Sterile neutrino search at NEOS Experiment

Abstract: An experiment to search for light sterile neutrinos was conducted at a reactor with a thermal power of 2.8 GW located at the Hanbit nuclear power complex. The search was done with a detector consisting of a ton of Gd-loaded liquid scintillator in a tendon gallery approximately 24 m from the reactor core. The measured antineutrino event rate is 1976 per day with a signal to background ratio of about 22. The shape of the antineutrino energy spectrum obtained from eight-month data-taking period is compared with a hypothesis of oscillations due to active-sterile antineutrino mixing. It is found to be consistent with no oscillation. An excess around 5 MeV prompt energy range is observed as seen in existing longer baseline experiments. The parameter space of $\sin^{2}2\theta_{14}$ down below 0.1 for $\Delta m^{2}_{41}$ ranging from 0.2 eV$^{2}$ to 2.3 eV$^{2}$ and the optimum point for the previously reported reactor antineutrino anomaly are excluded with a confidence level higher than 90%.

• The paper employs short-baseline reactor antineutrino measurements with a Gd-loaded liquid scintillator at 24 meters from a 2.8 GW reactor to probe active-sterile mixing.
• The paper finds no definitive evidence for 3+1 neutrino oscillations while noting a persistent 5 MeV excess in the prompt energy spectrum.
• The paper constrains the mixing parameter sin²2θ₁₄ to below 0.1 for Δm²₄₁ between 0.2 and 2.3 eV², paving the way for more precise future experiments.

Overview of the Sterile Neutrino Search at the NEOS Experiment

The paper "Sterile Neutrino Search at the NEOS Experiment" explores the potential for sterile neutrinos by analyzing neutrino oscillations at short baselines. Conducted at the Hanbit nuclear power complex in Korea, the experiment uses a detector positioned 24 meters from a reactor core with a thermal power of 2.8 GW. The detector, equipped with a Gd-loaded liquid scintillator, captures inverse beta decay (IBD) events, allowing for intricate investigation of active-sterile neutrino mixing parameters.

Methodology and Results

The NEOS experiment capitalizes on the well-established methodology of using short-baseline reactor experiments to probe potential neutrino oscillations. The NEOS detector achieved an antineutrino event rate of approximately 1976 events per day, with a strong signal-to-background ratio of about 22. Over an eight-month data gathering period, the experiment scrutinized the shape of the antineutrino energy spectrum, looking for distortions indicative of active-sterile neutrino oscillations.

Notably, the NEOS data does not support a definitive presence of 3+1 neutrino oscillations. However, an excess in the event rate around the 5 MeV prompt energy range was observed, resonating with findings from previous long-baseline experiments. The analysis defines the mixing parameter $\sin^{2}2\theta_{14}$ to be less than 0.1 within the $\Delta m^{2}_{41}$ range of 0.2 to 2.3 eV² at a 90% confidence level.

Theoretical and Practical Implications

The absence of a significant oscillation signal restricts the parameter space for sterile neutrinos, providing constraints relevant to theories suggesting the existence of sterile neutrinos as a solution for anomalies in neutrino physics, like the reactor antineutrino anomaly (RAA) and the LSND anomaly. The limits established by the NEOS experiment coincide with those of the Bugey-3 experiment in overlapping parameter spaces, thus reinforcing the constraints on light sterile neutrinos.

The observed 5 MeV excess persists as an intriguing anomaly across various experiments, possibly suggesting additional unexplored physics or inaccuracies in the current reactor flux models. Future dedicated measurements and enhanced reference spectra could elucidate these inconsistencies.

Future Directions

Despite the no-evidence conclusion for 3+1 neutrino oscillations, the NEOS experiment lays groundwork for further explorations. Upcoming experiments with superior baseline configurations and $L/E$ resolutions are anticipated to offer increased sensitivity, potentially uncovering even subtler signals. Acknowledging that previous cosmological limits challenge the existence of sterile neutrinos at such scales, the NEOS experiment underscores the necessity for high precision and calibration improvements in future neutrino research endeavors.

The NEOS experiment's findings hold significant implications for both the short-baseline and the broader neutrino research community. Future investigations will likely continue refining the parameter space for sterile neutrinos, whilst attempting to resolve spectral anomalies, ultimately contributing to a more comprehensive understanding of neutrino physics.