Sterile Neutrino Search at the NEOS Experiment

Area of Science:

• Particle Physics
• Nuclear Physics
• Cosmology

Background:

• The existence of sterile neutrinos is a theoretical possibility that could explain neutrino masses and other phenomena.
• Reactor experiments are sensitive probes for detecting light sterile neutrinos through their potential oscillation effects.

Purpose of the Study:

• To search for evidence of light sterile neutrinos by analyzing antineutrino oscillations.
• To constrain the mixing parameters (sin²2θ₁₄ and Δm₄₁²) of a potential 3+1 neutrino model.

Main Methods:

• Utilized a 1-ton Gadolinium-loaded liquid scintillator detector placed 24m from the Hanbit nuclear reactor core.
• Collected data over an eight-month period, measuring the antineutrino event rate and energy spectrum.
• Compared the observed antineutrino energy spectrum with predictions from active-sterile neutrino mixing models.

Main Results:

• Measured an antineutrino event rate of 1976 per day with a signal-to-background ratio of 22.
• Observed an excess in the prompt energy spectrum around 5 MeV, consistent with previous experiments.
• Found no strong evidence supporting the 3+1 neutrino oscillation hypothesis.

Conclusions:

• The experiment sets limits on the sterile neutrino mixing parameter sin²2θ₁₄ < 0.1 for Δm₄₁² between 0.2 and 2.3 eV² at 90% confidence level.
• The observed 5 MeV excess warrants further investigation in the context of sterile neutrino searches.