Fermions (named after Enrico Fermi ), all particles which satisfy the Fermi -Dirac statistics in the physical sense. After the spin - statistics theorem, they have a half-integer spin, ie, etc. Figuratively speaking, fermions are those particles that make up matter.
The fermions are:
- Among the elementary particles: leptons (eg the electron and the neutrino ) and the quarks.
- Among the composite particles: including all which are made up of an odd number of quarks, such as all baryons, which include the proton and the neutron.
Fermions differ from the bosons, which satisfy the Bose -Einstein statistics and after the spin- statistics theorem have an integer spin. An elementary particle in three spatial dimensions is always either a fermion or a boson.
In very thin layers, ie two-dimensional systems, there are, bosons and fermions, the so-called anyons that meet its own quantum statistics.
Fermions obey the exclusion principle Pauli'schen, which states that two fermions can not accept the same place at the same time an identical quantum state. In general, the quantum mechanical wave function of two or more similar fermions must be completely antisymmetric in exchange of two fermions, ie the sign changes ( phase factor -1).
When applied to the electrons in an atom explains the Pauli principle that can not satisfy all of the electrons in the same ground state, but in pairs to populate the various atomic orbitals of an atom. Be explained only by this property of the systematic structure of the periodic table of chemical elements.
In the standard model of particle physics, there is no elementary fermions with a spin greater than 1/2. A property of fermions with spin 1/2 is that their quantum mechanical wave function changes sign after a rotation of 360 °; after a rotation of 720 ° (ie, twice shot entirely ) the initial state is restored. This can be vividly with a clock compare: after a rotation of the hour hand to 720 ° you back the same day.
In extended about supersymmetry model of elementary particles, there are other elementary fermions. On each boson is calculated as a fermion supersymmetric partner, called Bosino, so that the spin is different in each case by ± 1/2. The superpartners of the bosons are identified by the suffix- ino in the name, it means, for example, the corresponding fermion to the (hypothetical) graviton then gravitino.
Strictly speaking, a fermionic field is initially assigned as super affiliates in the interaction picture each bosonic field. In the mass, the observable image or predicted particles give each as linear combinations of these fields. The number and the relative proportion of contributing to the mixtures components on the side of the fermionic super- partner does not comply with the conditions on the original bosonic page. In the simplest case (without or with only slight mix ) but can a boson (like the above-mentioned graviton ) a particular fermion or Bosino (such as the gravitino ) are assigned.
So far, none of the postulated supersymmetric partner particles has been demonstrated experimentally. You must therefore have such a high degree that they do not occur under normal conditions. It is hoped that the new generation of particle accelerators can prove at least some of these fermions directly or indirectly. With the lightest supersymmetric particle ( LSP), it is hoped to find a candidate for the dark matter of the universe.