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The Standard Modèle of the Physique of the particles is a theory which describes the interactions strong, weak and electromagnetic, as well as the whole of the elementary particles which constitute the Matière. Developed between the years 1970 and 1973, it is a Quantum theory of the fields which is naturally compatible with the principles of the quantum Mécanique and of the relativity. To date (2007) the whole of the experimental tests of the three fundamental forces described by the standard model revealed an good agreement with the predictions. For as much, the standard model is not a complete theory of the fundamental interactions mainly because it does not describe the force of Gravitation. particle, force, Médiateur , i.e. it distinguishes from the families of particles by the forces to which they are sensitive, each force being exerted by means of mediators exchanged by the particles which are subjected there. --->

The standard model has 29 free parameters (including 10 to describe the parameters of mass of the neutrinos) which amongst other things describe the masses of the elementary particles like their various couplings. The value of each one of these parameters is not fixed by principles first but must be in experiments given.

Short history

Following Ernest Rutherford which showed that the Atome S consisted of a core, agglomerate of Proton S and of Neutron S , around of which turned of the electron S , of many experiments of atomic collisions took place, revealing hundreds of particles. To find itself there, the physicists tried to classify these particles.

To start, they made the distinction between particles (or quanta) of matter and fields. Then they classified the matter particles, by far most, in three categories according to their mass:

• the Lepton S (of the Greek leptos = light), like the electron or the Neutrino;
• the Meson S (of the Greek mesos = average), like the meson π;
• the Baryon S (of the Greek barys = heavy), like the proton or the neutron.

Protons and neutrons were described as Nucléon S because of their crucial role in the atomic nuclei and their close masses. The others let us baryons were called hyperons .

The physicists noted in addition that to each one of these particles a Antiparticule corresponded of the same mass, but whose other characteristics were opposite (for example, with the proton corresponds a Antiproton of negative electric charge, and to the electron corresponds a Positron of positive electric charge…)

They discovered then that mesons and baryons were in fact of the composed particles, that they gathered then under the term of Hadron S (of the Greek hadros = extremely).

They thus led to the Standard Model, organized around the triptych ( quantum of matter , quantum field , quantum of associated field ) already mentioned above.

Matter particles or quanta

The elementary particles of matter are Fermion S. the fermions obey the Statistique of Fermi-Dirac; they are thus of Spin half-entirety (2k + 1)/2 and are subjected to the Principe of exclusion of Pauli.

The matter elementary particles are divided in leptons and Quark S , according to three generations which does not differ one from the other than by the mass, higher with each generation. Only the particles of first generation form the ordinary matter. Indeed, the protons are made of two quarks up and a down , while the neutrons are made of a quark up and two down . The particles of second and third generations are unstable and disintegrate quickly in particles of first generation, lighter.

Here a table gathering by generation the various leptons and quarks. Not to overload this table, the antiparticles are not represented there.

Table of the principal particles First generation

Second generation

Third generation

(*): With the difference in the case of the quantum electrodynamics the weak and strong loads are not numbers strictly speaking but representations of the groups $SU\; (2)$ and $SU\; (3)$ which mathematically respectively describes the weak interaction and the strong interaction. Thus for example $\backslash \; bold\; \{1\}$ indicates the commonplace representation what means that the particle is not not charged under the corresponding group.

The Quark S cannot exist separately. They are generally presented in the form of pairs quark-antiquark (mesons), or of trios of quarks (baryons them), but it is not always the case: recent experiments revealed formed particles of four quarks and a antiquark, wrongly called pentaquarks .

Fundamental forces of the universe

They are four:

1. the force of Gravitation: she is exerted on all the particles proportionally with their Masse;

2. the electromagnetic Force: she is exerted on the matter particles electrically charged;
3. the weak nuclear force: it relates to only certain quarks and leptons and is responsible for the radioactivities β^- and β⁺.
4. the Force of color, which is exerted between the quarks, and from which the Nuclear force derives, which ensures the cohesion of the atomic nucleus;

These four forces are described respectively by four theories:

1. the General relativity,

2. the quantum electrodynamic ,
3. the électrofaible theory (makes some, it associates weak force and electromagnetic force and thus includes the quantum electrodynamics),
4. the quantum Chromodynamique,

three last being gathered in the “standard model”.

Particles or quanta of field

For each fundamental force, there exist particles, known as of field, supports of these forces. These are Boson S, i.e. that they obey the Statistique of Bump-Einstein. The bosons have a whole Spin and can coexist between them in the same quantum state.

The particles of field can be real or virtual . In this last case, they last one extremely short existence and are observed indirectly by their action, which primarily consists in transmitting the fundamental forces. It is besides why these virtual particles are also called particles messengers or mediators .

Different the bosons are:

• the Graviton (of spin 2), mediator of the force of gravitation (it was not observed until now);
• the Photon “γ” (of spin 1, and mass and zero load), mediator of the electromagnetic force;
• 3 Boson S intermediaries (of spin 1 and high mass), known as also weak bosons , mediators of the weak force: bosons “ W ⁺”, “ W ^- ” and “ Z ⁰”;
• 8 Gluon S (of spin 1 and null mass), mediators of the force of color.

For these particles, it is necessary to add one or more bosons of Higgs (of Spin 0, which are scalar fields), supposed to confer to them Masse on the other particles by a mechanism of spontaneous Brisure of symmetry called within this framework the Mécanisme of Higgs. These bosons yet were not officially detected, although one suspects of having seen their trace in certain collisions observed with CERN. Their existence will be in theory definitively established or refuted within the framework of the new experiments installation at the LHC which will be put in servicing 2007.

Defects of the standard model

Three families of fermions

The standard model does not predict why there exist three generations of fermions carrying the same loads, but in a range of very different mass. The mass of the quark U is about MeV whereas that of the T is about 170 GeV. In addition, nothing says that there do not exist other families. It should be noted that until now no theory beyond the standard model explains in a precise way the existence of these three families. The Unitarité of the Matrice CKM is a significant test of the existence of another generation of fermions.

Problems of gauges

The Lagrangian one of gauge of the standard model is composed of three internal symmetries to the particles $U\; (1)$, $SU\; (2)$ and $SU\; (3)$. In the same way that for the families of fermions, nothing prohibits the existence of under groups of symmetries. This is besides a expensive subject with the theories of great unification, which make it possible in theory to explain these symmetries by including them like sub-groups of a group broader than the three first. The mathematical group $SU\; (5)$ could have been appropriate and it is on him which the theory of the Great Unification ( GUT in English) rested. But this symmetry of gauge complicated the standard model while obliging to postulate 24 bosons.

Critics of the standard model

According to Alain Connes, " nobody thinks that the standard model is it fine word of the history especially because of the very great number of free parameters that it contient."

Notes and references of the article

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