The neutrino is a Elementary particle standard model of the Physique of the particles.

It has a Spin ½, it is thus a Fermion.

A long time its mass was supposed to be null. However, of the recent experiments (Super-Kamiokande) showed that this one, although very small, is different from zero.

The existence of the neutrino was postulated for the first time by Wolfgang Pauli to explain the continuous spectrum of the Beta decay as well as apparent the not-conservation of the kinetic moment.

History

In 1930, confronted with the problem of the Spectrum in energy of the Disintegration $\backslash \; beta$, Pauli invents the neutrino to satisfy the principle of conservation of energy. Fermi gives him the name of neutrino in 1933 by incorporating it in its theory weak interaction. The neutrino is discovered in 1956, (makes of it $\backslash \; overline\; \{\backslash \; nu\_e\}$) by Queens and Cowan near a nuclear reactor. In 1962 the $\backslash \; nu\_\; \{\backslash \; driven\}$ is discovered in Brookhaven. In 1990, the LEP, with CERN, shows that there are only three families of neutrinos. The neutrino $\backslash \; tau$ $\backslash \; nu\_\; \{\backslash \; tau\}$ is discovered in 2000 in experiment DONUT.

Physical characteristics

The neutrinos are elementary particles belonging to the leptons (fermions of spin ½ stripped of color, gathered in 3 families (savors)).

There exist three kinds, or savors of neutrinos:

• the electronic neutrino or neutrino electron $\backslash \; nu\_e$,
• the muon neutrino or neutrino Muon $\backslash \; nu\_\; \{\backslash \; driven\}$,
• the tau neutrino or neutrino Tau $\backslash \; nu\_\; \{\backslash \; tau\}$.

They are called according to the Lepton which is associated to them in the standard model. The neutrino has a zero load and its helicity is left (the spin points in the direction opposed to the movement; right helicity for the antineutrino) as showed it the experiment of Mr. Goldhaber and his colleagues in 1958. In the minimal standard model, the neutrinos do not have mass. However many experiments carried out since 1968 brought to the conclusion which the neutrinos are massive by the description of the mechanism of oscillation. The mass of the neutrinos is very weak, heaviest would have a mass lower than 0,23 eV according to the results of satellite WMAP with the current cosmological models and the results experiments on the oscillations.

The neutrinos not having an electric charge nor of color they only interact by weak Interaction (the Gravité although presents is negligible). Their cross section of interaction (its probability of interacting) is thus very weak because it is about a force with short range.

The cross section of a neutrino of 1 GeV compared with that of an electron and a of the same proton energy is roughly in the report/ratio $10^\; \{-\; 14\}\; /10^\; \{-\; 2\}\; /1$. On 10 billion neutrinos of 1 Mev which cross the Earth, only one will interact. One would need a thickness of a Année-lumière of Plomb to stop half of the neutrinos which cross it. The detectors of neutrinos thus contain typically hundreds of tons of a built material in such way that some atoms per day interact with the neutrinos entering. In a Supernova which crumbles, the Density in the core becomes sufficiently high (10¹⁴grams/cm ³) so that the produced neutrinos can be retained one brief moment.

Recent experiments, in particular that of Super-Kamiokande in 1998 (which accepted the Nobel Prize of physics 2002 on this occasion) and that led to the Observatoire of Neutrinos of Sudbury since 1999, showed that the neutrinos can, via a phenomenon called Oscillation of the neutrino , continuously to transform form of savor into another. This phenomenon is not possible that if the neutrinos have a mass and that this one is different for each savor. The discovery of this phenomenon made it possible to provide a solution to the Problème of the solar neutrinos.

Another problem in Astrophysique which related to the neutrinos is that of the dark Matière, the “missing” mass of the Univers according to certain theories. Indeed, the universe seems to contain much more matter than that which is detectable by the Rayonnement than it emits. This matter which does not emit a light, from where the term matter sinks , is however detectable by the influence nelle Gravitation which she exerts on the visible matter like the star S and the Galaxie S, and, until recently, one thought that if the neutrinos had a mass they could perhaps constitute the dark matter. However, according to current knowledge, the mass of the neutrinos is well too small so that the neutrinos can contribute to a significant fraction of the hypothetical dark matter.

According to current knowledge, the neutrinos were born there is approximately 15 billion years, shortly after the birth of the universe. Since, the universe did not cease extending, to cool and the neutrinos made their way. Theoretically, they form today a cosmic bottom of Rayonnement of temperature equal to 1,9 Kelvin (- 271,2 degrees Celsius). The other neutrinos which one finds in the universe are created during the life of stars or during the explosion of the Supernova E.

The major part of the energy released during the collapse of a supernova is radiated with far in the shape of produced neutrinos when the Proton S and the electron S combine in the core to form Neutron S. These collapses of supernova produce immense quantities of neutrinos. The first experimental proof of this was provided in 1987, when neutrinos coming from the Supernova 1987a were detected by the experiments American Japanese woman and Kamiokande and IMB.

Detectors of neutrinos

There are several types of detectors of neutrinos. Each one is composed of a great quantity of material located in an underground cave designed to protect it from the cosmic radiation.

• the detectors with chlorine were the first employees and are composed of a tank filled with Carbon tetrachloride (CCl). In these detectors, a neutrino converts a Atome Chlore into an atom of Argon. The fluid must be purged periodically with gas Hélium which removes argon. Helium must then be cooled to separate it from argon. These detectors had the major disadvantage that it was impossible for them to determine the direction of the entering neutrino. It was the detector with the chlorine of Homestake, in the South Dakota, containing 520 tons of liquid, which detected the first time the deficit of the neutrinos coming from the Sun and which led to the Problème of the solar neutrinos.

• the detectors with gallium are similar to the detectors with chlorine but are more sensitive to the neutrinos of weak energy. In these detectors, a neutrino converts the Gallium into Germanium which can then be detected chemically. Again, this type of detector does not provide any information on the direction of the neutrino.
• the detectors with ordinary water, such as Super-Kamiokande, contain a large pure water tank surrounded by detectors very sensitive to the light, tubes photomultipliers. In these detectors, a neutrino transfers its energy to an electron which moves then more quickly than the Lumière in this medium, but more slowly than the light in the vacuum, as the Theory of relativity envisages it. This produced a “known Shock wave optical” under the name of radiation Cherenkov which can be detected by the tubes photomultipliers. The advantages of this detector are to record the neutrino as soon as it enters the detector, and the possibility of obtaining information on the direction of the neutrino. It is this type of detector which recorded the “start” of neutrinos of the supernova 1987a.
• the detectors with heavy Eau employ three types of reactions to detect the neutrinos: same reaction as the detectors with light water, a reaction implying the collision of a neutrino with the neutron of a deuterium core, which releases an electron, and a third reaction in which the neutrino breaks a deuterium core in proton and neutron without itself to change nature. The results of these reactions can be detected by tubes photomultipliers and detectors of neutrons. This type of detector is in function in the Observatoire of neutrinos of Sudbury.

Current experiments

In 2004, various experiments of physics of the particles seek to improve knowledge on the oscillation of the neutrinos. In addition to the neutrinos created by the nuclear reactions in the Sun and those coming from the beta decay in the nuclear plants, the physicists also study neutrinos created in the particle accelerator (for example, the Japanese experiment K2K). The advantage of this type of experiment is to control flow and the moment when the particles are sent. Moreover, one knows their energy and the distance which they traverse between their production and their detection. One can thus place oneself at the extremums of the oscillations where the measurement of the parameters of oscillation is most precise.

Telescopes with neutrinos

Our sky was always observed using the Photons with energies very different energy from the waves radios to the gamma rays. The use of another particle to observe the sky would make it possible to open a new window on the Universe. The neutrino is for that a perfect candidate:

• it is stable and is not likely to disintegrate during its course;
• it is neutral and is thus not deviated by the magnetic fields. It is thus possible to roughly locate the direction of its source.
• it has a very weak cross section of interaction and can thus extirpate dense zones of the universe like the accesses of a Black hole or the heart of the cataclysmic phenomena (it should be specified that the Photon S which we observe of the celestial objects come us only from the surface of the objects and not of the heart).
• it interacts only by weak Interaction and thus transports information on the nuclear phenomena of the sources, contrary to the Photon which is resulting from electromagnetic processes.

A new complementary astronomy is thus being created since ten years.

One of the possible principles for such a telescope is to use the Earth like target making it possible to stop the astrophysical neutrinos. When a muon neutrino crosses the Earth, it is likely weak to interact and thus to generate a Muon. This Muon, if it has an energy beyond a hundred GeV, is aligned with the neutrino and is propagated on ten kilometers in the Earth. If it were created in the earth's crust, it will be able to leave the Earth and to be propagated in the sea where the telescopes with neutrinos would be installed. This going muon more quickly than speed of light in water, it generates light Tcherenkov, the equivalent for the light of the supersonic bang. It is about a bluish cone of light. This type of telescope with neutrinos consists of a three-dimensional network of detectors of photons (of the Photomultiplicateur S) which makes it possible to rebuild the Tcherenkov cone, and thus the trajectory of the muon and the incidental neutrino, and thus the position of the source in the sky. The current angular resolution is about the degree.

These telescopes with neutrinos are deployed in a great volume of liquid water or ice so that the light emitted by the muon is perceptible. Dimensions about the kilometer cubes to have a sufficient sensitivity to weak cosmic flows. They must be placed under kilometers of water for, on the one hand, being in the absolute darkness, and, on the other hand, to have a shielding with the cosmic rays which constitute the principal background noise of the experiment.

The telescopes with neutrinos, these immense volumes located at the funds of water and looking under our feet, constitute a major stage in the development of astrophysics of the particles and should allow new discoveries in astrophysics, cosmology, black matter and oscillations of neutrinos.

Neutrinos beyond the Standard Model

Since it is known that the neutrinos have a mass, the theorists developed many theories known as " with-delà" standard Model in order to explain this mass. One of the most promising models is the model of the " see-saw" , or " balançoire" in French. Into this model one introduces into the right theory of the neutrinos of Chiralité (one thus extends the contents in particles of the Standard Model from where name " with-delà") that one supposes very massive (well beyond the électrofaible scale). This last assumption is justified by the fact that one never observed them until now and by considerations of symmetry. Thus, one manages to explain the low mass of the left neutrinos, those which one observes until now. There exists indeed a very strong bond between the mass of the left neutrinos and that of the right neutrinos: they are inversely proportional. Thus more the right neutrinos are heavy, plus the left neutrinos are light. This model rests on the fact that one regards the neutrinos as particles of Majorana, fact which will be cancelled or confirmed in the next years by experiment NEMO studied double disintegration β without neutrino. One of the attractions of this model is that it could make it possible to explain asymmetry (rather to say dissymmetry since one speaks about " Crack of symmetry " , according to the language of Prigogine) matter/antimatter of our Universe. Indeed, one always wonders why the Universe contains (rather) matter, without (almost no) antimatter. Processes resulting from disintegration of the right neutrinos during time when the Universe was very young make it possible to include/understand this phenomenon. The implied processes are called the leptogénèse and the Baryogénèse.

Current confusion

It is heard sometimes that the neutrinos would move in certain mediums more quickly than the light . This sentence should not in no case to be interpreted like a violation of the restricted Relativité, which prohibits to exceed the Speed of light: in fact, the speed of the neutrinos in water is higher than that of the light in water, but lower than that of the light in the vacuum.

See too

• Oscillation of neutrinos;
• Problem of the solar neutrinos;
• Problem of the atmospheric neutrinos;

External bonds

• Characteristic of the neutrinos (Particle Dated Group)
• Dossier: the neutrino, élusive particle
• the neutrino of its discovery to its applications

Simple: Neutrino

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