The up quark or u quark (symbol: u) is the lightest of all

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, a type of

elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, antiqu ...

, and a significant constituent of

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 partic ...

. It, along with the

down quark The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up q ...

, forms the

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...

s (one up quark, two down quarks) and

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s (two up quarks, one down quark) of

atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden experiments, Geiger–Marsden gold foil experiment. After th ...

. It is part of the first generation of matter, has an

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electron ...

of + ''e'' and a bare mass of . Like all

s, the up quark is an elementary

fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks ...

with spin , and experiences all four

fundamental interaction In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and elect ...

gravitation In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...

electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...

weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...

s, and

strong interaction The strong interaction or strong force is a fundamental interaction that confines Quark, quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is calle ...

s. The

antiparticle In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite charge (physics), physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also ...

of the up quark is the up antiquark (sometimes called ''antiup quark'' or simply ''antiup''), which differs from it only in that some of its properties, such as charge have equal magnitude but opposite sign. Its existence (along with that of the down and strange quarks) was postulated in 1964 by Murray Gell-Mann and

George Zweig George Zweig (; born May 30, 1937) is a Russian-American physicist. He was trained as a particle physicist under Richard Feynman. He introduced, independently of Murray Gell-Mann, the quark model (although he named it "aces"). He later turned his ...

to explain the Eightfold Way classification scheme of

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite particle, composite subatomic particle made of two or more quarks bound state, held together by the strong interaction. They are analogous to molecules tha ...

s. The up quark was first observed by experiments at the Stanford Linear Accelerator Center in 1968.

History

In the beginnings of particle physics (first half of the 20th century), hadrons such as

s and

pion In particle physics, a pion (or a pi meson, denoted with the Greek alphabet, Greek letter pi (letter), pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the li ...

s were thought to be

s. However, as new hadrons were discovered, the ' particle zoo' grew from a few particles in the early 1930s and 1940s to several dozens of them in the 1950s. The relationships between each of them were unclear until 1961, when Murray Gell-Mann and

Yuval Ne'eman Yuval Ne'eman ( he, יובל נאמן, 14 May 1925 – 26 April 2006) was an Israeli Theoretical physics, theoretical physicist, military scientist, and politician. He was Ministry of Science, Technology and Space, Minister of Science and D ...

(independently of each other) proposed a hadron classification scheme called the Eightfold Way, or in more technical terms, SU(3) flavor symmetry. This classification scheme organized the hadrons into isospin multiplets, but the physical basis behind it was still unclear. In 1964, Gell-Mann and

(independently of each other) proposed the

quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies Flavour (particle physics), ...

, then consisting only of up, down, and strange quarks. However, while the quark model explained the Eightfold Way, no direct evidence of the existence of quarks was found until 1968 at the Stanford Linear Accelerator Center. Deep inelastic scattering experiments indicated that protons had substructure, and that protons made of three more-fundamental particles explained the data (thus confirming the

). At first people were reluctant to describe the three bodies as quarks, instead preferring

Richard Feynman Richard Phillips Feynman (; May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superflu ...

's parton description, but over time the quark theory became accepted (see '' November Revolution'').

Mass

Despite being extremely common, the bare mass of the up quark is not well determined, but probably lies between 1.8 and . Lattice QCD calculations give a more precise value: . When found in

meson In 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 fundamental particles in the universe are ...

s (particles made of one quark and one

antiquark 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 ...

) or

baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron list of particles, family of particles; hadrons are composed o ...

s (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks becomes greater because of the

binding energy In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...

caused by the gluon field between each quark (see

mass–energy equivalence In physics, mass–energy equivalence is the relationship between mass and energy in a system's rest frame, where the two quantities differ only by a multiplicative constant and the units of measurement. The principle is described by the physicis ...

). The bare mass of up quarks is so light, it cannot be straightforwardly calculated because relativistic effects have to be taken into account.

History

Mass

See also

References

Further reading