page_title: Standard Model
section_title: Fermions
hierarchical_section_title: Standard Model / Particle content / Fermions
caption_attribution_description: English: This diagram illustrates the interactions among all particles in the Standard Model, including the presently hypothetical Graviton. Bosons are given vivid colors, while Fermions are given neutral shades. Massive particles are represented by balls with a 3D shading gradient, massless particles are represented by circles with solid fill. Lines represent a fundamental interaction between the two particles comprising the endpoints, and loops represent a self-interaction. The layout was determined by selecting the most symmetric result of arrangements constructed via trial-and-error after establishing a set of "rules": lines may intersect each other, but never intersect a node (particle); and the lines must be straight, having no curvature whatsoever (except for the loops representing self-interactions). Note that the illustration has very obvious fivefold symmetry (pentagon and pentagram); perhaps this implies that the underlying physical theory of a Standard Model containing the Graviton gives rise to this pattern and may exhibit fivefold symmetry itself.
context_page_description: The Standard Model of particle physics is the theory describing three of the four known fundamental forces in the universe, as well as classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists around the world, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, confirmation of the top quark, the tau neutrino, and the Higgs boson have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.
  Although the Standard Model is believed to be theoretically self-consistent and has demonstrated huge successes in providing experimental predictions, it leaves some phenomena unexplained and falls short of being a complete theory of fundamental interactions. It does not fully explain baryon asymmetry, incorporate the full theory of gravitation as described by general relativity, or account for the accelerating expansion of the Universe as possibly described by dark energy.
context_section_description: The Standard Model includes 12 elementary particles of spin ​¹⁄₂, known as fermions. According to the spin–statistics theorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle.
  Fermions are classified according to how they interact (or equivalently, by what charges they carry). There are six quarks (up, down, charm, strange, top, bottom), and six leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino). Each class is divided into pairs of particles that exhibit a similar physical behavior called a generation (see the table).
  The defining property of quarks is that they carry color charge, and hence interact via the strong interaction. The phenomenon of color confinement results in quarks being very strongly bound to one another, forming color-neutral composite particles called hadrons that contain either a quark and an antiquark (mesons) or three quarks (baryons). The lightest baryons are the proton and the neutron. Quarks also carry electric charge and weak isospin. Hence they interact with other fermions via electromagnetism and the weak interaction. The remaining six fermions do not carry color charge and are called leptons. The three neutrinos do not carry electric charge either, so their motion is directly influenced only by the weak nuclear force, which makes them notoriously difficult to detect. By contrast, by virtue of carrying an electric charge, the electron, muon, and tau all interact electromagnetically.
  Each member of a generation has greater mass than the corresponding particles of lower generations. The first-generation charged particles do not decay, hence all ordinary (baryonic) matter is made of such particles. Specifically, all atoms consist of electrons orbiting around atomic nuclei, ultimately constituted of up and down quarks. On the other hand, second- and third-generation charged particles decay with very short half-lives and are observed only in very high-energy environments. Neutrinos of all generations also do not decay and pervade the universe, but rarely interact with baryonic matter.
caption: Extended breakdown of particle interactions in the Standard Model if the hypothetical graviton were to be included.
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