The standard model of the Physical , universally admitted and very precisely checked by the Expérience, postulates that the Univers is a gigantic building set. Each object is composed of an assembly of a few fundamental blocks: the elementary particles .

The idea wanting that the Matière is made up of fundamental components is very old. The Greek S of the Antiquité introduced the word “ atom ”, which means “ indivisible ”, to refer to such components.

We know now that the Matière is made up of objects called Atome S . Those were originally considered as being indivisible, i.e. like the smallest particle which is. However, it was discovered that the atom was composed of smaller particles: the electron, the Proton and the Neutron. Later, still, it was discovered that these two last are themselves only assemblies of particles even smaller, the Quark S. This article reviews these particles as well as the others which it is necessary to introduce to describe precisely their interaction S.

Accelerators

In the Years 1930, the scientists thought that the electrons, the protons and the neutrons were the smallest objects in what the matter could be divided. They were indicated as “elementary particles” thinking that they were indivisible; new “atoms” according to the original term.

To study the interaction of the neutrons and the protons in the core of the atom, the physicists built particle accelerator . In an accelerator, particles are accelerated by electric fields with an aim of making them enter in collision. The energy of these collisions produces all kinds of particles which are then detected.

For the accelerators, it was necessary a few decades to realize that there was still another level of structure inside the protons and the neutrons. Those were composed of under-particles which one baptized Quark S. the protons and the neutrons are built starting from three quarks each one. These composite particles are almost always represented in a perfectly spherical form but the latter represents only the area of the space beyond which composite nature these particles becomes visible. In the standard model, proton and neutron strictly speaking do not have a form .

Until now, no substructure was discovered with the quarks and the electrons. They are thus the new elementary particles.

But the history does not stop with these quarks and the electrons. The observation of several hundreds of different, composite and often unstable particles, made it possible to the physicists to deduce the existence from a certain number of other elementary particles. The description of the basic components of nature and their interactions is summarized in a called physical theory the “standard model” of the particles.

The standard model

This standard model explains what the matter is done and how its components interact. In the standard model, there exist three main categories of elementary particles: quarks, the Lepton S and the Boson S of gauge.

All the known particles are made up of quarks and leptons (collectively named Fermion S), and they interact by exchanging bosons of gauge. Thus, all the matter of the Universe, the water molecules to the galaxies while passing by the living organisms, is made of quarks and leptons. But it is not all the history there. The quarks have properties quite different from the leptons  ; and moreover, for each kind of particle, there exists a particle of corresponding antimatter.

Antimatter

For each type of particle, there is a Antiparticule - it is what is called the symmetry. The antiparticles are in all points similar to the corresponding particles - except that all their characteristics - except the mass - have an opposed sign, like the load. For example, a proton is positively charged whereas an antiproton is negatively charged. By combining antiprotons, antineutrons and anti-electrons, it is possible to create anti-atoms. Moreover, the physicists already endeavoured to build atoms of anti-hydrogen, more recently in significant amounts (50  000 atoms) in the laboratories of CERN.

When a matter particle and its antiparticle meet, they are destroyed completely and are transformed into energy. The collisions between particles and antiparticles thus produce much energy and are usually used in experiments within the accelerators.

The antimatter has one very short lifespan in our environnement : unless it is not insulated by magnetic fields, it meets the ordinary matter quickly and is destroyed then.

The first particle of antimatter was discovered in 1933. It was about a Positron (anti-electron) produced by the meeting between a Cosmic ray and an atomic nucleus of the atmosphere.

Quarks

In 1964, Murray Freezing-Mann and George Zweig discovered independently that hundreds of particles could be explained by combinations of only three elements. Freezing-Mann chooses the name “quarks” to indicate these elements. This word was invented by James Joyce in his novel Finnegans Wake (this novel abounds in imaginary words and viol voluntarily the linguistic rules). It is only with beginning of the year 70 that the physical reality of these quarks was proven, and that they reached the row of particles.

We know now that there are six kinds or savors of quarks. They were nicely baptized, by order of increasing masses: up, down, strange, charm, bottom and signal . Moreover, for each one of these quarks, there is a antiquark corresponding.

The quarks have the strange property to have a fractional electric charge. This load is of 2/3 for the quarks up, charm and signal and of - 1/3 for the quarks down, strange and bottom.

The quarks are sociable particles: one never finds of them one which is alone. They are held out of packages of two or three to form particles called Hadron S. For example, the proton is a high-energy particle made up of two quarks up and a quark down . As for the neutron, it is made of two quarks down and a quark up . This property makes that the particles observed in a free state have a whole a whole or null electric charge.

The formed particles of quarks and antiquarks are called Hadron S. They are divided into two classes:

  • the Baryon S, made of three quarks, like the neutrons ( N ) or the protons ( p ),
  • the Meson S, made of a quark and a antiquark.

Leptons

The other elementary particles forming the matter are the leptons. There are also six kinds, or savors of leptons, of which three have a negative electric charge and three are neutral. But, unlike the quarks, one lepton can only be found. One does not know in 2007 so fundamental bonds connect 6 savors of leptons and those of quarks.

The most known lepton is the electron ( E ). The two other leptons charged are the Muon (μ) and the tau (τ). They are much more massive than the electron. The three leptons without electric charge are the Neutrino S (ν). There is a savor of neutrino associated with each lepton charged: an electronic neutrino (ν E ), a muon neutrino (νμ ) and a neutrino tauonic (ντ ).

The existence of the electronic neutrino was predicted by Wolfgang Pauli in 1932, but it is only in 1956 that it was discovered. Meanwhile, the muon was observed (in 1936) in the reactions between the atmosphere and the cosmic rays. Nothing let predict its existence, at this point that Isidor Isaac Rabi, a physicist of the particles, accommodated the news while requiring: “  But which ordered this trick-là ?   ”. The surprise made place with a thorough research which was going to lead to discovered other leptons.

The neutrinos were very difficult to see because they almost do not interact with the matter. It is necessary to build underground observatories, far from any disturbance, to be able to detect some neutrinos per day. However, the Sun emits an enormous quantity of neutrinos. Billion neutrinos solar crosses your body at each second!

Three families of elementary particles

All the elementary particles which we saw until now are called Fermion S. the researchers realized that the elementary fermions could be classified in three families. Each family contains two quarks, one lepton charged and her neutrino. From one family to another, the properties of the particles are similar, except for their mass. These masses are increasingly high first with the third family.

The first family contains the most stable particles and most current: the quarks up and down , the electron and the ν E . In the second family, one finds the quarks charm and strange as well as the muon and the νμ. The quarks signal and bottom , the tauon and the ντ train the third family.

Absolutely all that exists results from the fitting from these 12 particles or their antiparticles.

Bosons of gauge

“How do they hold together? ” The answer results in the interaction from the four physical forces: the Revolved, the strong nuclear force, the weak nuclear force and the electromagnetic Force. These forces act on the elementary fermions by the exchange of bosons of gauge , the other class of elementary particles. One calls also bosons of gauge of the “  particles of rayonnement  ”.

There are 12 bosons of gauge in the standard model: the Photon, 8 Gluon S and 3 weak bosons. Moreover, one predicts the existence of the Graviton which was not observed yet. Each boson of gauge is associated with a force:

  • the photon transmits the electromagnetic force,
  • gluons them transmit the strong nuclear force,
  • the weak bosons transmit the weak nuclear force,
  • the role of the graviton is to transmit the gravitational force.

Let us note that the graviton does not form part of the standard model. Its existence is purely theoretical and no experiment still showed its presence.

The boson of Higgs

The standard model predicts the existence of a very special particle: the Boson of Higgs.

In the beginning, the theory of the standard model considered that all the elementary particles had a null mass. It was obviously nonin conformity with reality. The scientists could establish in experiments the masses of several particles with good precise details. Only the photon, let us gluons them and the graviton would be of null mass.

To correct the model, Peter Higgs proposed, towards the end of the Années 1960, to add another particle to it: a boson conferring the masses on all the other particles.

The basic idea is that the particles acquire a mass while interacting with an omnipresent field (the field of Higgs) carried by this famous boson of Higgs. This mechanism is now regarded as an essential part of the standard model and the existence of boson of Higgs is capital for the theorists. Moreover, the physicist Leon Lederman called it “the God particle” (the particle God). There is one problem: the boson of Higgs was never yet detected.

The detection of boson of Higgs is the current challenge of the Physique of the particles. If no laboratory that point reaches from here 2007, the new accelerator of the CERN (LHC) to Geneva, which will be in function on this date, should bring an definitive answer on the existence of boson of Higgs.

Beyond the standard model

The standard model is a good theory. Many experiments validated its predictions with incredible precise details and all the postulated particles were found. A theory, after the philosopher Karl Popper, is regarded as valid as long as she was not refuted. The standard model resists all the experimental refutations.

However, this theory does not explain all and several questions remain unanswered. For example: Why there are exactly 12 fermions and 4 forces? How the gravitation can be included in the model? The quarks and the leptons or has really fundamental a substructure (beyond the 10-18 meters)? Which are the particles which form the Matière sinks in the Universe?

To answer these questions, the physicists hope on the construction of new particle accelerators being able to probe increasingly large energies (physics known as Terascale ). Also, several theorists dream of a news and ultimate theory being able to unify all the physical phenomena. Several see the solution in the Théorie of the cords which stipulates that all the elementary particles are modes of vibration of a fundamental cord. This cord would exist into 10 (1st theory), 11 (the Théorie M), up to 26 dimensions (in 2 of the 5 theories pre theory M) nothing less.

See too

Internal bonds

  • List of particles
  • Material
  • periodic Matter
  • Table of the elements

External bonds

  • For more technical data:

    • Site of the group of data on the particles (Particle Dated Group), international collaboration of compilation, evaluation and data processing on the particles. Site-mirrors with Durham (the United Kingdom), Gënes (Italy), or with the CERN (Switzerland & France)

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