Final Search for Short-Baseline Neutrino Oscillations with the PROSPECT-I Detector at HFIR

Abstract

The PROSPECT experiment is designed to perform precise searches for antineutrino disappearance at short distances (7 - 9~m) from compact nuclear reactor cores. This Letter reports results from a new neutrino oscillation analysis performed using the complete data sample from the PROSPECT-I detector operated at the High Flux Isotope Reactor in 2018. The analysis uses a multi-period selection of inverse beta decay neutrino interactions with reduced backgrounds and enhanced statistical power to set limits on electron-flavor disappearance caused by mixing with sterile neutrinos with 0.2 - 20 eV$^2$ mass splittings. Inverse beta decay positron energy spectra from six different reactor-detector distance ranges are found to be statistically consistent with one another, as would be expected in the absence of sterile neutrino oscillations. The data excludes at 95% confidence level the existence of sterile neutrinos in regions above 3~eV$^2$ previously unexplored by terrestrial experiments, including all space below 10~eV$^2$ suggested by the recently strengthened Gallium Anomaly. The best-fit point of the Neutrino-4 reactor experiment's claimed observation of short-baseline oscillation is ruled out at more than five standard deviations.

Figures (4)

• Figure 1: Operational status and baseline binning designation for each PROSPECT detector segment. The numbers and color of each segment defines its assigned baseline bin, while the colors of each segment's small boxes indicates its operational status for each of the five data taking periods (P1 through P5). Unnumbered non-fiducial segments are indicated in light gray.
• Figure 2: $L$/$\langle E_{\nu}\rangle$ spectrum of background-subtracted IBD signals (black points) and estimated correlated (solid red) and accidental (solid blue) backgrounds during PROSPECT-I reactor-on periods. $L$ and $\langle E_{\nu}\rangle$ are defined in the text. The null oscillation IBD signal prediction (dashed black) is also pictured. Error bars represent 1$\sigma$ statistical uncertainties.
• Figure 3: Ratio of $L$/$\langle E_{\nu}\rangle$ features between PROSPECT's IBD signal and its null oscillation prediction. As described in the text, predicted $\langle E_{\nu}\rangle$ spectra are weighted by the ratio between detector-integrated measured and predicted spectra, ($\frac{\mathrm{M}_{e}}{\mathrm{P}_{e}}$)P$_{l,e}$. Error bars indicate the statistical uncertainty in the measured IBD spectrum, M$_{l,e}$. Ratios expected due to oscillations at the PROSPECT data (magenta line) and Neutrino-4 (blue line) best-fit points are also depicted.
• Figure 4: Phase space for 3+1 sterile neutrino oscillations excluded by the multi-period PROSPECT-I dataset. Exclusion (solid) and sensitivity (dotted) curves, presented for 95% confidence level (CL), were generated using the Gaussian CLs method. Exclusion (dashed) at 5$\sigma$ CL is also shown. Shaded suggested phase space regions from the Gallium Anomaly Barinov:2022wfh (light pink) and Neutrino-4 neutrino4_prd (red) are also pictured.