particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

, a generation or family is a division of the

elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. The Standard Model presently recognizes seventeen distinct particles—twelve fermions and five bosons. As a c ...

s. Between generations, particles differ by their flavour quantum number and

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

, but their electric and strong interactions are identical. There are three generations according to the

Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...

of particle physics. Each generation contains two types of

lepton In particle physics, a lepton is an elementary particle of half-integer spin (Spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-li ...

s and two types of

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...

s. The two leptons may be classified into one with

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

−1 (electron-like) and neutral (neutrino); the two quarks may be classified into one with charge − (down-type) and one with charge + (up-type). The basic features of quark–lepton generation or families, such as their masses and mixings etc., can be described by some of the proposed

family symmetries In particle physics, the family symmetries or horizontal symmetries are various discrete, global, or local symmetries between quark-lepton families or generations. In contrast to the intrafamily or vertical symmetries (collected in the convention ...

Overview

Each member of a higher generation has greater mass than the corresponding particle of the previous generation, with the possible exception of the

neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...

s (whose small but non-zero masses have not been accurately determined). For example, the first-generation

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

has a mass of only , the second-generation

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...

has a mass of , and the third-generation

tau Tau (; uppercase Τ, lowercase τ or \boldsymbol\tau; ) is the nineteenth letter of the Greek alphabet, representing the voiceless alveolar plosive, voiceless dental or alveolar plosive . In the system of Greek numerals, it has a value of 300 ...

has a mass of (almost twice as heavy as a

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

). This mass hierarchy causes particles of higher generations to decay to the first generation, which explains why everyday

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic pa ...

(

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s) is made of particles from the first generation only. Electrons surround a nucleus made of

s and

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s, which contain up and down quarks. The second and third generations of charged particles do not occur in normal matter and are only seen in extremely high-energy environments such as

cosmic rays Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar ...

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...

s. The term ''generation'' was first introduced by

Haim Harari use both this parameter and , birth_date to display the person's date of birth, date of death, and age at death) --> , death_place = , death_cause = , body_discovered = , resting_place = , resting_place_coordinates ...

in Les Houches Summer School, 1976. Neutrinos of all generations stream throughout the universe but rarely interact with other matter. It is hoped that a comprehensive understanding of the relationship between the generations of the leptons may eventually explain the ratio of masses of the fundamental particles, and shed further light on the nature of mass generally, from a quantum perspective.

Fourth generation

Fourth and further generations are considered unlikely by many (but not all) theoretical physicists. Some arguments against the possibility of a fourth generation are based on the subtle modifications of precision

electroweak In particle physics, the electroweak interaction or electroweak force is the unified description of two of the fundamental interactions of nature: electromagnetism (electromagnetic interaction) and the weak interaction. Although these two forc ...

observables that extra generations would induce; such modifications are strongly disfavored by measurements. There are functions used to generalize terms for introduction in a new quark that is an isosinglet and is responsible for generating Flavour-Changing-Neutral-Currents' (FCNC) at tree level in the electroweak sectors. Nonetheless, searches at high-energy colliders for particles from a fourth generation continue, but as yet no evidence has been observed. In such searches, fourth-generation particles are denoted by the same symbols as third-generation ones with an added prime (e.g. ''b′'' and ''t′''). A fourth generation with a 'light' neutrino (one with a mass less than about ) was ruled out by measurements of the decay widths of the

Z boson In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , and ...

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...

Large Electron–Positron Collider The Large Electron–Positron Collider (LEP) was one of the largest particle accelerators ever constructed. It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland. LEP collided electr ...

(LEP) as early as 1989. The lower bound for a fourth generation neutrino (''ν'_τ'') mass as of 2010 was at about 60 GeV (millions of times larger than the upper bound for the other 3 neutrino masses). As of 2024, no evidence of a fourth-generation neutrino has ever been observed in

neutrino oscillation Neutrino oscillation is a quantum mechanics, quantum mechanical phenomenon in which a neutrino created with a specific lepton lepton number, family number ("lepton flavor": electron, muon, or tau lepton, tau) can later be Quantum measurement, mea ...

studies either. Because even in the third generation (tau) neutrino ''ν_τ'', mass is extremely small (making ''ν_τ'' the only third-generation particle that outside highly most energetic conditions will not readily decay), a fourth-generation neutrino ''ν'_τ'' that observes the general rules for the known 3 neutrino generations should both be easily within current particle accelerators' energy levels, and occur during the regular and highly predictable switching-of-generations (oscillation) neutrinos perform. If the

Koide formula The Koide formula is an unexplained Empirical relationship, empirical equation discovered by Yoshio Koide in 1981. In its original form, it is not fully empirical but a set of guesses for a model for masses of quarks and leptons, as well as Cabibbo ...

continues to hold, the masses of the fourth generation charged lepton would be 44 GeV (ruled out) and ''b′'' and ''t′'' should be 3.6 TeV and 84 TeV respectively (The maximum possible energy for protons in the

LHC The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists, and ...

is about 6 TeV). The lower bound for a fourth generation of quark (''b′'', ''t′'') masses as of 2019 was at 1.4 TeV from experiments at the LHC. The lower bound for a fourth generation charged lepton (''τ) mass in 2012 was 100GeV, with a proposed upper bound of 1.2 TeV from unitarity considerations.

Origin

The origin of multiple generations of fermions, and the particular count of ''3'', is an unsolved problem of physics.

String theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...

provides a cause for multiple generations, but the particular number depends on the details of the compactification of the

D-brane In string theory, D-branes, short for Dirichlet membrane, are a class of extended objects upon which open strings can end with Dirichlet boundary conditions, after which they are named. D-branes are typically classified by their spatial dimensi ...

intersections. Additionally, grand unified theories in 10 dimensions compactified on certain

orbifold In the mathematical disciplines of topology and geometry, an orbifold (for "orbit-manifold") is a generalization of a manifold. Roughly speaking, an orbifold is a topological space that is locally a finite group quotient of a Euclidean space. D ...

s down to 4 D naturally contain 3 generations of matter. This includes many heterotic string theory models. In standard quantum field theory, under certain assumptions, a single fermion field can give rise to multiple fermion poles with mass ratios of around and potentially explaining the large ratios of fermion masses between successive generations and their origin. The existence of precisely three generations with the correct structure was at least tentatively derived from first principles through a connection with gravity. The result implies a unification of gauge forces into SU(5). The question regarding the masses is unsolved, but this is a logically separate question, related to the Higgs sector of the theory.

References

{{reflist, 25em Quarks

Overview

Fourth generation

Origin

See also

References