Elementary particles

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Elementary particles of the Standard Model

! Quarks	! Exchange particles
! Leptons	! Higgs boson

Elementary particles are indivisible subatomic particles and the smallest known building blocks of matter . From the point of view of theoretical physics , they are the lowest levels of excitation of certain fields . According to today's knowledge, which has been secured by experiments and summarized in the standard model of elementary particle physics , there is

six types of quarks each with three different color charges ,
six types of leptons , namely three charged leptons and three neutrinos ,
twelve types of gauge bosons , viz
- a photon for the electromagnetic interaction
- eight gluons for the strong interaction ,
- three bosons ( Z ⁰ , W ⁺ , W ^- ) for the weak interaction
and the Higgs boson .

This results in 37 elementary particles. Taking the antiparticles into account, the number of known elementary particles increases to 61, since there are another 18 anti-quarks and six anti-leptons (on the other hand, the antiparticles of the eight gluons are already included, the photon , Z ⁰ and Higgs boson are each theirs own antiparticle and W ⁺ / W ^- are their mutual antiparticles).

The matter and the force and radiation fields of the strong, the weak and the electromagnetic interaction consist of these particles in different compositions and states. In the case of the gravitational field and the gravitational waves , the underlying particles - the gravitons (G) - have so far been hypothetical ; in the case of dark matter , they are still completely unknown.

The named particles are small in the sense

that one has not yet been able to obtain any clues for a non-zero diameter from experiments. Theoretically, they are therefore assumed to be point-like.
that, according to the current state of knowledge, they are not composed of even smaller sub-units.
that even a small object of everyday life already contains trillions (10 ²¹ ) of these particles. For example, the head of a pin already consists of 10 ²² electrons and 10 ²³ quarks.

Clarification of the term

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

Until the discovery of the quarks, all types of hadrons were also considered elementary particles, e . B. the core building blocks proton , neutron , the pion and many more. Because of the large number of different species, one spoke of the "particle zoo". Even today, the hadrons are often referred to as elementary particles, although according to the standard model they are all composed of quarks. B. also have a measurable diameter of the order of 10 ⁻¹⁵ m. To avoid confusion, the elementary particles listed above according to the standard model are sometimes referred to as fundamental elementary particle or fundamental particles called.

History and overview

matter

Until well into the 20th century, philosophers and scientists alike disputed whether matter was a continuum that could be subdivided infinitely, or whether it was made up of elementary particles that could not be further broken down into smaller pieces. Such particles were called “atom” from ancient times (from Greek ἄτομος átomos , “indivisible”), the name elementary particle (or English elementary particle ) did not appear before the 1930s. The earliest known philosophical considerations about atoms come from ancient Greece ( Democritus , Plato ). Based on scientific knowledge, this term was first filled with today's content around 1800, when, after John Dalton's work in chemistry, the insight began to prevail that every chemical element consists of particles that are identical to one another. They were called atoms; this name has held up. The diverse manifestations of the known substances and their possibilities of transformation could be explained by the fact that the atoms combine in different ways to form molecules according to simple rules . The atoms themselves were considered immutable, especially indestructible. From 1860 onwards this picture led to a mechanical explanation of the gas laws in the kinetic gas theory through the disordered heat movement of many invisible small particles. From this u. a. the actual size of the molecules can be determined: they are many orders of magnitude too small to be visible in the microscope.

Nevertheless, in the 19th century this picture was referred to as a mere "atom hypothesis " and criticized for reasons of principle (see article Atom ). It only found general approval in the context of modern physics at the beginning of the 20th century . Albert Einstein achieved a breakthrough in 1905. He derived theoretically that the invisible small atoms or molecules, due to their thermal movement, collide irregularly with larger particles that are visible under the microscope, so that these too are in constant motion. He was able to quantitatively predict the type of motion of these larger particles, which was confirmed from 1907 by Jean-Baptiste Perrin through microscopic observations of Brownian motion and the sedimentation equilibrium. This is considered to be the first physical proof of the existence of the molecules and atoms.

At the same time, however, the observations on radioactivity showed that the atoms, as they were defined in chemistry, cannot be regarded in physics as either immutable or indivisible. Rather, the atoms can be subdivided into an atomic shell of electrons and an atomic nucleus , which in turn is composed of protons and neutrons . Electron, proton and neutron were then considered to be elementary particles, soon together with numerous other types of particles that were discovered in cosmic rays from the 1930s ( e.g. muon , pion , kaon as well as positron and other types of antiparticles ) and from 1950 in experiments at particle accelerators.

Due to their large number and confusing properties and relationships to one another, all these types of particles were grouped under the name “particle zoo”, and there were widespread doubts whether they could all really be elementary in the sense of not being composed . The first characteristic for a classification was the distinction between hadrons and leptons in the 1950s . The hadrons like protons and neutrons react to the strong interaction , the leptons like the electron only to the electromagnetic and / or weak interaction . While the leptons are still considered elementary today, “smaller” particles, the quarks , could be identified in the hadrons from the 1970s . The six types of quarks are the really elementary particles according to the Standard Model , from which, together with gluons, the numerous hadrons of the particle zoo are built.

Fields

Physical fields such as the gravitational field, the magnetic field and the electric field were and are viewed as a 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 worked out by Albert Einstein in 1905 in the form of the light quantum hypothesis . According to this, 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 in an electromagnetic radiation field exactly as if it would collide with a single particle there. In 1926 this electromagnetic quantum was given the name photon .

To 1930 on the basis of quantum mechanics , the quantum electrodynamics developed that describes the emergence of a photon emission in the process and its destruction in the absorption process. In the context of this theory, it emerges that the known static electric and magnetic fields are also due to the effect of photons, which, however, are generated and destroyed as so-called virtual particles . The photon is the field quantum of the electromagnetic field and the first known exchange particle that causes one of the fundamental forces of physics to come about.

This resulted in two further developments: The formation and destruction of particles such as electrons and neutrinos observed in beta radioactivity was interpreted as an excitation or weakening of an "electron field" or a "neutrino field", so that these particles are now also viewed as field quanta of their respective field (see quantum field theory ). On the other hand, exchange particles were searched for and found for other basic forces: the gluon for the strong interaction (proven 1979), the W boson and Z boson for the weak interaction (proven 1983). For gravity, the fourth and by far the weakest of the fundamental interactions , there is still no recognized quantum field theory. Although all particles are subject to gravity, the effects that are theoretically to be expected from reactions of the elementary particles are considered to be unobservable. Gravitation is therefore not dealt with in the standard model, especially as an associated field quantum, the graviton , has so far been purely hypothetical.

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

List of elementary particles

Division into fermions and bosons

Elementary particles
Elementary fermions ("matter particles")		Elementary bosons
Leptons	Quarks	Calibration bosons ("force particles")	Higgs boson
ν _e , ν _μ , ν _τ , e ^- , μ ^- , τ ^-	d, u, s, c, b, t	g , γ , W ^± , Z ⁰	H ⁰

First of all, a distinction is made between the two classes of fermions and bosons for elementary particles (as well as for composite particles) . Fermions have a half-integer spin and obey a law of conservation of particle number, so that they can only arise or perish together with their antiparticles. Bosons have an integer spin and can be created and annihilated individually. With a view to the conservation of matter in everyday life and in classical physics, the fermions among the elementary particles are therefore often seen as the smallest particles of matter and are also referred to as matter particles . The bosons among the elementary particles, on the other hand, are associated with fields because a field strength can vary continuously in classical physics. Bosons are therefore often referred to as quanta of force or radiation fields, or briefly as field quanta. However, in quantum field theory, the fermions are also field quanta of their respective fields. Of the elementary particles in the Standard Model, the leptons and quarks belong to the fermions and the exchange particles as well as the Higgs boson (and - if it exists - the graviton) belong to the bosons.

Leptons

Leptons are the elementary particles of matter with spin that are not subject to the strong interaction. They are fermions and take part in the weak interaction and, if electrically charged, in the electromagnetic one. ${\ displaystyle {\ tfrac {1} {2}}}$

Electric charge	generation
Electric charge	1	2	3
−1	Electron (s)	Muon (μ)	Tauon (τ)
0	Electron neutrino (ν _e )	Muon neutrino (ν _μ )	Tauon neutrino (ν _τ )

There are three electrically charged leptons (charge = −1e): the electron (e), the muon (μ) and the tauon (or τ lepton) (τ) and three electrically neutral leptons: the electron neutrino (ν _e ) , the muon neutrino (ν _μ ) and the tauon neutrino (ν _τ ). The leptons are arranged in three generations or families : (ν _e , e), (ν _μ , μ) and (ν _τ , τ). Each family has its own number of leptons, which is always preserved except for neutrino oscillations .

For each of these types of leptons there is a corresponding type of antiparticle , which is generally identified by the preceding syllable anti- . Only the antiparticle of the electron, which was the first antiparticle discovered, is called the positron . It never occurs in observations that when an antilepton is generated, a lepton is not also generated or another antilepton is not destroyed. It describes this situation as conservation of lepton number (also Leptonenladung called) are employed for each lepton and for each antilepton , the total value of remains constant. The preservation of the number of leptons applies to all processes of creation and destruction of leptons and antileptons. Theories beyond the Standard Model have speculated about possible violations of this law, but they have not yet been observed and are therefore hypothetical. ${\ displaystyle L}$ ${\ displaystyle L = + 1}$ ${\ displaystyle L = -1}$ ${\ displaystyle L}$

The only stable leptons are the electron and the positron. Muons and tauons decay spontaneously by transforming themselves into a lighter lepton with the same electrical charge, a neutrino and an antineutrino, via the weak interaction. Alternatively, tauons can decay into a neutrino and hadrons.

Quarks

Quarks are the elementary particles of matter with spin , which in addition to the weak and electromagnetic interaction are also subject to the strong interaction. They are fermions and, in addition to weak isospin (depending on their chirality ) and electrical charge, also carry a color charge . ${\ displaystyle {\ tfrac {1} {2}}}$

Electric charge	generation
Electric charge	1	2	3
+ ² ⁄ ₃ e	up (u)	charm (c)	top (t)
- ¹ ⁄ ₃ e	down (d)	strange (s)	bottom (b)

There are three types of quarks with the electric charge e: down (d), strange (s) and bottom (b), and three types of quarks with the electric charge e: up (u), charm (c) and top (t) . Thus, one also knows three generations or families for quarks : (d, u), (s, c) and (b, t). As with the leptons, the families differ greatly in their masses. Conversions of quarks take place due to the weak interaction, preferably within a family (e.g. c ⇒ s). These conversions are described by the quark mixing matrix. ${\ displaystyle - {\ tfrac {1} {3}}}$ ${\ displaystyle + {\ tfrac {2} {3}}}$

When creating or destroying quarks or antiquarks, the same strictness applies to the preservation of the baryon number (also called baryon charge) as with the leptons (see above ): if you set for each quark and each antiquark , the total value of the baryon number remains with all known physical ones Processes constant. The choice of the value is explained by the fact that the core building blocks protons and neutrons were each assigned the baryon number 1 long before it was discovered that they were made up of three quarks. Here, too, theories beyond the Standard Model speculate about possible violations of the conservation of the baryon number, but they have not yet been observed and are therefore hypothetical. ${\ displaystyle B}$ ${\ displaystyle B = {\ tfrac {1} {3}}}$ ${\ displaystyle B = - {\ tfrac {1} {3}}}$ ${\ displaystyle B}$ ${\ displaystyle {\ tfrac {1} {3}}}$

Quarks are never observed freely, but only as bound components of the hadrons (see section “Compound particles” below).

Exchange particles (gauge bosons)

Particle	Rest energy (GeV)	Spin ( ) ${\ displaystyle \ hbar}$	Electric charge ( ) ${\ displaystyle e}$	mediated interaction
photon	0	1	0	electromagnetic force
Z ⁰ boson	approx. 91	1	0	weak force
W ⁺ boson	about 80	1	+1
W ^- boson	about 80	1	−1
Gluons	0	1	0	strong force (color force)
( Graviton )	0	2	0	Gravity

The exchange particles are the bosons that mediate the interactions between the aforementioned elementary particles of the fermion type . The name gauge boson is explained by the fact that the Standard Model is formulated as gauge theory , where the requirement for local gauge invariance means that interactions with exchange particles are predicted that have spin 1, i.e. are bosons .

The graviton has not yet been proven in experiments and is therefore hypothetical. However, it is often listed in connection with the other exchange particles, which reflects the hope that in future particle physics models the gravitational interaction can also be treated in terms of quantum field theory. The graviton properties given in the table below correspond to what is to be expected according to the general theory of relativity .

photon

As the field quantum of the electromagnetic field, the photon is the longest known gauge boson. It can be created or destroyed by any particle with an electrical charge and mediates the entire electromagnetic interaction . It has neither mass nor electrical charge. Because of these properties, the electromagnetic interaction has an infinite range and can have a macroscopic effect.

W and Z bosons

There are two W bosons with opposite electrical charges and the neutral Z boson. They can be created and destroyed by any particle with a weak isospin or weak hypercharge and mediate the weak interaction . They are therefore responsible for all the transformation processes in which a quark changes into another type of quark, or a lepton into another type of lepton. They have a large mass, which limits their range as exchange particles on the order of 10 ⁻¹⁸ m. This extremely short range is why the weak interaction appears weak. Unlike the photon, the W bosons also carry weak isospins themselves . Thus they can also interact with each other via the weak interaction.

Gluon

Gluons can be generated and destroyed by the particles with a colored charge and mediate the strong interaction between them . In addition to the quarks, the gluons themselves also carry a color charge, each in combination with an anti-color charge. The possible mixtures fill an eight-dimensional state space, which is why one usually speaks of eight different gluons. Two of the eight dimensions belong to states in which the gluon carries the anti-color charge exactly matching the color charge; these gluons are their own antiparticles. The gluons have no mass and neither electrical charge nor weak isospin. As carriers of colored charges, they also interact with one another. This property is the cause of the confinement , which effectively limits the range of the strong interaction to about 10 ⁻¹⁵ m. This is roughly the diameter of the hadrons made up of quarks (such as protons and neutrons) and also the range of the nuclear force that holds the protons and neutrons together in the atomic nucleus.

The Higgs boson

The Higgs boson is an elementary particle predicted by the Standard Model , which was discovered at the European nuclear research center CERN . It can be generated and destroyed by all particles with mass and is the field quantum of the omnipresent Higgs field , which gives these particles their mass in the first place. The Higgs boson has spin 0 and is not a gauge boson.

Particles composed of elementary particles

Compound particles
Particle group		Examples	Explanation
Hadrons			consist of quarks (and gluons )
Mesons			Hadrons with integer spin ( bosons )
	Quarkonia	J / ψ , Υ , ...	heavy quark and its antiquark
	other q q	π , K , η , ρ , D , ...	generally a quark and an antiquark
	exotic	Tetraquarks , Glueballs , ...	partly hypothetical
Baryons			Half-integer spin hadrons ( fermions )
	Nucleons	p , n , N resonances	Baryons from u and d quarks with isospin ¹ ⁄ ₂
	Δ-baryons	Δ ⁺⁺ (1232), ...	Baryons from u and d quarks with isospin ³ ⁄ ₂
	Hyperons	Λ , Σ , Ξ , Ω	Baryons with at least one s-quark
	other	Λ _c , Σ _c , Ξ _b , ...	Baryons with heavier quarks
	exotic	Pentaquarks , ...	consisting of more than three quarks
Atomic nuclei			Baryons bound by strong interaction
	normal	d , α , ¹² C , ²³⁸ U , ...	consist of protons and neutrons
	exotic	Hyper nuclei , ...	other systems
Atoms			electromagnetically bound
	normal	H , He , Li , ...	consist of an atomic nucleus and electrons
	exotic	Positronium , muonium , ...	other systems

Particles composed of quarks (and gluons) are called hadrons . Until the discovery of the quarks and the development of the Standard Model from around 1970, they were considered elementary particles and are still often referred to as such today. Hadrons are divided into two categories: mesons and baryons .

Atomic nuclei are also made up of quarks and bound by the strong interaction, but are not referred to as hadrons.

Mesons

Mesons have integer spin, so they are bosons . They are binding states of a quark and an antiquark. All mesons are unstable. The lightest meson is the pion , which, depending on the electrical charge, transforms into leptons or photons ("decays"). In the Yukawa theory, pions are considered to be exchange particles of the nuclear forces with which protons and neutrons are bound in the atomic nucleus.

Baryons

Baryons have half-integer spin, so they are fermions . They are bond states from three quarks (analogous to antibaryons from three antiquarks). The only stable baryons are the proton and the antiproton. All others are unstable in and of themselves and eventually transform into a proton or antiproton, possibly via intermediate steps. The most important baryons are the proton and the neutron . Since they are the components of the atomic nucleus, they are collectively referred to as nucleons .

Atomic nuclei

Atomic nuclei are bound systems of baryons due to the strong interaction. Normally they consist of protons and neutrons - only such atomic nuclei can be stable. The smallest stable system of this kind is the atomic nucleus of heavy hydrogen, which is called the deuteron and consists of one proton and one neutron, i.e. six quarks. Usually the proton is also one of the atomic nuclei, since it represents the nucleus of the hydrogen atom . If one or more nucleons are replaced by other baryons, one speaks of hyper nuclei . Due to the short range of the strong interaction, the mean distance between the baryons in the atomic nucleus is not much larger than their diameter.

Atoms

Atoms are bound by the electromagnetic interaction, which usually consist of a (heavy) atomic nucleus and (light) electrons. If a nucleon in the atomic nucleus and / or an electron in the shell is replaced by particles of a different kind, an unstable exotic atom is created . In the 19th century, before the internal structure of atoms was discovered, the atoms themselves were sometimes referred to as the elementary particles of the chemical elements.

Stability and lifespan

Of the elementary particles of the Standard Model, only the electron, the positron, the photon and neutrinos are stable in a free, isolated state.

In the case of quarks and gluons, it is difficult to speak of stability because they cannot be isolated. They only appear in several together in hadrons. In it, they are constantly being transformed from one species to another by the strong interaction that holds them together. The stability of the proton or many other atomic nuclei is only valid as a whole, but not for the individual quark or gluon contained therein. A neutrino of one of the three types of neutrinos shows a periodically changing mixture of the three types with the neutrino oscillation , but certain mixtures of the different types of neutrinos, the three mass eigenstates , are stable. (The same applies to the respective antiparticles.)

The other elementary particles and their antiparticles are unstable in the ordinary sense of the word: they spontaneously transform into other particles of lower mass. The radioactive decay law applies , and based on radioactive decay , one speaks of the decay of the particles here too , especially since one particle always gives rise to two or three others. However, the decay products were not in any way already present in the original particle. Rather, it is destroyed in the disintegration process, while the disintegration products are regenerated. The average lifetime of the unstable elementary particles is between 2 · 10 ⁻⁶ s (muon) and 4 · 10 ⁻²⁵ s (Z boson).

The stability of elementary particles such as the electron, or of bound systems such as the proton, atomic nucleus or atom, is generally explained in the Standard Model by the fact that there is no path of decay that is not forbidden by one of the general conservation laws. It follows from the law of conservation of energy that the sum of the masses of the decay products cannot be greater than the mass of the decaying particle or system. With the conservation law of the electric charge it follows that electron and positron are stable because there are no lighter particles with the same charge. For the stability of the proton (and other nuclei, but also of the antiproton etc.) one of the two conservation laws for the baryon number or the lepton number must also be used. Otherwise the positron (the electron in the case of a negative electrical charge) would be a possible decay product for all positively charged elementary particles. However, the separate conservation laws for quarks and leptons are abolished in some theoretical models beyond the standard model. The stability of the proton is therefore checked in experiments. Decays of protons have not yet been observed; the average lifespan of the proton, if it is finite at all, is at least 10 ³⁵ years according to the current status (2017) .

Properties of all elementary particles

The following applies in the standard model:

All elementary particles can be created and destroyed. Apart from their force-free movement through space, creation and annihilation are the only processes in which they participate. These are therefore also the basis of every interaction. Otherwise, however, the particles are completely unchangeable in their internal properties. In particular, they are not divisible and have no excited states.

All elementary particles of the same kind are identical ; H. indistinguishable. At best, one can distinguish between the states that such particles are currently assuming. On the other hand, it is fundamentally impossible to determine which of several identical particles had or will assume a certain state at an earlier or later point in time (see Identical Particles ).

All charged elementary particles have antiparticles that are completely identical to them in all properties, except that they carry opposite charges. The four uncharged elementary particles photon, Z ⁰ boson, Higgs boson and two gluons are their own antiparticles. One particle and one antiparticle of the same kind can annihilate each other. Nothing but all of your energy, momentum and angular momentum are retained. These are transferred to newly created particles (see pair annihilation , pair creation ).

All elementary particles appear point-like. They only assume states in which they have a spatially extended probability of their presence (see wave function ). With increasing expenditure of energy, however, this type of spatial expansion can be pushed below any previously ascertainable limit without any change in the internal properties of the particle. The corresponding experiments have advanced furthest with electrons and have reached the range 10 ⁻¹⁹ m.

All elementary particles remain members of the same type of particle until the next interaction . The neutrinos are a certain exception: A neutrino occurs in the form of one of the three observable types mentioned above, but has partially converted into another of these types by the next interaction of an interaction ( neutrino oscillation ). This periodically changing mixture of the three observed species is explained by the fact that there are theoretically three unchangeable neutrino types with different, precisely defined masses, while the three observed neutrino types are three certain mutually orthogonal linear combinations thereof. Strictly speaking, the three observed species do not have a sharply defined mass, but a mass distribution.

The invariable internal properties of every elementary particle are

its rest energy ( mass ),
its spin (intrinsic angular momentum, which always has the same size, possibly also in the rest system of the particle. The value zero only applies to the Higgs boson.)
its internal parity (defined as positive for particles and negative for antiparticles)
its lepton number (value +1 for every lepton, −1 for every antilepton, zero for all other particles)
its baryon number (value (for historical reasons) for every quark, for every antiquark, zero for all other particles) ${\ displaystyle + {\ tfrac {1} {3}}}$ ${\ displaystyle - {\ tfrac {1} {3}}}$
its electric charge (if it has the value zero, the particle is not involved in the electromagnetic interaction .)
its weak isospin (if it has the value zero and the particle also has no electrical charge, the particle is not involved in the weak interaction .)
its color charge (if it has the value zero, the particle is not involved in the strong interaction .)

Generation and destruction as the basis of all processes

The standard model only envisages the creation and destruction of elementary particles as possible processes. First three examples to explain this far-reaching statement:

Deflection of an electron: A simple change in the direction of flight of an electron is resolved into a process of annihilation and creation: the electron in its initial state is annihilated and an electron with the momentum in the new direction is generated. Since electrons are indistinguishable particles, the question of whether “it is still the same electron” is meaningless. Nevertheless, this process is usually paraphrased linguistically in such a way that "the" electron has only changed its flight direction. The standard model only allows this process, which combines annihilation and generation, if an exchange particle is also involved. This is either absorbed (destroyed) or emitted (generated) and in any case has such values of energy and momentum that both quantities are retained overall. The exchange particles in question in this example are the photon, the Z boson and the Higgs boson. All others are ruled out: gluons are out of the question because the electron is a lepton and therefore does not carry a color charge; W bosons are ruled out because of the strict conservation of the electrical charge, because they are charged; when they arise or disappear, their charge would have to appear in one of the other two particles involved. But the electron has the same charge before and after the deflection.

Decay of a Z boson into an electron-positron pair: A Z boson is destroyed, an electron and an anti-electron (positron) are generated. The total electrical charge is retained because the electron-positron pair is together neutral, like the original Z boson.

Conversion of a down quark into an up quark: The down quark is destroyed, the up quark is generated, an exchange particle must be generated or destroyed. In this case it not only has to compensate for the (possible) change in momentum and energy of the quarks, but also the conversion of the electrical charge from to . This means that only the W boson with the correct charge sign comes into question: if it is generated, it has the charge , otherwise . Again, this combination of annihilation and creation of quarks is linguistically referred to as the conversion of a quark into a quark of a different type. (This process is the first step of beta radioactivity . The emitted W ^- boson is not stable, but is destroyed in a second process step, whereby a suitable pair of fermions is generated. In beta radioactivity, it is an electron, the beta radiation, and an electron antineutrino.) ${\ displaystyle - {\ tfrac {1} {3}}}$ ${\ displaystyle + {\ tfrac {2} {3}}}$ ${\ displaystyle -1}$ ${\ displaystyle +1}$

All of these are examples of a “three-way vertex”, because three particles are always involved in these elementary process steps, two fermions and one boson each. In this context, the word vertex stands for a certain combination of creation and destruction processes. It comes from the graphic symbolic language of Feynman diagrams , in which each particle is represented by a short line. The lines of the particles involved in a process meet at a common point, the vertex, where they end (for annihilation) or begin (for creation). Lines for fermions (including antifermions) must always appear in pairs, either for leptons or for quarks, but not mixed. The third line must always describe a boson. Particles and antiparticles must be involved in such a way that the total number of leptons or baryons is retained. There are also 3-way vertices and 4-way vertices with only bosons. For other sizes that must be retained for each vertex, see conservation law .

The action of one fermion on another, e.g. B. the mutual repulsion of two electrons is described as a two-step process, i.e. with two 3-way vertices: in one vertex, an electron generates a photon, which is absorbed by the other electron in the other vertex. It is said that the electrons exchange a photon, from which the term exchange particle is derived. In general, every interaction between two fermions consists in the fact that exchange particles are exchanged. According to the rules of quantum field theory, the exchange particle evades direct observation; it remains a virtual particle . Regardless of this, it transmits momentum and energy from one particle to another and thus causes z. B. the change in the flight directions of the particles. This is an observable effect, as it is caused by a force in classical physics .

Interactions and charges

The Standard Model deals with three fundamental interactions :

The strong interaction comes from the color charge . Only quarks and gluons carry a color charge.

The electromagnetic interaction is based on the electrical charge . All electrically charged particles, i.e. all quarks, half of the leptons, both W bosons take part in it, and additionally the photon , although it is uncharged.

The weak interaction has no charge in analogy to the electric charge or the color charge. All particles whose weak isospin or whose electrical charge is different from zero take part in it, i.e. all particles except for gluon, photon and the right-handed neutrino .

The fourth basic force, gravity , acts on all elementary particles because all particles have an energy. In particle physics, however, it is mostly left out of consideration because of its low strength, especially since there is still no quantum theory of gravity. So is z. B. the graviton , the associated field quantum, so far purely hypothetical.

Mass (rest energy)

Based on Einstein's equation E = mc ² , the mass of a particle corresponds to an energy value, the rest energy . Since in particle physics an energy is usually given in electron volts (eV), the unit of mass is eV / c ² . As a rule, natural units are used , in which case the quotient “c ² ” can be omitted from the specification and the mass can be specified in eV.

The masses of the elementary particles range from 0 eV / c ² ( photon , gluon ) to 173 GeV / c ² ( top quark ). For example, the mass of the proton is 938 MeV / c ² , that of the electron 0.511 MeV / c ² . With values of at most 1 eV / c ² , the neutrinos have the lowest non-zero masses. In the Standard Model , they were initially considered massless until neutrino oscillations were observed in 1998 . From the oscillation one can conclude that the three types of neutrinos have different masses. But they are so small that exact values could not yet be determined.

Spin

All elementary particles except the Higgs boson have an intrinsic angular momentum other than zero , also called spin. This can only occur in integer or half-integer multiples of the quantum of action and is called the spin quantum number of the particle. The spin is an intrinsic property of the particles, its amount cannot be changed, only its orientation in space can be changed. Leptons and quarks have the exchange particles , the Higgs boson . In general, the particles with integer spin form the particle class of bosons, those with half- integer spin the particle class of fermions. Bosons can be created and destroyed individually, such as B. individual light quanta; Fermions, however, only in pairs as particles and antiparticles. For further consequences of this fundamentally important distinction see boson and fermion . ${\ displaystyle \ hbar}$ ${\ displaystyle J}$ ${\ displaystyle J = {\ tfrac {1} {2}}}$ ${\ displaystyle J = 1}$ ${\ displaystyle J = 0}$ ${\ displaystyle J}$

More quantum numbers

Further quantum numbers of quarks and leptons characterize their affiliation to one of the six species and further conserved quantities, e.g. B. Isospin , Strangeness , Baryon number , Lepton number . Composite hadrons are the symbol simplified or o. Ä. In which the quantum number of the pins is necessary for the parity , which for the G-parity and the for the charge conjugation . ${\ displaystyle I}$ ${\ displaystyle S}$ ${\ displaystyle A}$ ${\ displaystyle L}$ ${\ displaystyle I ^ {G} (J ^ {PC})}$ ${\ displaystyle J ^ {PC}}$ ${\ displaystyle J}$ ${\ displaystyle P}$ ${\ displaystyle G}$ ${\ displaystyle C}$

Antiparticle

There are antiparticles for every kind of particle. In some properties, the particle and the corresponding antiparticle exactly match, e.g. B. in the mass, in the amount of spin, in the lifetime. They differ in the sign of all charges for which a conservation law applies. This concerns z. B. the electrical charge, the baryon and lepton charge. For example, the proton is positively charged and the antiproton is negative.

Particles without such retained charges, namely the photon and the Z boson, are their own antiparticle. The neutrinos are not included, because they are only electrically neutral, but carry the positive lepton charge as particles and the negative lepton charge as antiparticles. Neutrinos are therefore not identical to antineutrinos and also behave differently in the experiment. The two W bosons are a particle-antiparticle pair. A gluon is charged with one color charge and one anti-color charge, so that the associated antigluon is already included in the group of gluons.

Since a pair of particles and antiparticles taken together is neutral with regard to each of the charges obtained, such pairs can arise “out of nowhere” as long as the necessary energy is available to generate their masses ( pair formation ). For example, a photon (lepton number 0, electrical charge 0) can become a lepton (lepton number 1, electrical charge −1) and an antilepton (lepton number −1, electrical charge +1). From a minimum energy of 1.02 MeV it is an electron-positron pair, from 212 MeV a muon-antimuon pair is also possible. The opposite reaction also takes place: while electron and positron are each stable due to the retention of the lepton number or the retention of electrical charge, when they come together they annihilate each other within nanoseconds ( annihilation ) and leave behind nothing but their entire - in the form of suitable other elementary particles Energy content, i.e. at least 1.02 MeV, as well as - if not equal to zero - their total momentum and total angular momentum.

Hypothetical elementary particles

Further particles were postulated in theoretical models, some of which are plausible, but some are very speculative. These include:

A fourth generation of quarks and leptons.
The Axion and its super partners Axino and Saxion appear in extensions of quantum chromodynamics .
The lepto quark or X boson mediates between quarks and leptons and could be responsible for the excess of matter compared to the antimatter .
The sterile neutrino .
The graviton is expected as the mechanism of gravitational interaction for a theory of quantum gravity .
Supersymmetric theories postulate the existence of a whole class of bosonic “ super partners ” for the previously known fermionic particles and vice versa.

Remarks

↑ However, if the neutrinos are Majorana fermions , then these would be identical to their antiparticles.

Quotes

“The Standard Model is much more than a theoretical model of elementary particles and their interactions. It claims the rank of a self-contained theory of all phenomena observed in the world of elementary particles. For the initiated, the theory can be presented in a few lines, thus forming a kind of global formula that theoretical physicists such as Albert Einstein or Werner Heisenberg searched for without success in the past. "

- Harald Fritzsch

literature

C. Amsler et al. a .: Physics Letters. Row B. Amsterdam 667.2008.1. ISSN 0031-9163 ( Particle Data Group , PDG)
Summary tables. In: PDG. January 15, 2008, accessed September 30, 2008 .
Christoph Berger: Elementarteilchenphysik , Springer Verlag, 2nd edition 2006, ISBN 3-540-23143-9
Klaus Bethge and Ulrich E. Schröder : Elementary particles and their interactions - an overview. WILEY-VCH, Weinheim 2006. ISBN 3-527-40587-9
Harald Fritzsch : Elementary Particles. Building blocks of matter. Beck, Munich 2004. ISBN 3-406-50846-4
Henning Genz : Elementary Particles. Fischer, Frankfurt a. M. 2003. ISBN 3-596-15354-9
Bogdan Povh et al. a .: particles and nuclei. An introduction to the physical concepts. Springer, Berlin 2006. ISBN 3-540-36685-7
Jörn Bleck-Neuhaus: Elementary Particles . From the atoms to the standard model to the Higgs boson. 2nd, revised edition. Springer, Berlin Heidelberg 2013, ISBN 978-3-642-32578-6 , doi : 10.1007 / 978-3-642-32579-3 .

Web links

Wiktionary: Elementary particles - explanations of meanings, word origins, synonyms, translations

Fundamentals of particle physics
German Particle Physics Outreach and Info Pages
Particle physics in the world of physics
Austrian Particle Physics Outreach Pages (of the Austrian Physical Society )
Siegmund Brandt: In search of the smallest things (slides from a popular science lecture, pdf, 5.5 MB)
The Particle Data Group

Individual evidence

↑ Erhard Scheibe : The Philosophy of Physicists . 2nd Edition. CH Beck, Munich 2012, ISBN 978-3-406-54788-1 .
↑ Higgs particle is discovery of the year. In: sueddeutsche.de. December 21, 2012, accessed March 9, 2018 .
↑ Harald Fritzsch : Elementary particles - building blocks of matter. Beck, Munich 2004, ISBN 3-406-50846-4 , p. 13.

[1] However, if the neutrinos are Majorana fermions , then these would be identical to their antiparticles.

[2] Erhard Scheibe : The Philosophy of Physicists . 2nd Edition. CH Beck, Munich 2012, ISBN 978-3-406-54788-1 .

[3] Higgs particle is discovery of the year. In: sueddeutsche.de. December 21, 2012, accessed March 9, 2018 .

[4] Harald Fritzsch : Elementary particles - building blocks of matter. Beck, Munich 2004, ISBN 3-406-50846-4 , p. 13.