Axion

Birth of the axion

The problem strong CP

As “T Hooft showed it, the quantum Chromodynamique predicts that some strong interactions will violate the symmetry of load and parity. This violation was however never observed. In combination with the effects of the weak Interaction, the parameter which quantifies this violation, noted

\ bar \ Theta

, constituted a fundamental constant only accessible by measurement. In addition, of the interactions violating this parity in an important way would have provided to the Neutron S a Dipole moment intense. The observations, which prove the absence or the very low intensity of this effect, impose that the parameter

\ bar \ Theta

is very weak, even no one.

However, in theory, this parameter $\ bar \ Theta$ can take any value between 0 and 2π, and there existed no reason so that it is low or zero: it is what was called the “problem strong CP”.

The resolution of the problem

A first solution, natural, consisted in supposing that at least a Quark standard Modèle was of null Masse. Then, the parameter $\ bar \ Theta$ deviendait negligible and the problem of its value would be regulated. Nevertheless, measurements very quickly invalidated this theory by showing that no quark observed had of null or almost null mass.

In 1977, Roberto Peccei and Helen Quinn proposed a more elegant solution, by postulating a new symmetry which would lead naturally to a model QCD in which symmetry CP is not violated. A little later Frank Wilczek and Steven Weinberg noted that this symmetry could be interpreted differently: it is possible to make of this parameter $\ bar \ Theta$ a field, i.e. a Particule. They named it “axion”.

By registering the axion within a broader theoretical framework, by associating it with the spontaneous crack of a “symmetry of Peccei-Quinn” which causes the formation of the particle, one can make bringings together with the Boson of Goldstone, which makes axion a pseudo-boson of Nambu-Goldstone. It appears thus that the theory of the quantum Chromodynamique, provided with the axion, does not present more any violation of Symétrie CP.

It is as to note as the axions could be necessary to the Théorie of the cords.

Supposed properties

The axion is supposed to be a Boson. By basing on the cosmological influence that the axions would have, one can limit some their physical properties supposed: in particular, they would not have any electric Charge, a very small mass (of 10⁻⁶ with 10⁻² eV/c²) and a cross Section of interactions strong and weak reduced.

These properties imply that the axions interact little or not with the matter.

Their properties make that the axions can change into Photon S and vice versa when they are subjected to a fort Magnetic field — this phenomenon is put at contribution in the experiments aiming at detecting them: it is the Effet Primakoff. In March 2006, of the Italian researchers discovered unexpected rotations in the Polarization of photons subjected to strong magnetic fields. Some speculate whereas they are the consequence of the existence of the axions, but several problems are raised.

At the end of 2006, Piyare Jain and Gurmukh Singh affirmed to have detected a particle having the properties of the axion using photographic emulsions in July 2006. However, this discovery of particles of 6-20 one lifespan MeV about 10^-13 S, would not tally with the theoretical place of the axion in the cosmological model most common. Moreover, these experiments could not be reproduced and although the axion was regularly predicted, the fact of not having observed it in spite of collaboration between different particle accelerator carried out the physicists to give in doubt his existence.

In the supersymmetric theories, the axion has two Superpartenaire S:

a scalar superpartenaire: the saxion, analog of the Dilaton within the theoretical framework of Kaluza-Klein;
a superpartenaire ic Fermion: the axino.

In particular, the Masse of the axion is function of the temperature: in paramount plasma, it is predicted that the axion had a null mass, then that the particle became increasingly massive with the cosmic Inflation.

Experimental research

Many experiments tried to highlight the axion, having predicted its properties.

Project PVLAS

Italian project PVLAS projects polarized light through a Magnetic field of 5 Tesla, in the search of a rotation of the direction of polarization. If the axions exist, then the Photon S could interact and change into axions or virtual axions. It would follow a light anomaly of polarization from there.

This effect is very subtle and difficult to detect: to be able to measure it, the light carries out billion go-and-come through the magnetic field, which would cumulate the possible anomalies.

The most recent experiments revealed an abnormal rotation, which could be interpreted by the existence of an axion of mass ranging between 1 and 1,5 meV. One proposes nevertheless other possible sources explaining these observations, which utilize known phenomena.

Project ADMX

Project ADMX (Axion Dark Matter Experiment), led to Lawrence Livermore National Laboratory, research of the axions slightly interacting in the halation of black Matter supposed to wrap our Galaxy.

In the presence of a intense Magnetic field, the objective is to transform an axion into photon Micro-onde. The process is supported by the use of a resonant cavity from 460 to 810 MHz, presumedly adapted to the predicted mass of the axion.

Other experiments

Another manner of detecting the axions is to undertake experiments “through the wall”: by sending a beam of light through an intense magnetic field, one transforms some of them into axions. Those interact little with the matter, and can pass a thick wall whereas the photons are absorbed or reflected quickly. Other side of such a wall, a second magnetic field makes it possible the axions to change into photons, which one will be able to possibly observe.

The first transformation like the second have an extremely weak output, such an experiment requires consequently very intense luminous flows.

Cosmological implications

The theory suggests that the Big Bang created a multitude of axions. Because of a single coupling to the field Instanton during the paramount universe, a friction appears for the period when the Masse is acquired following the cosmic Inflation. The paramount axions lose all their kinetic energy thus.

If they have a low mass, which deprives them of any other mode of disintegration, it is predicted that the universe would be filled with rich condensates of Bump-Einstein of paramount axions at very low temperature. The axions could thus explain the black Matière missing. Several experiments took place in 2006 to determine the existence of the axions, but the sensors used are, in practice, incompetents to detect these particles. Experiment ADMX, calling upon a cavity microwaves, made it possible to eliminate from the axions of mass 10^-6 eV. The axions of higher mass than that predicted by Jain and Singh affirm to have found could not thus exist in the current universe and would not constitute a valid explanation to the black matter.

See too

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