The elementary particles , also called fundamental particles are defined as subatomic particles they can not be divided into smaller substructures, ie particles that are not composed of other particles.
In this sense, an atom is formed by fermions , the family of elementary particles with mass whose combination gives rise to the baryonic matter or ordinary matter . Specifically, an atom would be formed by two subtypes of fermions : leptons and quarks :
- Electrons : they are an elementary particle of the lepton type . Orbit around the atomic nucleus.
- Neutrons and protons : are subatomic particles formed by a combination of quarks( hadrons ). They agglutinate and form the nucleus of the atom.
From classical subatomic particles to quarks
In the search for the particles that make up the material, one of the first to offer an atomic theory with scientific basis was John Dalton , whose ideas were collected in the so-called postulates of Dalton , published in the first years of the nineteenth century, between 1803 and 1807.
For Dalton, the atom was the fundamental particle of matter , but throughout the nineteenth century, and mainly throughout the twentieth century, they were discovering subatomic particles . First the electrons and then the protons and neutrons , unseating the atom as a fundamental particle.
Electrons, neutrons and protons were taken again as if they were the real elementary particles, until the discovery of the quarks showed that protons and neutrons were not elementary particles either, but could be decomposed into other particles.
- In 1897 JJ Thomson discovered the electron , today considered an elementary particle of the lepton type. Until this discovery, the atom was considered an indivisible particle.
- In 1905 Albert Einstein discovered the photon , now considered a type of boson(particle without mass that carries energy or outside interaction).
- In 1977 Ernest Rutherford discovered the proton and that most of the mass of an atom is concentrated in the nucleus.
- In 1932 James Chadwick discovered the neutron .
- In 1964 Murray Gell Mann proposed the existence of quarks , the elementary particle that forms neutrons and protons.
Thus, an atom is not currently defined as the elementary particle of matter, but as the smallest unit of baryonic matter (or ordinary matter) that has properties of a chemical element .
Fermions: the elementary particles of matter
With the data and current experiments, two major types of elementary particles have been described: fermions and bosons .
The fermions are particles with mass and elementary particles of matter are considered. For their part, bosons are massless particles that carry the elementary forces : gravity, electromagnetic force and strong and weak nuclear forces .
Fermions include other subtypes of particles, leptons and quarks . Leptons and quarks interact with each other thanks to the interaction forces that transport the bosons and form the subatomic particles : electrons, neutrons and protons.
The electrons are still considered an elementary particle in themselves, specifically a type of lepton, it has not been able to divide into smaller particles. Neutrons and protons are formed by a combination of three different quarks each .
In short, leptons and quarks are the smallest known elementary particles of matter.
The so-called standard model explains all the ordinary matter of the Universe with these particles or fermions, plus 5 types of bosons (4 gauge bosons, which carry the 4 elementary forces , and the Higgs boson, which would explain how the fermions acquire mass ).
We would have to discover the graviton , the hypothetical particle that gravity would carry. If discovered, the known elementary particles would add a total of 18 , including all the fermions and bosons.
Leptons are defined as fermions or particles with mass that do not experience strong nuclear interaction . There are six types of leptons, three with electric charge -1 ( electron, muon and tau ) and three without charge and much smaller mass ( neutrinos ).
- Electron : it is the only particle that continues to be considered fundamental since the subatomic particles were discovered. It has electric charge -1 and spin 1/2.
- Muon : is similar to the electron but heavier.
- Tau : is similar to the electron but even heavier. The muon and the tau are very unstable; in nature they appear for a very short time.
- Neutrino electron : particle without charge and very little mass, so it is very difficult to detect. But they can carry a lot of energy that they release when they collide with another particle and so they can be detected indirect
- Neutrino muon : particle with no charge and little mass but much heavier than the neutrino electron. They occur in the decay or disintegration of atomic particles, for example in beta decay .
- Tau neutrino : no load and heavier than the neutrino muon.
Electron, muon and tau are called “flavors,” and the neutrino electron / electron, muon / neutrino muon and tau / tau neutrino pairs are called weak doublets . The leptons, therefore, have 3 flavors and 3 doublets of particles.
Each lepton doublet consists of an electrically charged lepton, for example the electron, and a lepton of the same taste but without charge and much less mass, for example the neutrino electron
In addition, since each particle has its antiparticle , each lepton has its antilepton . For example, the antiparticle of the electron is the positron , and the antiparticle of the electron neutrino is the electronic antineutrino .
They are another group of fermions that, unlike the leptons, experience the strong nuclear force . One of the most peculiar characteristics of quarks is that their electric charge has non-integer values; The electric charge of the quarks is -1/3 or +2/3, depending on the type of quark.
Quarks are also the only particles that interact with all types of fundamental interactions : electromagnetic, strong nuclear interaction, weak nuclear interaction and gravity.
The quarks come in six flavors . 3 quarks have positive charge of +2/3 (u, cyt), and 3 have negative charge of -1/3 (d, syb).
- u (up) : positive electric charge of +2/3
- d (down) : load -1/3
- c (charm) : load +2/3, like the quark up but with more mass.
- s (strange) : load -1/3, like the heaviest down quark.
- t (top) : load +2/3, like quarks up and charm but even heavier.
- b (bottom) : charge -1/3, like the quarks dys but still heavier.
As with leptons, each type of quark has its antiquark (antiup, antidown, etc).
Thanks to strong nuclear interactions, quarks interact strongly with each other and form the hadrons , the elementary particles that make up nuclear matter. There are two types of hadrons, mesons and baryons .
The protons and neutrons of the atomic nuclei are two types of baryons formed by a combination of up and down quarks . The other quarks are unstable in nature and exist in very specific conditions for short periods of time.
- Neutron : formed by the combination of three quarks: 2 quarks d (down) and 1 quark u (up) . The sum of the two charges -1/3 of the quarks d, plus the charge +2/3 of the quark u, result in the neutral electric charge typical of the neutron.
- Proton : formed by the combination of three quarks: 2 quark or plus 1 quark d .
It is often said that quarks are the smallest particles in which matter can be divided , but in reality we can not know the size of the elementary particles, our technology does not allow it, but we can measure its mass, although in physics of particles the mass is usually understood in terms of energy, since matter is ultimately formed by energy .
In this sense, quarks are heavier than any lepton. The energy of the quarks goes from 4.5 GeV of the bottom quark to 0.003 GeV of the quark up, while the electron has a much lower energy: 0.000511 GeV (0.511 MeV).
And the neutrino has even less energy, 0.000000001 GeV, and even less mass, equivalent to one millionth of the mass of the electron .
Bosons: force-carrying particles
Bosons are massless particles responsible for what are known as fundamental interaction forces : electromagnetic force, gravitational force, strong nuclear force and weak nuclear force.
There are five types of confirmed bosons and a sixth, the graviton, still to be confirmed, which are grouped into three categories:
- Gauge bosons or vector bosons : gluon (strong nuclear force), photon (electromagnetic force), boson Z and boson W (weak nuclear force).
- Scalar bosons: Higgs boson (its interaction with the fermions is responsible for the fermions acquire mass ).
- Tension bosons : Graviton (hypothetical, to be confirmed)
According to the standard model of elementary particles , ordinary matter or baryonic matter (as opposed to dark matter) is formed by fermions (particles with mass) that interact with each other through the bosons (particles without mass).
Fermions are the particles that transport mass and bosons are the particles that transport interaction forces (gravity, electromagnetism, strong and weak nuclear forces). The mass of the fermions would be due to the interaction with the Higgs boson.
There are two types of fermions: leptons and quarks . Electrons are a type of lepton and the combination of quarks gives rise to hadrons , including neutrons and protons. Thus, an atom is formed by fermions of the two types:
- Electrons, a type of lepton
- Hadrons (neutrons and protons) formed by quarks