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What is a neutrino?

 Automatic translation  Automatic translation Updated April 4, 2015

Neutrinos are subatomic particles belonging to leptons, components of matter called ordinary with quarks and electrons (see attached table).
The neutrino has a mass supposed to be null, but it has never been measured, however, it is recognized that it is not zero.
The neutrino is not sensitive to the strong interaction (nuclear force), for cons, it is sensitive to the weak interaction responsible for the disintegration of the atom, and perhaps to the electromagnetic interaction.
Hundreds of billions of neutrinos pass through our bodies every second, even a gigantic wall of lead can not stop neutrinos, which explains why they are very difficult to detect. However, occasionally, a neutrino collides with matter, they are the ones that scientists watch for their detectors.
Neutrinos are emitted in abundance by the stars during the collapse of a supernova.
Neutrinos travel at nearly the speed of light and interact very weakly with matter.
There are three flavors of neutrinos:
- The electron neutrino (νε), discovered in 1956 by Frederick Reines (1918 - 1998) and Clyde Cowan (1919 - 1974), accompanying the emission of an electron. It is emitted during the decay β-, i.e. during the transformation of a neutron into a proton.
- The muon neutrino (νμ) discovered in 1962 at Brookhaven. It is emitted during the decay of a muon (heavy electron).


- The tau neutrino (ντ) discovered in 2000 at Fermilab in Batavia, near Chicago. It is emitted during the decay of a tauon.
Only the electron neutrino is stable, others are unstable and decay very quickly to reach a stable particle.
Radioactive decay is the transformation of matter into energy, the number of radioactive nuclei decreases with time, it is governed by chance and its law is statistical.
Neutrino detectors are typically located deep underground or under the sea to avoid as much as possible, the sound of cosmic background. In the detector chlorine, a possible impact of a neutrino converts a chlorine atom in an argon atom. A detector gallium, a neutrino can convert a gallium atom, germanium atom.
The OPERA detector at the Gran Sasso in Italy, is used for particle physics experiments designed to study the neutrino oscillation phenomenon.

NB: inside an atom  there are nucleons, i.e. protons and neutrons, inside which there are quarks. The nucleus is surrounded by an electron cloud. The nature of matter is much more complex than previously thought in the 20th century. We now know that the particle world is extremely rich. To understand the infinitely great, man creates infernal machines (Tevatron, LHC,...), more and more powerful for "peel" the matter, to the confines of the infinitely small.

 constituents of matter, neutrinos

Image: Four of these elementary particles would be sufficient in principle to build the world around us: the up and down quarks, the electron and the electron neutrino. The others are unstable and decay to reach these four particles. The sources of neutrinos in nature are the Earth, the atmosphere, the Sun, supernovae, nuclear reactors and humans via the few mg of radioactive potassium in it. A human being emits ≈340 million neutrinos per day.

Neutrino and beta emission


In the world of subatomic particles that make up matter, we manipulate the smallest energies of nature, and extremely small lengths of the order of 10−15 to 10−17 meters, well below the size of an atom that is 10−10 meter. But we know that an atom is composed of 99.99% vacuum and it is at this scale that neutrinos are. The particles are not visible but they are detectable, however, if it applies sufficient energy, of the order of gigaelectronvolt (GeV). Energy and mass are two aspects of the same physical phenomenon, according to Einstein's famous equation (E = mc2), the mass can be converted into energy and vice versa. Because of this equivalence, mass and energy can be measured with the same unit. On the scale of particle physics there is the electronvolt (eV).
Radioactivity is a natural phenomenon that occurs in the nucleus in the depths of atoms. Nucleons are not all stable, they disintegrate from one state to another state of equilibrium. The decay is the transformation of matter into energy (E = mc2). Disintegrating the nuclei emit particles of different energies.
There are 3 types of decays:
- The alpha decay (α) emits charged particles (2 neutrons and 2 protons), responsive to the magnetic field. These particles do not pass through a sheet of paper.
- The gamma decay (γ) emits a particle, a non-visible photon has an energy of 1 GeV, 1 million times more energy than visible light photon. These particles only stop in front of a lead plate. These gamma photons have zero electromagnetic load and are therefore insensitive to the magnetic field.


- Beta decay (β) concerns neutrinos. This will happen during the decay of a nucleus, such as cobalt-60 that will transmute nickel 60 and during this transmutation, there will be emission of an electron and a neutrino or antineutrino.
It is the measure of the beta decay energies in 1931, led Wolfgang Pauli (1900 - 1958) to propose that the "missing" energy was taken away by another new particle, the neutrino.
The neutron is not yet discovered, it will be discovered by the British physicist James Chadwick (1891 - 1974) in 1932.
Beta minus decay, is the emission of an electron and an antineutrino accompanying the transformation of a neutron into a proton.
Beta plus decay is the transformation of a proton into a neutron, with emission a positron and a neutrino.
This is the weak nuclear force that is responsible for the decay of a neutron into a proton or a proton into a neutron without changing the number of nucleons. To balance the load, an electron or a positron is expelled from the nucleus. The emission of the electron is accompanied by an electronic antineutrino ∇e whereas the positron is accompanied by an electron neutrino νe.
Some beta minus emitters exist in nature:
- tritium 3 (3H+) which transforms into helium 3 (3He2+)
- carbon 14 (14C)  upon absorption of neutrons by nitrogen 14 (14N) in the stratosphere and troposphere upper layers.
- potassium 40 (40K) which transforms into calcium 40 (40Ca).

 beta decay

Image: example of beta decay to the radioactive isotope cobalt 60 (60Co). Cobalt core 60, contains 33 neutrons (gray) and 27 protons (red), it has an excess of 6 neutrons. A neutron will be transformed into a proton. Cobalt core 60 becomes more stable and is transformed into nickel 60 (60Ni+) with 28 protons (1 more) and 32 neutrons (1 less), but still 60 nucleons. During the disintegration, two particles are created, an electron and an electronic antineutrino. There are two variants of beta decay, beta minus decay, as here with cobalt 60 which is transmuted into nickel 60 with the emission of an electron and beta plus decay that emits a positron and an electron neutrino.

The lepton is an elementary particle on which the strong nuclear force has no influence. The electron, positron and neutrino are low mass particles grouped under the name of lepton, which in Greek means [Light]. Leptons are fermions (building blocks of the material), that is, the spin 1/2 particles. Charged leptons are the electron and the positron, the muon and its anti-particulates, tauon, and its anti-particles. In the standard model of particle physics, there are twelve flavors of elementary fermions: six quarks and six leptons. Unlike quarks leptons interact through the strong interaction. The electronvolt (eV symbol) is a unit of energy or mass. Its value is defined as the kinetic energy gained by an electron accelerated from the repose by a difference potential of 1 volt. An electron volt is about 1 eV = 1.602 176 565 (35) × 10−19 joule (J). According to special relativity E = m · c2, we deduce: 1 eV / c2 = 1.783 × 10−36. The mass of the electron is 511 keV / c2, the proton 938 MeV / c2 and that the neutron is 940 MeV / c2. An electron is an elementary particle of the atom, it is a universal constituent of matter that has a negative electric charge sign denoted e. The electron has a charge considered indivisible, e = 1.59 x 10-19 coulomb and a mass m = 9 x 10-28 gramme. An electron volts, eV = 1.602 x 10−19 joule. The concept of an indivisible amount of electric charge is developed from 1838 by the British naturalist Richard Laming (1798  − 1879). The Greeks noticed that amber attracted small objects when rubbed with fur. The Greek word ἤλεκτρον (electron) refers to amber. The electron was the first elementary particle demonstrated in many experiments from the late nineteenth and early twentieth centuries. When the neutrino passes through matter, its property (flavor) is changed. The neutrino oscillation is a phenomenon of quantum mechanics, in which a neutrino appeared with some flavor lepton (electron, muon or tau) can later get a different flavor. Theoretically, a neutrino can oscillate in three flavors.

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