CE$ν$NS Search with Cryogenic Sapphire Detectors at MINER: Results from the TRIGA reactor data and Future Sensitivity at HFIR

TL;DR

This work reports a search for coherent elastic neutrino-nucleus scattering (CEνNS) using cryogenic sapphire detectors at MINER near a TRIGA reactor, detailing detector design, calibration, background modeling, and a statistical analysis of reactor-on versus reactor-off data. The analysis shows no significant CEνNS signal, with a best-fit strength $\rho = 0.26 \pm 1534.74\,(stat) \pm 0.05\,(sys)$ and a very modest significance of $0.007 \pm 0.022\,(stat) \pm 0.001\,(sys)$, dominated by reactor-induced backgrounds at low energy. A Geant4-based background simulation supports these findings, and the collaboration outlines a path to substantially improve sensitivity through relocation to HFIR (85 MW$_{th}$) with upgraded shielding and a larger detector payload, projecting a 3$\sigma$ CEνNS detection in roughly 30 kg$\cdot$days. These prospects highlight the potential of MINER to constrain CEνNS cross sections and explore SM/BSM physics with reactor antineutrinos.

Abstract

We report on a search for coherent elastic neutrino--nucleus scattering (CE$ν$NS) using cryogenic sapphire (Al$_2$O$_3$) detectors deployed at the Mitchell Institute Neutrino Experiment at Reactor (MINER), located near the 1~MW$_\text{th}$ TRIGA research reactor at Texas A\&M University. The experiment operated with a primary detector mass of 72~g and achieved a baseline energy resolution of $\sim 40$~eV. Using exposures of 158~g-days (reactor-on) and 381~g-days (reactor-off), we performed a statistical background subtraction in the energy region of 0.25--3~keV. A GEANT4 simulation has been performed to understand the reactor-correlated background present in the data and it agrees with our observations. The resulting best-fit ratio of the observed CE$ν$NS rate to the Standard Model prediction after rejecting the reactor induced background from the data with the help of simulation, is $ρ= 0.26\pm 1534.74~\mathrm{(stat)} \pm 0.05~\mathrm{(sys)}$ with a significance of $0.007 \pm 0.022~\mathrm{(stat)} \pm 0.001~\mathrm{(sys)}$. This low significance indicates a high background rate at low energies. To have enhanced sensitivity, the MINER collaboration plans to relocate the experiment to the 85~MW$_\text{th}$ High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). With improved shielding, increased detector mass, and higher antineutrino flux, the upgraded setup is projected to achieve a 3$σ$ CE$ν$NS detection within 30~kg$\cdot$days of exposure.

Figures (12)

• Figure 1: (a) Photograph of the primary (4 mm ) and veto (10 mm) sapphire detector with the phonon sensors photolithographically patterned on the surface. (b) Schematic of the cavity packed with layered shielding. (c) Detector shielding (top view) showing the placement of the detector stack inside the dilution fridge.
• Figure 2: (a) A representative phonon pulse template, (b) noise performance, (c) a phonon pulse, and (d) the uncalibrated energy spectrum observed in the primary detector.
• Figure 3: X-ray peaks at 5.89 keV and 6.49 keV from the $^{55}$Fe calibration source as observed in the primary detector. The double Gaussian fit function is shown in magenta colour to extract the mean positions.
• Figure 4: Energy spectra of events collected during 158 g-days (reactor ON) and 381 g-days (reactor OFF) within the 0–20 keV energy range. The two prominent peaks correspond to the 5.89 keV and 6.49 keV X-rays from the $^{55}$Fe calibration source.
• Figure 5: Schematic illustration of (a) a single scatter (signal-like), (b) a multiple scatter (background-like) event within the detector stack, and (c) time coincidence definition of the primary detector.