Upper quark

Physical terms

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All neutrons are composed of three quark Composed of, Antineutron Is composed of three corresponding Antiquark Composed of, such as proton ， neutron 。 The proton consists of two upper quarks and one Lower quark The neutron is composed of two lower quarks and one upper quark. By 1967, scientists through SLAC linear accelerator Ongoing Deep inelastic scattering Experiments provide evidence for the existence of such quarks for the first time.

Chinese name: Upper quark
Foreign name: up quark
Application: Atomic physics

Symbol: u
Proposed time: 1964
Meaning: Neutron component

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history

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stay Particle physics At the early stage of its development (the first half of the 20th century), proton 、 neutron 、 π meson wait hadron Are treated as elementary particles. However, with the discovery of more new hadrons, Particle circle It also expanded from several particles in the 1930s and 1940s to dozens of particles in the 1950s. However, physicists do not know the relationship between them. In 1961, Murray Gelman And Yufar Niman Respectively and independently propose Octet The classification system of SU(3) taste Symmetry 。

This classification system organizes various hadrons into Isospin multiplet However, the physical theory behind it is not clear. In 1964, Gelman and George Zweig Independently Quark model , which initially only describes the upper quark, lower quark and Odd quark However, although the quark model explains the octet, no experiment directly confirms the existence of quarks. In 1968 Stanford Linear Accelerator Center ， Deep inelastic scattering experiment It is revealed that the proton has an internal structure and is composed of three more basic particles, so the quark model is recognized as correct.

At first, people were reluctant to accept the fact that they preferred Richard Feynman Of Partial submodel 。 As time flows, quark theory is gradually accepted.^[1]

Research background

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The existence of the upper quark was first proposed in 1964, when Gell Mann and Zweig made Quark model 。 By 1967, through SLAC linear accelerator Ongoing Deep inelastic scattering Experiments provide evidence for the existence of such quarks for the first time.^[1]

Composition

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A neutron consists of two lower quarks and an upper quark according to Particle physics Of Standard Model Theory , the upper and lower quarks are composed of nucleon The basic elements of, proton With two upper quarks and one Lower quark , and neutron There is an upper quark and two lower quarks. Up quark (u for short) is the first generation quark A kind of classification. It has+(2/3) e charge and is also the lightest quark, with a net mass between 1.5 and 4 MeV.^[1]

Physical and chemical properties

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In fact, one nucleon The mass of Gluon Force dominates, not the mass of quarks themselves. The spin of the upper quark is 1/2; Isospin is 1/2; The isospin component is 1/2; Overload+1/3; Singular number, charm number, base number and top number are all 0.^[2]

quality

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Although the upper quark is very common, physicists do not know its naked mass. Its value is about 1.8 and 3.0 MeV/ c² Used between Lattice chromodynamics The calculation can give a relatively accurate value: 2.01 ± 0.14 MeV/ c² 。

The mass of quarks cannot be directly measured experimentally, so it is quite difficult to determine the quantity. stay meson or baryon Li, quark Effective quality It will become very large, which is derived from the relationship between quarks Gluon field Caused by binding energy Note that special relativity allows the conversion between energy and mass. Since the naked mass of the upper quark is very small, it cannot be calculated directly.^[2]

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