Sterile Neutrinos Evade The NEOS Detector

Results from a new experiment shows that finding sterile neutrinos may be more challenging than we thought.

AsianScientist (Mar. 29, 2017) – The Neutrino Experiment for Oscillation at Short Baseline (NEOS) has failed to find evidence for the elusive ‘sterile neutrino.’ These results have been published in Physical Review Letters.

Neutrinos are elementary particles that have no electric charge and extremely small masses. They are created only in extreme conditions such as the environment of the Sun, nuclear reactors and supernova explosions.

All neutrinos detected to date come in three types, or flavors: electron neutrino, muon neutrino, and tau neutrino. Neutrinos can change from one type to another, through a phenomenon called neutrino oscillation. In 2015, Professors Takaaki Kajita and Arthur B. McDonald won the Nobel Prize in Physics for their discover of neutrino oscillation. Four Asian teams were also recognized for their work on neutrinos with the 2016 Breakthrough Prize in Fundamental Physics.

Interestingly, previous experiments measured these oscillations and found an anomaly in the data: the number of measured neutrinos is around seven percent lower than the predicted value. Researchers have proposed that these disappearing neutrinos transform into a fourth type of neutrinos, called sterile neutrinos.

In the search for sterile neutrinos, researchers at the Center for Underground Physics at the Institute for Basic Science led the NEOS study. The experiment was conducted in the Hanbit Nuclear Power Plant in Yeonggwang, South Korea, a standard nuclear reactor that is expected to produce 5.1020 neutrinos per second as by-products of the reaction that generates nuclear energy.

Firstly, the scientists had to overcome the problem of background signals which could hinder the neutrino detection. One solution was to install the detector underground, as close as possible to the core of the reactor, where the beta decay reaction is taking place. In this case, the neutrino detector was installed 24 meters from the core in a structure called a tendon gallery. The detector was protected by several layers of lead blocks which shield the detector from gamma rays, and borated polyethylene to block neutrons.

The team measured the number of electron neutrinos using a detector that produces a light signal when a neutrino interacts with it. They then compared their results with data obtained from other experiments and theoretical calculations. In some cases NEOS results agreed with the previous data, but in other cases they differed. For example, the data show that there is an unexplained abundance of neutrinos with energy of 5 MeV (Mega-electron Volts), dubbed ‘the 5 MeV bump,’ much higher than the one predicted from theoretical models.

The experiment succeeded in measuring electron neutrinos with great precision and low background signals. However, sterile neutrinos were not detected and remain some of the most mysterious particles of our Universe. The results also show that it is necessary to set up new limits for the detection of sterile neutrinos, since the oscillations that convert electron neutrinos into sterile neutrinos are probably less than previously thought.

“These results do not mean that sterile neutrinos do not exist, but that they are more challenging to find than what was previously thought,” explained Dr. Oh Yoomin, one of the authors of this study.

Other groups hoping to detect neutrinos are the Super-Kamiokande group in Japan, the Daya Bay Group in China and the Jiangmen Underground Neutrino Observatory (JUNO) set to be completed in 2020.

The article can be found at: Ko et al. (2017) Sterile Neutrino Search at the NEOS Experiment.


Source: Institute for Basic Science.
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