The exceptional smallness of the neutrino masses requires special explanation. The investigation of the neutrino oscillations, the neutrinoless double β-decay and β-spectrum of 3 H-decay, will allow one to obtain important information on the neutrino masses, neutrino mixture, and neutrino nature (Dirac or Majorana?). The investigation of the neutrino oscillations that is going on all over the world is a new field of research. The evidence for this phenomenon, predicted many years ago, was obtained recently. Because of the smallness of neutrino masses, new physical phenomenon, neutrino oscillations, the periodical transitions between different types of neutrinos in vacuum or in matter, become possible. The neutrino masses are many orders of magnitude smaller than the masses of their family partners (electron, muon, tau). The neutrinos are also exceptional particles with respect to their internal properties. They have enormous penetration properties and give us the unique possibility to investigate the internal structure of the nucleon, the internal invisible region of the sun where solar energy is produced. The neutrinos play a special role in the fields of particle physics and astrophysics. Bilenky, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 XVIII Conclusion It took the enormous Super-Kamiokande detector, filled with 50,000 tons of ultrapure water, to gather sufficient data: 5,400 atmospheric neutrino interactions.S.M. The explanation? Muon neutrinos were oscillating into different types of neutrinos, causing a slight excess of electron neutrinos and a deficit of muon neutrinos. Scientists working on a variety of experiments, notably the Super-Kamiokande experiment in Japan, found that many fewer muon neutrinos were arriving than predicted. In fact, it was deviation from this ratio that provided some of the evidence that neutrinos oscillate, or change flavor, as they travel. Thus, about two-thirds of atmospheric neutrinos are muon neutrinos and antineutrinos, and the remainder are electron neutrinos and antineutrinos. A muon is also unstable, so it will usually decay into an electron, electron antineutrino, and muon neutrino. These are unstable particles made of two quarks, and they rapidly decay into muons and muon antineutrinos (or antimuons and muon neutrinos). Short-lived particles called mesons form, most of them pions (disappointingly, they are not made of pie). When they strike an atomic nucleus in our atmosphere, there is a cascade of particles. These particles are typically protons, though they can also be helium or heavier nuclei. They form when a cosmic ray, an energetic particle from space, crashes into Earth’s atmosphere. Credit: Takaaki Kajita in the Proceedings of the Japan Academy, Series B, Physical and Biological Sciences ( 10.2183/pjab.86.303)Ītmospheric neutrinos are typically produced around 15 kilometers above Earth’s surface. These neutrinos are typically produced around 15 kilometers above the ground. This diagram shows cosmic rays interacting with an air nucleus in the atmosphere, producing atmospheric neutrinos.
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