Oct 2025) exemplifies advanced low-threshold measurement. With a primary 72 g detector exposed for 158 g-days (reactor-on) and 381 g-days (reactor-off), energy resolutions of $\sim$40 eV enable precise background subtraction in the 0.25–3 keV window. A GEANT4-based simulation confirms that reactor-induced backgrounds (primarily fast neutrons and gammas) dominate observed rates.

Key findings:

• The best-fit ratio $\rho$ of observed CEνNS rate to the Standard Model prediction is $$0.26 \pm 1534.74~(\mathrm{stat}) \pm 0.05~(\mathrm{sys})$$.
• Detection significance is $$0.007 \pm 0.022~(\mathrm{stat}) \pm 0.001~(\mathrm{sys})$$, indicating the signal is statistically consistent with background.
• Plans to relocate to the 85 MW$_\text{th}$ HFIR at ORNL will multiply the neutrino flux and detector mass (scaling tower to multi-primary configuration), aiming for 3$\sigma$ CEνNS detection within $30~\mathrm{kg}\cdot\mathrm{days}$ exposure and improved background suppression.

A summary of CEνNS results at MINER:

Reactor Power	Detector Mass	Threshold	Best-fit $\rho$	Significance	Future Sensitivity
1 MW$_\text{th}$ (TRIGA)	72 g	~40 eV	$0.26 \pm 1534.74$	$0.007 \pm 0.022$	Plan: 3$\sigma$ at 30 kg·days (HFIR)

4. ALP Searches and Beyond-Standard Model Physics

MINER’s sapphire detector array was used to search for axion-like particles (ALPs), focusing on axion-photon ($g_{a\gamma\gamma}$) and axion-electron ($g_{aee}$) couplings (Mirzakhani et al., 29 Apr 2025). Positioned 4 m from the reactor core, the experiment achieves sensitivity via high reactor photon flux and ultra-low backgrounds. The ALP search excludes couplings down to $g_{a\gamma\gamma} \sim 10^{-5}$ and $g_{aee} \sim 10^{-7}$, exploiting the low-energy threshold and proximity to the core; energy depositions below 3 keV remain blinded for subsequent CEνNS analysis. The experiment demonstrates the utility of reactor-based, cryogenic, low-threshold platforms for probing non-WIMP dark matter scenarios.

5. Quantum Decoherence, Oscillation Hierarchy, and Lorentz Violation

MINER’s reactor configuration, baseline flexibility, and detector energy resolution make it well suited to probe quantum decoherence and mass hierarchy effects:

• Decoherence studies (Gouvêa et al., 2020, Gouvêa et al., 2021) use the wave-packet width $\sigma$ to parameterize loss of coherence in neutrino oscillations. Current reactor experiments constrain $\sigma > 1.0 \times 10^{-4}$ nm (Daya Bay, RENO) and $\sigma > 2.1 \times 10^{-4}$ nm (Daya Bay, RENO, KamLAND combined). This establishes that decoherence is subdominant on experimental baselines explored so far and that standard oscillation parameters are robust with respect to possible wave-packet separation effects.
• Medium-baseline reactor studies exploit Fourier analysis of $L/E$-binned spectra to separate hierarchy-dependent oscillatory indicators (RL + PV). Energy-dependent weighting in the Fourier transform, such as $w(E) = \exp(-0.08 E^2/\mathrm{MeV^2})$ (Ciuffoli et al., 2012), suppresses the influence of high-energy spectrum tails, improving sensitivity to the mass hierarchy signal and reducing model dependence.
• Lorentz invariance violation, via neutrino–antineutrino oscillations modeled in the Standard-Model Extension (SME) framework (Diaz et al., 2013), is constrained by analyzing energy-dependent spectral distortions against SME coefficients. MINER’s platform is adaptable to similar analyses and can contribute to tightening limits on CPT and Lorentz-violating parameters.

6. Background Measurements, Simulation Strategies, and Monitoring

The environment near a reactor core entails elevated challenges in background control. MINER characterizes backgrounds through:

• In situ measurements with HPGe detectors (gamma), copper foil activation (neutron), and scintillator muon counters.
• Detailed MCNP and GEANT4 simulations, incorporating geometry, shielding layers, material composition, and importance sampling strategies for efficient particle tracing.
• Dynamic monitoring strategies: development of segmented liquid-scintillator shields, dedicated iZIP-type detectors for recoil discrimination, ⁶Li- and PTP-doped scintillators for neutron/gamma pulse shape identification.
• Continuous validation of background subtraction via reactor-on/off cycles and simulation/data comparison.

7. Future Directions and Synergies

MINER’s future program includes:

• Relocation to higher-power reactors (e.g., HFIR 85 MW$_\text{th}$), increased detector mass, and enhanced shielding to significantly improve CEνNS and ALP sensitivity.
• Expansion to five-detector sapphire towers, corresponding to 725 g total mass in ALP searches and multi-primary detector configurations for CEνNS.
• Synergetic use of results to constrain or probe new physics in oscillator, spectral, and particle dark sector regimes.
• Potential for integrating multi-baseline, dual-detector arrangements to minimize systematic uncertainties in oscillation and sterile neutrino parameter space mapping.

MINER provides an integrated experimental platform for reactor neutrino physics, uniquely combining low-threshold cryogenic detection, high-flux reactor sources, and a flexible geometry for efficient probing of CEνNS, sterile neutrino oscillations, quantum decoherence, Lorentz violation, and axion-like particle phenomenology.