26 May 2016
The Reactor Experiment for Neutrino Oscillation (RENO) collaboration announced new results including their first measured value of a neutrino squared-mass difference and a more precisely measured value of the smallest neutrino mixing probability. Both of them are essential for understanding the unknown nature of neutrinos. The RENO experiment has observed energy and baseline distance dependent disappearance of reactor neutrinos that are emitted from six reactors at the Hanbit (known as Yonggwang) nuclear power plant in South Korea, on the way to their 1.4 km distant detector.
Neutrinos are almost invisible and massless particles, traveling at close to the speed of light and interacting with matter so weakly. The Super-Kamiokande and SNO experiments found transformation among the neutrinos and thus existence of their masses for the first time. T. Kajita and A. McDonald received the 2015 Nobel Physics Prize due to their discovery of the neutrino transformation and mass.
The RENO reported the first results on firm determination of the smallest neutrino mixing probability, so-called “mixing angle theta one-three”, in April 2012. The new results give a more precise value of the probability, and will appear in the Physical Review Letters on May 27, 2016. The systematic error is reduced from 0.019 to 0.006, and the total error becomes 0.010 from 0.023. The experiment observes an interesting spectral deviation in the energy region centered at 5 MeV, from the prediction of reactor neutrino models. The excess of events constitutes about 3% of the total observed reactor neutrinos.
“This 5 MeV excess was seen from the beginning and reported in the 2012 Neutrino Conference at Kyoto”, says Soo-Bong Kim of Seoul National University, spokesperson of the RENO experiment. “The excess turned out to be proportional to the reactor power with more data, and the 3.5 σ excess in the reactor neutrino spectrum was announced in the 2014 Neutrino Conference at Boston. This observation suggests needs for reevaluation and modification of the current reactor neutrino model.”
The RENO uses two identical detectors located at 290m and 1380m on either side of the center of six reactor cores. Because of the unexpected structure around 5 MeV, the neutrino oscillation amplitude and frequency are determined from the measured far-to-near ratio of the reactor spectra. The two identical neutrino detectors allow a relative measurement and considerably reduce several systematic errors, through comparison of the observed neutrino rates and spectra. The RENO collaboration has measured an effective squared-mass splitting of neutrino to a precision of 9.5% based on the measured periodic disappearance expected from neutrino oscillations. This indicates that the mass difference between the heaviest and lightest neutrinos is roughly a billionth of an electron mass.
A precise measurement of the mixing angle would provide an important information on determining the CP violation phase which might be associated with the asymmetry between matter and antimatter in the Universe. The RENO’s ultimate precision of the mass splitting is expected to be ±5%, and will be nearly sensitive to determination of neutrino mass ordering, one of the most mysterious nature of neutrino.