Quark is a fundamental particle in the Standard Model that has an electric charge that is a multiple of e / 3 and is not observed in the free state, but is part of hadrons (strongly interacting particles such as protons and neutrons ). Quarks are structureless, point particles; this was verified up to a scale of about 10–16 cm [5] , which is about 20 thousand times smaller than the proton size .
| Quark ( q ) | |
|---|---|
 Proton as a structure of two u-quarks and one d-quark | |
| Composition | fundamental particle |
| Family | fermion |
| Generation | there are quarks of all 3 generations |
| Involved in interactions | gravitational [1] , weak , strong , electromagnetic |
| Antiparticle | antiquark ( q ) |
| Number of types | 6 [2] ( lower , upper , strange , charmed , lovely , true ) |
| Quantum numbers | |
| Electric charge | Multiple e / 3 |
| Color charge | r, g, b |
| Baryon number | 1/3 [3] |
| Spin | ½ [4] ħ |
| Theoretically grounded | M. Gell-Mann and, independently of him, J. Zweig in 1964 [5] |
| Detected | SLAC (~ 1968) |
Currently, there are 6 different “varieties” (often referred to as “ flavors ”) of quarks, the properties of which are given in the table. In addition, for the gauge description of the strong interaction , it is postulated that quarks also have an additional internal characteristic called “ color ”. Each quark corresponds to an antiquark - an antiparticle with opposite quantum numbers .
The hypothesis that hadrons were built from specific subunits was first put forward by M. Gell-Mann and, independently of it, J. Zweig in 1964 [5] .
Quark Properties
| Symbol | Title | Charge | Weight | |
|---|---|---|---|---|
| Russian | English | |||
| First generation | ||||
| d | lower | down | - 1/3 | 4.8 ± 0.5 ± 0.3 MeV / s ² |
| u | upper | up | + 2/3 [6] | 2.3 ± 0.7 ± 0.5 MeV / s² |
| Second generation | ||||
| s | strange | strange | - 1/3 | 95 ± 5 MeV / s² |
| c | charmed | charm [7] ( charmed ) | + 2/3 | 1275 ± 25 MeV / s² |
| Third generation | ||||
| b | charming | beauty ( bottom ) | - 1/3 | 4180 ± 30 MeV / s² |
| t | true | truth ( top ) | + 2/3 [8] | 174 340 ± 650 MeV / c² [9] |
Due to reasons unknown so far, quarks are naturally grouped into three so-called generations (they are presented in the table). Quarks have a fractional electric charge [10] , and in each generation one quark has a charge and the other . Quarks of one generation would be indistinguishable if not for the Higgs field [11] . The division into generations also extends to leptons .
Quarks are involved in strong , weak , electromagnetic, and gravitational [1] interactions. Strong interactions ( gluon exchange) can change the color of a quark, but do not change its flavor. Weak interactions, on the contrary, do not change color, but can change flavor. Unusual properties of strong interaction lead to the fact that a single quark cannot move away at any significant distance from other quarks, which means that quarks cannot be observed in free form (a phenomenon called confinement ) [12] . Only “colorless” combinations of quarks - hadrons can fly apart. Quarks are asymptotically free at high energies.
The mathematical apparatus of the theory of quarks is based on the experimentally confirmed assumption that the interactions of quarks are invariant under the group of isospin transformations [13] .
Quark and antiquark can annihilate . The same type of uncharged quarks annihilate, as a rule, with the emission of two photons (that is, through electromagnetic interactions ). For example, the neutral pi meson π 0 , which is a combination of light quarks and antiquarks decays by electromagnetic annihilation. Other quarkoniums , heavier than the neutral pion ( J / ψ meson , ϒ meson , etc.), can annihilate with the participation of a strong interaction of two or three gluons , depending on the total spin , although such processes are usually suppressed by the rule Okubo - Zweig - Iizuki [14] . At high energies, in the hadron collisions, an increase in the cross section of the processes of weak (that is, involving weak interaction ) annihilation of quarks and antiquarks into a virtual or real W ± or Z 0 boson is observed [15] . It should be noted that annihilating quarks and antiquarks do not have to be of the same type; Thus, the dominant decay of the charged pi meson π + → μ + ν μ is due to weak annihilation of a heterogeneous quark pair d u into a virtual W + boson, which then decays into a pair of leptons [16] . Inverse annihilation processes of the production of quark-antiquark pairs are also observed.
The fractional charge of quarks manifests itself during the production of hadron jets in the annihilation of e + e - at high energies [17] .
Quarks are generated by gluons only by a quark – antiquark pair [18] .
Quark Reality
Due to the unusual property of strong interaction - confinement - often non-specialists ask the question: how are we sure that quarks exist if no one ever sees them in free form? Maybe they are just a mathematical abstraction , and the proton does not consist of them at all?
The reasons why quarks are considered real objects are as follows:
- Firstly, in the 1960s, it became clear that all the numerous hadrons obey a more or less simple classification: they unite into multiplets and supermultiplets themselves. In other words, the description of all these multiplets requires a very small number of free parameters. That is, all hadrons have a small number of degrees of freedom : all baryons with the same spin have three degrees of freedom, and all mesons have two. Initially, the quark hypothesis was precisely this observation, and the word “quark”, in fact, was a short form of the phrase “sub-hadronic degree of freedom”.
- Further, taking the spin into account, it turned out that each such degree of freedom can be assigned a spin ½ and, in addition, an orbital moment can be assigned to each pair of quarks - as if they are particles that can rotate relative to each other. From this assumption a harmonious explanation arose for the whole variety of hadron spins, as well as their magnetic moments .
- Moreover, with the discovery of new particles, it turned out that no modifications of the theory were required: each new hadron successfully fit into the quark structure without any rearrangements (except for adding new quarks).
- How to check that the charge of quarks is really fractional? The quark model predicted that when the high - energy electron and positron are annihilated, not the hadrons themselves will be born, but first the quark-antiquark pairs, which then turn into hadrons. The result of calculating the course of such a process depended directly on the charge of the generated quarks. The experiment fully confirmed these predictions [19] .
- With the advent of the era of high-energy accelerators, it became possible to study the distribution of momentum inside, for example, a proton. It turned out that the momentum in the proton is not evenly distributed over it, but is concentrated in parts in separate degrees of freedom. These degrees of freedom were called partons (from the English part - part). Moreover, it turned out that partons, in a first approximation, have spin ½ and the same charges as quarks. As the energy increased, it turned out that the number of partons increased, but such a result was expected in the quark model at ultrahigh energies [20] [21] .
- With increasing accelerator energy, it was also possible to try to knock out a single quark from a hadron in a high-energy collision. The quark theory gave clear predictions of how the results of such collisions should have looked - in the form of jets . Such jets were indeed observed in the experiment. Note that if the proton did not consist of anything, then there would certainly be no jets .
- In high-energy hadron collisions, the probability that hadrons scatter to a certain angle without destruction decreases with increasing angle. The experiments confirmed that, for example, for a proton the speed is exactly the same as expected for an object consisting of three quarks [22] .
- In collisions of protons with high energies, annihilation of a quark of one proton with an antiquark of another proton is experimentally observed with the formation of a muon – antimuon pair ( the Drell – Yang process ) [23] .
- The quark model from the standpoint of the interaction of quarks with each other using gluons explains well the splitting of the masses between the decuplet members [24] .
- The quark model explains well the splitting of masses between [25] .
- The quark model predicts for the ratio of the magnetic moments of the proton and neutron the value which is in good agreement with the experimental value of −1.47. For the ratio of the magnetic moments of hyperon and proton, the theory of quarks predicts the value , which also is in good agreement with the experimental value of −0.29 ± 0.05 [26] .
- There are many other experimental confirmations of the quark model of the structure of hadrons [27] .
In general, we can say that the quark hypothesis and everything that follows from it (in particular, QCD ) is the most conservative hypothesis regarding the structure of hadrons, which is able to explain the available experimental data. Attempts to do without quarks encounter difficulties in describing all those numerous experiments that were very naturally described in the quark model.
The quark model was recognized by the physical community in 1976 [28] .
Open Questions
With regard to quarks, questions remain for which there is no answer yet:
- why exactly three colors?
- why exactly three generations of quarks?
- Is the coincidence of the number of colors and the number of generations accidental?
- Is it coincidence that this number coincides with the dimension of space in our world?
- where does such a spread in quark masses come from?
- what are quarks made of? (see Preon ) [5]
- How do quarks add up to hadrons [29] ?
However, the story of hadrons and quarks, as well as the symmetry between quarks and leptons, suggests that quarks themselves may consist of something simpler. The working name for hypothetical particles-constituent quarks is preons . From the point of view of these experiments, so far no suspicion of the non-point structure of quarks has arisen. However, attempts to construct such theories are made independently of experiments. There are no serious successes in this direction yet.
Another approach is to build a theory of the Great Unification . Such a theory would benefit not only in combining strong and electroweak interactions, but also in a single description of leptons and quarks. Despite active efforts, it has not yet been possible to build such a theory.
Alternative Models
- The Sakata model (Shoichi Sakata), also known as the Fermi - Young - Sakata model . The basis is p, n, Λ and their antiparticles. Describes all the mesons and baryons known at the time of publication. [30] Subsequently, the basis expanded to 4 particles. [31]
- Baryonic non-baryonic nonets. [32]
Title
The word “quark” was borrowed by Gell-Mann [5] from the novel by J. Joyce Finnegans Wake [33] , where in one episode the gulls shout: “Three quarks for Muster Mark!” (Usually translated as “Three quarks for Masters / Müster Mark! ”). The word “quark” in this phrase is supposedly onomatopoeic to the cry of seabirds. There is another version (put forward by R. Jacobson ), according to which Joyce learned this word from German during his stay in Vienna. In German, the word Quark has two meanings: 1) cottage cheese, 2) nonsense. But the German word came from the West Slavic languages ( Czech tvaroh , Polish twaróg - “cottage cheese”) [34] . According to the story of Irish physicist , during a stay in Germany at an agricultural exhibition, Joyce heard the advertising slogan “Drei Mark für Musterquark” (“three brands for exemplary cottage cheese”), which he later rephrased for the novel [35] .
J. Zweig called them Aces , but this name did not take root and was forgotten - perhaps because there were four aces , and there were three quarks in the original model.
See also
- Quark-gluon plasma [36]
- Quarkonium - a meson consisting of a quark and an antiquark of the same type
- Preons are hypothetical particles that quarks and leptons might consist of.
- Quark star - a hypothetical neutron star with extreme density and a degenerate state of matter
- Infinite nesting of matter
Notes
- ↑ 1 2 The amazing world inside the atomic nucleus. Questions after the lecture .
- ↑ Quarks and the octal path
- ↑ quarks
- ↑ QUARKS Quark structure of hadrons
- ↑ 1 2 3 4 5 QUARKS • Great Russian Encyclopedia .
- ↑ Basic concepts and laws of physics and properties of elementary particles of matter Leo Perch Electromagnetic interaction Neutral particles.
- ↑ Particle physics on the eve of the launch of the Large Hadron Collider V. A. Rubakov Popular science lecture for students, LPI, September 25, 2008
- ↑ Quark classification of hadrons and their properties
- ↑ E.E. Boos, O. Brandt, D. Denisov, S.P. Denisov, P. Grannis. Top quark (on the 20th anniversary of the discovery) // Uspekhi Fiz . - 2015 .-- T. 185 . - S. 1241-1269 . - DOI : 10.3367 / UFNr.0185.201512a.1241 .
- ↑ Along the Ocean of the Unknown: The Illusion of Simplicity
- ↑ "Particle on the edge of the Universe." Chapter from Sean Carroll's book Symmetries of Weak Interactions
- ↑ Igor Ivanov, Ph.D. in Physics and Mathematics (Institute of Mathematics SB RAS, Novosibirsk, and University of Liege, Belgium). The anatomy of one piece of news, or How physicists actually study elementary particles: Why quarks are not free . - Elements.ru.
- ↑ Quark Theory, 1971 , p. 40.
- ↑ Gerasimov S. B. Zweig rule // Physical Encyclopedia : [in 5 volumes] / Ch. ed. A.M. Prokhorov . - M .: Great Russian Encyclopedia, 1999. - T. 5: Stroboscopic devices - Brightness. - S. 418. - 692 p. - 20,000 copies. - ISBN 5-85270-101-7 .
- ↑ Annihilation
- ↑ Khlopov M. Yu. Annihilation // Physical Encyclopedia : [in 5 volumes] / Ch. ed. A.M. Prokhorov . - M .: Soviet Encyclopedia, 1988. - T. 1: Aaronova - Bohm effect - Long lines. - S. 85-86. - 707 p. - 100,000 copies.
- ↑ Quark model of hadrons Quark flight
- ↑ Gauge Bosons Gluons
- ↑ Introduction to Quarks and Partons, 1982 , p. 246.
- ↑ AV Belitsky, AV Radyushkin. Unraveling hadron structure with generalized parton distributions // Phys. Rept. - 2005. - No. 418 . - P. 1–387. - arXiv : hep-ph / 0504030 . arXiv : hep-ph / 0504030
- ↑ High Energy Nuclear Physics, 1980 , p. 23.
- ↑ Elements - science news: ALICE results on the asymmetry of protons and antiprotons put an end to a long-standing dispute Archived February 3, 2012 on Wayback Machine
- ↑ Introduction to Quarks and Partons, 1982 , p. 306.
- ↑ Introduction to Quarks and Partons, 1982 , p. 369.
- ↑ Introduction to Quarks and Partons, 1982 , p. 379.
- ↑ Quark Theory, 1971 , p. 116.
- ↑ Akhiezer A.I. , Rekalo M.P. Quark model and hadron interaction processes // Problems of Theoretical Physics. Collection dedicated to Nikolai Nikolaevich Bogolyubov in connection with his sixtieth birthday. - M., Nauka , 1969. - Circulation 4000 copies. - c. 197-216
- ↑ Quarks - half a century Aleksey Levin “Trinity Option” No. 11 (155), June 3, 2014 From distrust to acceptance
- ↑ Игорь Иванов. Детектор ALICE изучает тонкие эффекты в рождении адронов . Сложные вопросы в физике элементарных частиц (2 августа 2013). Дата обращения 9 августа 2013. Архивировано 30 августа 2013 года.
- ↑ S. Sakata. On a composite model for new particles Progr. Theor. Phys. 16 (1956), 686
- ↑ Y. Katayama, K. Matumoto, S. Tanaka, E. Yamada. Possible unified models of elementary particles with two neutrinos. Progr. Theor. Phys. 28 (1962), 675
- ↑ CZ Yuan, XH Mo, P. Wang. Baryon-antibaryon nonets
- ↑ Теория кварков, 1971 , с. 33.
- ↑ В. В. Иванов. Ранние коптские заимствования в славянском // Славянская языковая и этноязыковая системы в контакте с неславянским окружением. — М. : Языки славянской культуры, 2002. — С. 57—58.
- ↑ H. Leutwyler. Insights and puzzles in particle physics // H. Fritzsch and M. Gell-Mann, eds. Fifty Years of Quarks. — Singapore: World Scientific, 2014. — arXiv : 1410.4000 .
- ↑ Крошечные капли кварк-глюонной плазмы образуются и в несимметричных ядерных столкновениях
Literature
- Jean Letessier, Johann Rafelski, T. Ericson, PY Landshoff. Hadrons and Quark-Gluon Plasma. — Cambridge University Press, 2002. — 415 p. — ISBN 9780511037276 .
- Боголюбов Н.Н., Логунов А.А., Оксак А.И., Тодоров И.Т. Общие принципы квантовой теории поля. — Москва: Наука, 1987. — С. 3, 226-228, 362, 363, 366, 412, 414-416, 420, 421, 423, 425, 428, 561, 562, 571, 572, 574, 614. — 616 с.
- Намбу Ё. Кварки. — М. : Мир , 1984. — 225 с.
- Клоуз Ф. Введение в кварки и партоны. — М. : Мир , 1982. — 438 с.
- Никитин Ю. П., Розенталь И. Л. Ядерная физика высоких энергий. — М. : Атомиздат , 1980. — 232 с.
- Коккедэ Я. Теория кварков. — М. : Мир , 1971. — 341 с.
Links
- Экспериментальная информация о кварках на сайте Particle Data Group