The easiest way to conceive a neutrino is to imagine ian electron which has lost its electrical charge.
For that reason, the neutrinos of radioactivity are called electron-neutrinos by physicists. This appellation allows to distinguish rhem from to two other kinds of neutrinos, muon-neutrinos and tau-neutrinos.
The electron-neutrino, together with the electron, is one of the two actors of beta radioactivity, an almost invisible actor, extremely difficult to detect. In the most common case of beta-minus radioactivity, it is not the neutrino which occurs instead, but its antiparticle, tan antineutrino-electron. The antineutrino-electron is the partner of the positive electron or positron.
The electron-neutrino is the electron partner in the family of the 4 fundamental constituents of matter. The other two members of this family are the so-called up and down quarks, the elementary constituents of protons and neutrons and therefore the ultimate constituents of the atomic nucleus. Electron, electron-neutrino, quarks up and down quark form in the eyes of physicists the first generation of elementary particles.
What distinguishes the quarks from the electron and its neutrino is that the former are sensitive to the very strong interactions that cement nuclei. Thus quarks are difficult to observe because they remain captive in nuclear matter.
Electrons and neutrinos are not sensitive to the strong interaction. They take off to discover the world ! But neutrinos are furthermore electrically neutral : they do not have the electrical charge that allows electrons to form the electronic cloud that surrounds the atom nucleus. They are only sensitive to the forces responsible in particular of the beta radioactivity, that the physicists call weak interactions.
It has long been assumed that neutrinos have no mass (like photons). Recent experiments have shown that they have a extremely small mass. To give an idea, the mass of the neutrino-electron is much less than the millionth of the mass of the electron, a particle already very light.
Practically massless corpuscles, neutrinos travel at the speed of light. This is how in 1987, was observed, together with a tiny flash of light coming from a distant supernova explosion, the arrival of a whiff of neutrinos ... after a journey of 180,000 light years !
Like the electrons, neutrinos have two possible states of rotation (or spin) that are called left and right-handed helicity, when the axis of rotation is chosen parrallel to the direction of motion. But neutrinos interacts only through the weak forces, which distinguishes the two helicity states (a phenomenon that physicists call "parity violation"). Only left-handed neutrinos are produced and observed. Right-handed neutrinos do not interact.
Neutrinos interacting with matter only through these weak forces, their probability (called cross-section) to interact is extremely low. To give an idea of this smallness, in their experiment leading to the discovery of the neutrino in 1956, Reines and Cowan measured a cross section of 0,063 billionth of a billionth of barn.
The barn is the unity used by physicists to measure cross-sections. For instance, the cross-section of a neutron capture by a uranium-238 nucleus is of the order of a few barns. The neutrino energies in radioactive decays are generally below 1 MeV. Even the cross section of a 1 MeV neutrino , well above these Reines and Cowan measured, remains extremely low. This explains why neutrinos coming from radioactive decay occuring in the sun are able to cross through the Earth.
Physicist have found two other species of neutrinos, the muon-neutrino and the tau-neutrino. The muon neutrino is the companion of the muon, a heavy electron observed in cosmic rays. The tau-neutrino is the companion of the tau, a super-heavy electron (it weighs twice as much as the proton) discovered in 1975 by the American physicist Martin Perl.
A last aspect of these fascinating particles ! Under the effect of the weak interaction, neutrinos can transform themselves into other species. Physicists say that they "oscillate". So a neutrino born neutrino-electron neutrino in the sun can me detected as a muon-neutrino at its arrival on Earth. An observed deficit of electron-neutrino coming from the sun is one of the evidence for neutrino oscillations.
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