Elementary particles

Elementary particles: This term is used to designate the smallest parts of matter, at the beginning of the century they were believed to be atoms , but advances in the area of ​​electronics and radioactivity have shown otherwise.


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  • 1 History
    • 1 The fundamental forces
    • 2 Types of elementary particles:
  • 2 Quarks
    • 1 Composition
    • 2 The search for quarks
    • 3 Meaning of quark
    • 4 Experimental discovery
  • 3 Substructure
  • 4 Antiquark
  • 5 See also
  • 6 Sources


In 1897 physicist JJ Thomson discovered the electron , a negatively charged particle, that is in the outer layers of the atom . In 1920 a positively charged particle called a proton was discovered, later in 1932 a neutrally charged particle called a neutron was discovered . These three classes of particles made up Bohr’s atomic model. Through this model, the atoms that make up matter, that is, the chemical elements, could be discovered.

But this theory did not explain the result of certain experiments, until some scientists postulated the existence of smaller particles. AM Dirac postulated that there was a particle with the same mass of the electron , but with a positive charge, the positron . This theory was demonstrated in [[1932] with cosmic radiation, when an electron and a positron collide, destroy each other, and produce annihilation radiation. The particles that disappear when colliding are called antiparticles, antiprotons were discovered in 1955, and antineutrons in 1956 . These works could find the antimatter , and later create it in the laboratories.

Scientists in charge of studying matter on a small scale have discovered many new particles, but most have a very short life and quickly become other particles or transform into radiation.

The properties of elementary particles are studied by bombarding nuclei with atomic or other particles, after accelerating them to give them great energy. The particles used are obtained from cosmic radiation or other times from an accelerator. Then they go through a detector where they collide with other particles, the collisions of these and the bodies resulting from these violent interactions, are seen as traces on a photographic plate, or as liquid in a bubble chamber, or as a cloud in a bubble chamber. fog or expansion.

At present it is known that these particles (more than one hundred) are made up of smaller ones called quarks, framed in 3 families. In other words, matter is made up of leptons (electron and its electronic neutrino) and quarks (up and down, in the proton and neutron , that is, in the nucleons).

With the detection of quark top in 1995 the task of explaining the origin of the mass of elementary particles begins for particle physicists. The first obstacle is the confirmation of the existence of the higgs particle, for which Europe is working to build the large particle accelerator LHC (CERN).

The fundamental forces

-The gravitational interaction or Gravity acts on all the particles conferring cohesion to the matter. It is long-range and is the dominant interaction between the bodies of the universe.

-Electromagnetic interaction is based on the attraction experienced by different sign charges. It is the interaction that allows atoms to be held together.

-The strong interaction acts inside the atomic nucleus and, despite the charges of the same sign repel, it allows to keep the protons and neutrons together .

-the weak interaction is responsible for the radioactive decay of some nuclei. At very high energies the weak and electromagnetic interaction seem to be intimately linked.

Types of elementary particles:


Electron, Muon, Tau and Neutrinos electronic, muonic, tauonic


Gluons, Photons, Bosons, Vectors, Bosons Z, Higgs and Gravitons (the latter have not been observed experimentally)

Quarks: There are six different types of quarks that particle physicists have named as follows:





top (top)


Quark structure neutron


Quark: Each of the six elementary particles, which with their antiparticles form the baryons and the mesons. The quarks are the only fundamental particles that interact with the four fundamental forces. The quarks are gluon-like particles in weight and size, this it is assimilated in the cohesion force that these particles exert on themselves. They are spin 1/2 particles, so they are fermions. Together with the leptons, they form visible matter.


Matter is made up of quarks and leptons, quarks form protons and neutrons, and leptons, things like electrons and their neutrino . There is also a third particle that acts by transmitting the forces that act on quarks and leptons, called bosons, which they form for example; the photon , which is in charge of transmitting the electromagnetic interaction. Hadrons have been found not to be elementary particles, and are made up of quarks.

There are 3 families of quarks and each one has two different types, each family has a mass that doubles the previous one. The first family consists of the up (u) and down (d) quarks, the quark up, has a mass of 4 million electron volts (MeV) approximately 1/250 times the mass of the proton, the down quark has 8 MeV.

The second family is formed by the quark strange (s) and charm (c) whose masses are 200 and 1,500 MeV respectively. The third family consists of the bottom (b) and top (t) quarks, the first has 5,500 MeV and the second 199 GeV. The electric charge of the quarks is fractional, the up quark has a fractional charge of 2/3 and the down quark of –1/3, the proton consists of an up quark and a down quark, which adds up to a total charge of 1 / 3, the neutron has two up quarks and one down quark, therefore it has zero charge. Only the first two families of quarks and leptons form ordinary matter, the rest are only fleetingly observed in large colliders.

The various quarks can be combined to create all known particles, except for leptons and bosons, so that the proton is udd, the neutron udd, etc. Furthermore, assuming that the mass of the dyu quarks is 4 MeV and that of the syc quarks, there is 150 MeV, a very good approximation of all the particles can be given. This does not explain why there are particles created only by 2 quarks, nor why they cannot be observed in isolation. It was necessary to introduce quantum numbers, such as color, whose values ​​are r, a, and v, and flavor, which characterize many particles.

The search for quarks

The structure of detectors used in laboratories around the world, dedicated to the study of new elementary particles, allows us to follow the path left by these in collisions and thus deduce the properties that characterize them. Since quarks cannot be isolated, their existence can only be deduced from traces of particles formed by three quarks (Bariones) or by a pair of quarks-antiquark (mesons).


The word was originally designated by Murray Gell-Mann as a meaningless word that rhymed with pork, [8] but without spelling. [9] Later, he found the word “quark” in a James Joyce book titled Finnegans Wake and hence his spelling was used:

Three quarks for Muster Mark! Sure he has not got much of a bark And sure any he has it’s all beside the mark.

From James Joyce’s Finnegans Wake

Gell-Mann said about this that:

In 1963 , when I assigned the fundamental constituents of nucleons the name quark, I had the first sound, without spelling, that might have been kwork. Then, in one of his occasional Finnegans Wake readings, by James Joyce, I came across the word quark in the phrase Three quarks for Muster Mark. So quark (which means, on the one hand, the cry of the seagull) was the clear attempt to rhyme with Mark, as with bark and other similar words. I had to find an excuse to pronounce it as well as kwork.

But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text often come from several sources at once, like the word portmanteau in Through the Looking Glass. Occasionally, the phrases that appear in the book are determined to refer to drinks in a bar. I argued, therefore, that one of the multiple resources of the phrase Three quarks for Muster Mark could be Three quarts for Mister Mark, in which case the pronunciation of “kwork” could be fully justified. In any case, number three fits perfectly in the path as the quark appeared in nature.

The phrase tres quarks (three quarks in English) fit particularly well (as mentioned in the quote) since at that time there were only three known quarks and so the quarks were in groups of three in the baryons.

In Joyce’s book, seabirds are given three quarks, quark takes on a meaning like the cry of the gulls (probably onomatopoeia, like quack (or what) for ducks). The word is also a play on words between Munster and its provincial capital Cork.

Experimental discovery

The notion of theoretical quark arises from the attempt to classify hadrons, now explained thanks to the quark model. Murray Gell Mann and Kazuhiko Nishijima independently performed that classification in 1964 . In the mid- 1960s there was some consensus that the proton was approximately 10–15 m in size with a smooth charge distribution within it. Analysis of certain properties of high-energy reactions of hadrons led Richard Feynman to postulate hadron substructures, which he called partons (because they were part of the hadrons).

The series of experiments at the SLAC (Stanford Linear Accelerator Center) between 1967 and 1973 aimed to study the electron-proton scattering and see the charge distribution on the proton . These experiments were very similar to those carried out by Rutherford years ago to confirm the existence of the atomic nucleus. SLAC is a linear particle accelerator where particles such as electrons can reach energies of up to 50 GeV, enough for them to pass through nucleons.

Theoretical analysis of inelastic collisions that took place between the electron and the proton had been worked on by James Bjorken. He considered several hypotheses to explain the shape function of the dispersion. Of all of them, the most speculative was to consider the proton made up of charged 1/2-spin point particles. When analyzing the data for different amounts of momentum transferred to the proton, Bjorken’s fit with such a hypothesis was found to be adequate. Quarks experimentally allowing had been discovered obtain the Nobel Prize in Physics in 1990 to Taylor Kendall and Alexandr Friedmann , leaders of the experiments at SLAC.

Later, other inelastic neutrino collision experiments at CERN served to confirm the SLAC results. Feynmann partons and quarks were confirmed to be exactly the same thing. With the 1973 test of asymptotic freedom in quantum chromodynamics performed by David Gross , Frank Wilczek, and David Politzer , the connection became stable. These scientists were awarded the Nobel Prize in Physics in 2004for this work. Kendall said of the finding: … the specific discovery was a discovery. We didn’t know if it would be there, and neither did anyone in this world – not the people who invented the quark, or the entire theoretical community. No one could say specifically and unequivocally: hey friends go around the quark. We hope it is in the nucleons.


New extensions of the standard model of particle physics indicate that quarks could be made up of substructures. This assumes that the elementary particles in the standard model of particle physics are composite particles; These hypotheses are being evaluated, although no such structure has currently been discovered. The so-called quark substructures are called preons.


The antiquark is the antiparticle that corresponds to a quark. The number of quark and anti-quark types in the matter is the same. They are represented with the same symbols as those, but with a bar above the corresponding letter, for example, if a quark is represented, an antiquark is written.


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