First Result of NEOS Experiment

Abstract

Neutrino Experiment for Oscillation at Short baseline (NEOS) targeted the search of singlet neutrinos as natural extension to Standard Model three neutrinos. The electron antineutrinos emerging from a reactor of 2.8-GW thermal power were detected in tendon gallery, 24 meters away from the reactor core. The experiment was performed at Hanbit Nuclear Power Plant in Young-Gwang, Korea for eight months. The baseline is optimized for the oscillation with mass-squared difference about 1 ${\rm eV^2}$. A ton of Gd-loaded liquid scintillator collected 1976 events per day with signal-to-background rate 22 during reactor-on period. The shape analysis exhibits a moderate spectrum consistent to those obtained in other reactor neutrino oscillation for about 1.5-km distance. The 5-MeV excess also appears in the spectrum shape of NEOS as seen commonly in Daya Bay, RENO and Double Chooz. Although our analysis drew an exclusion curve of high sensitivity, a singlet neutrino state has not been verified for a scale between the mass of atmospheric neutrino and the order of eV. However, the result of NEOS presents the direction to further search of sterile neutrinos. Possible strategies using the result and the prospect of future sterile neutrino search will be discussed.