Elementary particle

Other elementary particles are predicted by theories that go beyond the Standard Model. However, these are called hypothetical, because they have not yet been proven by experiments.

Until the discovery of quarks, all types of hadrons were also considered elementary particles, e.g. the nuclear building blocks proton, neutron, the pion and many others. Because of the large number of different types, one spoke of the "particle zoo". Even today, hadrons are often referred to as elementary particles, although according to the Standard Model they are all composed of quarks and also have, for example, a measurable diameter of the order of 10-15 m. To avoid confusion, the elementary particles listed above according to the Standard Model are occasionally referred to as fundamental elementary particles or fundamental particles.

Matter

Until the 20th century, it was disputed among philosophers as well as natural scientists whether matter was a continuum that could be infinitely finely divided, or was made up of elementary particles that could not be further divided into smaller pieces. Such particles have been called "atoms" from time immemorial (from Greek ἄτομος átomos, "the indivisible"), the name elementary particle not appearing until the 1930s. The earliest known philosophical reflections on atoms date back to Greek antiquity (Democritus, Plato). The term was first given its current meaning in the natural sciences around 1800, when John Dalton's work led to the realization that every chemical element consisted of identical particles. They were called atoms; this name has persisted. The manifold manifestations of the known substances and their possibilities of transformation could be explained by the fact that atoms combine in various ways to form molecules according to simple rules. The atoms themselves were regarded as unchangeable, especially as indestructible. From 1860 onwards, this picture led in the kinetic theory of gases to a mechanical explanation of the gas laws by the disordered thermal motion of many invisibly small particles. From this, among other things, the actual size of molecules could be determined: They are many orders of magnitude too small to be visible in the microscope.

Nevertheless, in the 19th century this picture was called a mere "atom hypothesis" and was criticized on principle grounds (see article Atom). It was not until the beginning of the 20th century that it found general acceptance within the framework of Modern Physics. A breakthrough was made by Albert Einstein in 1905, who deduced theoretically that the invisibly small atoms or molecules, due to their thermal motion, collide irregularly with larger particles already visible under the microscope, so that these are also in constant motion. He was able to predict quantitatively the nature of the motion of these larger particles, which was confirmed from 1907 by Jean-Baptiste Perrin through microscopic observations on Brownian motion and sedimentation equilibrium. This is considered the first physical proof of the existence of the molecules and atoms.

At the same time, however, observations on radioactivity revealed that atoms, as they had been defined in chemistry, cannot be regarded in physics as either immutable or indivisible. Rather, atoms can be divided into an atomic shell of electrons and an atomic nucleus, itself composed of protons and neutrons. As a result, the electron, proton and neutron were considered to be elementary particles, together with numerous other types of particles discovered in cosmic rays from the 1930s onwards (e.g. muon, pion, kaon, positron and other types of antiparticles) and in experiments at particle accelerators from 1950 onwards.

Because of their large number and confusing properties and relations to each other, all these types of particles were grouped together under the name "particle zoo", and there were widespread doubts as to whether they could all really be elementary in the sense of not being composed. The first feature to emerge for classification in the 1950s was the distinction between hadrons and leptons. Hadrons such as the proton and neutron react to the Strong Interaction, while leptons such as the electron react only to the Electromagnetic and/or Weak Interaction. While the leptons are still considered elementary today, from the 1970s onwards it was possible to identify "smaller" particles in the hadrons, the quarks. The six kinds of quarks are the really elementary particles according to the standard model, of which together with gluons the numerous hadrons of the particle zoo are built up.

Fields

Physical fields such as the gravitational field, the magnetic field and the electric field were and are considered to be continuum. That is, they have a certain field strength at every point in space, which can vary spatially and temporally in a continuous manner (i.e. without jumps). The discovery that elementary particles also play a role in the electromagnetic field was prepared by Max Planck in 1900 and elaborated by Albert Einstein in 1905 in the form of the light quantum hypothesis. According to this hypothesis, free electromagnetic fields that propagate as waves can only be excited or weakened in jumps of the size of an elementary quantum. That these electromagnetic quanta have all the properties of an elementary particle was recognized from 1923 as a result of the experiments of Arthur Compton. He showed that a single electron behaves exactly as if it were colliding with a single particle in an electromagnetic radiation field. In 1926, this electromagnetic quantum was given the name photon.

Around 1930, quantum electrodynamics was developed on the basis of quantum mechanics, which describes the creation of a photon in the emission process and its annihilation in the absorption process. Within the framework of this theory, it follows that the known static electric and magnetic fields are also due to the action of photons, which, however, are created and annihilated as so-called virtual particles. Thus the photon is the field quantum of the electromagnetic field and the first known exchange particle which causes the occurrence of one of the basic forces of physics.

Two further developments resulted from this: The creation and annihilation of particles such as the electron and neutrino observed in beta radioactivity was interpreted as the excitation or weakening of an "electron field" or a "neutrino field", so that these particles are now also regarded as field quanta of their respective fields (see quantum field theory). On the other hand, exchange particles were searched for and found for other fundamental forces: the gluon for the strong interaction (proven in 1979), the W-boson and Z-boson for the weak interaction (proven in 1983). For gravitation, the fourth and by far the weakest of the fundamental interactions, no accepted quantum field theory exists yet. Although all particles are subject to gravitation, the effects that can theoretically be expected in the reactions of elementary particles are considered to be unobservably small. Therefore, gravitation is not treated within the framework of the Standard Model, especially since a corresponding field quantum, the graviton, is purely hypothetical so far.

The Higgs boson is the field quantum of another new type of field that was inserted into the quantum field theory of the unified electromagnetic and weak interaction (electroweak interaction) in order to be able to formulate the fact that particles with mass exist in a theoretically consistent way. A new type of particle corresponding to these expectations was found in 2012 in experiments at the Large Hadron Collider near Geneva.