Measurement of the smallest neutrino mixing angle using reactor antineutrino events with neutron capture on hydrogen at RENO

• Sang Yong KIM (JNU)

Room: 305호 The reactor experiment for neutrino oscillation (RENO) has been taking data since August 2011 and successfully measured the smallest neutrino mixing angle, theta13(θ13). The measured value is obtained from the observed reactor anti-neutrino events with neutron captures on gadolinium (n-Gd) in the target detector region. RENO has also measured the mixing angle using an independent sample of neutron captures on hydrogen (n-H). Because of high environmental radioactivity below 3.5 MeV, the neutron captures on H emitting a 2.2 MeV gamma-ray are not easily detected. Satisfactory purification of liquid scintillator and use of low–radioactivity photomultiplier tube (PMT) glass in addition to an improved analysis allow successful extraction of the reactor signal against the high backgrounds. The identical near and far detectors are essential to significantly reducing the systematic uncertainties correlated between them. In particular, the (n-H) analysis utilizes events in both target and gamma-catcher and thus obtains a larger reactor antineutrino sample, twice more than that of (n-Gd). This independent measurement of θ13 is consistent with the precise measurement using the reactor events with a delayed signal of neutron capture on gadolinium (Gd), and provides a systematic cross-check on the measured value. In this talk, we will present a new measurement of theta13 using 2900 days of n-H data.