Editing Quark (section)

== Properties ==

=== Electric charge ===
{{See also|Electric charge}}
Quarks have [[fraction (mathematics)|fractional]] electric charge values – either &minus;{{sfrac|1|3}} or +{{sfrac|2|3}} times the [[elementary charge]] (''e''), depending on flavor. Up, charm, and top quarks (collectively referred to as ''up-type quarks'') have a charge of +{{sfrac|2|3}}&nbsp;''e''; down, strange, and bottom quarks (''down-type quarks'') have a charge of −{{sfrac|1|3}}&nbsp;''e''. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of −{{sfrac|2|3}}&nbsp;''e'' and down-type antiquarks have charges of +{{sfrac|1|3}}&nbsp;''e''. Since the electric charge of a [[hadron]] is the sum of the charges of the constituent quarks, all hadrons have integer charges: the combination of three quarks (baryons), three antiquarks (antibaryons), or a quark and an antiquark (mesons) always results in integer charges.<ref>
{{cite book
 |author=C. Quigg
 |chapter=Particles and the Standard Model
 |editor=G. Fraser
 |title=The New Physics for the Twenty-First Century
 |page=91
 |publisher=[[Cambridge University Press]]
 |year=2006
 |isbn=978-0-521-81600-7
}}</ref> For example, the hadron constituents of atomic nuclei, neutrons and protons, have charges of 0&nbsp;''e'' and +1&nbsp;''e'' respectively; the neutron is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark.<ref name="Knowing" />

=== Spin ===
{{See also|Spin (physics)}}
Spin is an intrinsic property of elementary particles, and its direction is an important [[Degrees of freedom (physics and chemistry)|degree of freedom]]. It is sometimes visualized as the rotation of an object around its own axis (hence the name "[[Wikt:spin|spin]]"), though this notion is somewhat misguided at subatomic scales because elementary particles are believed to be [[point particle|point-like]].<ref>
{{cite web
 |title=The Standard Model of Particle Physics
 |url=https://www.bbc.co.uk/dna/h2g2/A666173
 |publisher=BBC
 |year=2002
 |access-date=2009-04-19
}}</ref>

Spin can be represented by a [[euclidean vector|vector]] whose length is measured in units of the [[reduced Planck constant]] ''ħ'' (pronounced "h bar"). For quarks, a measurement of the spin vector [[vector projection|component]] along any axis can only yield the values +{{sfrac|''ħ''|2}} or −{{sfrac|''ħ''|2}}; for this reason quarks are classified as [[spin 1/2|spin-{{sfrac|1|2}}]] particles.<ref>
{{cite book
 |author=F. Close
 |title=The New Cosmic Onion
 |pages=80–90
 |publisher=[[CRC Press]]
 |year=2006
 |isbn=978-1-58488-798-0
}}</ref> The component of spin along a given axis – by convention the ''z'' axis – is often denoted by an up arrow ↑ for the value +{{sfrac|1|2}} and down arrow ↓ for the value −{{sfrac|1|2}}, placed after the symbol for flavor. For example, an up quark with a spin of +{{sfrac|1|2}} along the ''z'' axis is denoted by u↑.<ref>
{{cite book
 |author=D. Lincoln
 |title=Understanding the Universe
 |url=https://archive.org/details/understandinguni0000linc
 |url-access=registration
 |page=[https://archive.org/details/understandinguni0000linc/page/116 116]
 |publisher=[[World Scientific]]
 |year=2004
 |isbn=978-981-238-705-9
}}</ref>

=== Weak interaction ===
{{Main|Weak interaction}}
[[Image:Beta Negative Decay.svg|thumb|right|192px|upright|[[Feynman diagram]] of [[beta decay]] with time flowing upwards. The CKM matrix (discussed below) encodes the probability of this and other quark decays.|alt=A tree diagram consisting mostly of straight arrows. A down quark forks into an up quark and a wavy-arrow W[superscript minus] boson, the latter forking into an electron and reversed-arrow electron antineutrino.]]
A quark of one flavor can transform into a quark of another flavor only through the weak interaction, one of the four [[fundamental interaction]]s in particle physics. By absorbing or emitting a [[W boson]], any up-type quark (up, charm, and top quarks) can change into any down-type quark (down, strange, and bottom quarks) and vice versa. This flavor transformation mechanism causes the [[radioactive decay|radioactive]] process of [[beta decay]], in which a neutron ({{SubatomicParticle|neutron}}) "splits" into a proton ({{SubatomicParticle|proton}}), an [[electron]] ({{SubatomicParticle|electron}}) and an [[electron antineutrino]] ({{SubatomicParticle|electron antineutrino}}) (see picture). This occurs when one of the down quarks in the neutron ({{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}{{SubatomicParticle|down quark}}) decays into an up quark by emitting a [[virtual particle|virtual]] {{SubatomicParticle|W boson-}} boson, transforming the neutron into a proton ({{SubatomicParticle|up quark}}{{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}). The {{SubatomicParticle|W boson-}} boson then decays into an electron and an electron antineutrino.<ref name="SLAC">
{{cite web
 |title=Weak Interactions
 |url=http://www2.slac.stanford.edu/vvc/theory/weakinteract.html
 |work=Virtual Visitor Center
 |publisher=[[Stanford Linear Accelerator Center]]
 |year=2008
 |access-date=2008-09-28
 |archive-date=23 November 2011
 |archive-url=https://web.archive.org/web/20111123112925/http://www2.slac.stanford.edu/vvc/theory/weakinteract.html
 |url-status=dead
}}</ref>

{| style="margin:auto;" cellpadding="5%"
|-
| &nbsp; {{SubatomicParticle|Neutron}}|| → || &nbsp; {{SubatomicParticle|Proton}} ||+|| {{SubatomicParticle|electron}} ||+|| {{SubatomicParticle|electron antineutrino}} || (Beta decay, hadron notation)
|-
| {{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}{{SubatomicParticle|down quark}} || → || {{SubatomicParticle|up quark}}{{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}} ||+|| {{SubatomicParticle|electron}} ||+|| {{SubatomicParticle|electron antineutrino}} || (Beta decay, quark notation)
|}

Both beta decay and the inverse process of ''[[inverse beta decay]]'' are routinely used in medical applications such as [[positron emission tomography]] (PET) and in experiments involving [[neutrino detector|neutrino detection]].
[[Image:Quark weak interactions.svg|thumb|271px|left|The [[Coupling (physics)|strengths]] of the weak interactions between the six quarks. The "intensities" of the lines are determined by the elements of the [[CKM matrix]].|alt=Three balls "u", "c", and "t" noted "up-type quarks" stand above three balls "d", "s", "b" noted "down-type quark". The "u", "c", and "t" balls are vertically aligned with the "d", "s", and b" balls respectively. Colored lines connect the "up-type" and "down-type" quarks, with the darkness of the color indicating the strength of the weak interaction between the two; The lines "d" to "u", "c" to "s", and "t" to "b" are dark; The lines "c" to "d" and "s" to "u" are grayish; and the lines "b" to "u", "b" to "c", "t" to "d", and "t" to "s" are almost white.]]

While the process of flavor transformation is the same for all quarks, each quark has a preference to transform into the quark of its own generation. The relative tendencies of all flavor transformations are described by a [[matrix (mathematics)|mathematical table]], called the [[Cabibbo–Kobayashi–Maskawa matrix]] (CKM matrix). Enforcing [[Unitary operator|unitarity]], the approximate [[absolute value|magnitudes]] of the entries of the CKM matrix are:<ref name="PDG2010">
{{cite journal
 |author=K. Nakamura
 |display-authors=etal
 |collaboration=[[Particle Data Group]]
 |year=2010
 |title=Review of Particles Physics: The CKM Quark-Mixing Matrix
 |url=http://pdg.lbl.gov/2010/reviews/rpp2010-rev-ckm-matrix.pdf
 |journal=[[Journal of Physics G]]
 |volume=37 |issue= 7A|page=075021
 |bibcode=2010JPhG...37g5021N
 |doi=10.1088/0954-3899/37/7A/075021 |doi-access=free
}}</ref>
: <math alt="|V_ud| ≅ 0.974; |V_us| ≅ 0.225; |V_ub| ≅ 0.003; |V_cd| ≅ 0.225; |V_cs| ≅ 0.973; |V_cb| ≅ 0.041; |V_td| ≅ 0.009; |V_ts| ≅ 0.040; |V_tb| ≅ 0.999.">
\begin{bmatrix} |V_\mathrm {ud}| & |V_\mathrm {us}| & |V_\mathrm {ub}| \\ |V_\mathrm {cd}| & |V_\mathrm {cs}| & |V_\mathrm {cb}| \\ |V_\mathrm {td}| & |V_\mathrm {ts}| & |V_\mathrm {tb}| \end{bmatrix} \approx
\begin{bmatrix} 0.974 & 0.225 & 0.003 \\ 0.225 & 0.973 & 0.041 \\ 0.009 & 0.040 & 0.999 \end{bmatrix},</math>
where ''V''<sub>''ij''</sub> represents the tendency of a quark of flavor ''i'' to change into a quark of flavor ''j'' (or vice versa).<ref group="nb">The actual probability of decay of one quark to another is a complicated function of (among other variables) the decaying quark's mass, the masses of the [[decay product]]s, and the corresponding element of the CKM matrix. This probability is directly proportional (but not equal) to the magnitude squared (|''V''<sub>''ij''&nbsp;</sub>|<sup>2</sup>) of the corresponding CKM entry.</ref>

There exists an equivalent weak interaction matrix for leptons (right side of the W boson on the above beta decay diagram), called the [[Pontecorvo–Maki–Nakagawa–Sakata matrix]] (PMNS matrix).<ref>
{{cite journal
 |author1=Z. Maki
 |author2=M. Nakagawa
 |author3=S. Sakata
 |title=Remarks on the Unified Model of Elementary Particles
 |journal=[[Progress of Theoretical Physics]]
 |volume=28
 |issue=5
 |page=870
 |year=1962
 |bibcode=1962PThPh..28..870M
 |doi=10.1143/PTP.28.870
 |doi-access=free
}}</ref> Together, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear.<ref>
{{cite journal
 |author1=B. C. Chauhan
 |author2=M. Picariello
 |author3=J. Pulido
 |author4=E. Torrente-Lujan
 |title=Quark–Lepton Complementarity, Neutrino and Standard Model Data Predict {{nowrap|1=θ{{su|p=PMNS|b=13}} = {{val|9|+1|-2|u=°}}}}<!-- See Section 2 -->
 |journal=[[European Physical Journal]]
 |volume=C50 |issue=3 |pages=573–578
 |year=2007
 |arxiv=hep-ph/0605032
 |bibcode = 2007EPJC...50..573C
 |doi=10.1140/epjc/s10052-007-0212-z
 |s2cid=118107624
}}</ref>
{{clear}}

=== Strong interaction and color charge ===
{{See also|Color charge|Strong interaction}}
[[Image:Hadron colors.svg|right|thumb|upright|All types of hadrons have zero total color charge.|alt=A green and a magenta ("antigreen") arrow canceling out each other out white, representing a meson; a red, a green, and a blue arrow canceling out to white, representing a baryon; a yellow ("antiblue"), a magenta, and a cyan ("antired") arrow canceling out to white, representing an antibaryon.]]
[[File:Strong force charges.svg|200px|left|thumb|The pattern of strong charges for the three colors of quark, three antiquarks, and eight gluons (with two of zero charge overlapping).]]
According to [[quantum chromodynamics]] (QCD), quarks possess a property called ''[[color charge]]''. There are three types of color charge, arbitrarily labeled ''blue'', ''green'', and ''red''.<ref group="nb">Despite its name, color charge is not related to the color spectrum of visible light.</ref> Each of them is complemented by an anticolor – ''antiblue'', ''antigreen'', and ''antired''. Every quark carries a color, while every antiquark carries an anticolor.<ref>
{{cite web
 |author=R. Nave
 |title=The Color Force
 |url=http://hyperphysics.phy-astr.gsu.edu/hbase/forces/color.html#c2
 |work=[[HyperPhysics]]
 |publisher=[[Georgia State University]], Department of Physics and Astronomy
 |access-date=2009-04-26
}}</ref>

The system of attraction and repulsion between quarks charged with different combinations of the three colors is called [[strong interaction]], which is mediated by [[force carrier|force carrying particles]] known as ''[[gluon]]s''; this is discussed at length below. The theory that describes strong interactions is called [[quantum chromodynamics]] (QCD). A quark, which will have a single color value, can form a [[bound state|bound system]] with an antiquark carrying the corresponding anticolor. The result of two attracting quarks will be color neutrality: a quark with color charge ''ξ'' plus an antiquark with color charge −''ξ'' will result in a color charge of 0 (or "white" color) and the formation of a [[meson]]. This is analogous to the [[additive color]] model in basic [[optics]]. Similarly, the combination of three quarks, each with different color charges, or three antiquarks, each with different anticolor charges, will result in the same "white" color charge and the formation of a [[baryon]] or [[antibaryon]].<ref>
{{cite book
 |author=B. A. Schumm
 |title=Deep Down Things
 |pages=[https://archive.org/details/deepdownthingsbr00schu/page/131 131–132]
 |publisher=[[Johns Hopkins University Press]]
 |year=2004
 |isbn=978-0-8018-7971-5
 |url=https://archive.org/details/deepdownthingsbr00schu/page/131
}}</ref>

In modern particle physics, [[gauge symmetry|gauge symmetries]] – a kind of [[symmetry group]] – relate interactions between particles (see [[gauge theories]]). Color [[SU(3)]] (commonly abbreviated to SU(3)<sub>c</sub>) is the gauge symmetry that relates the color charge in quarks and is the defining symmetry for quantum chromodynamics.<ref name="PeskinSchroeder">Part III of
{{cite book
 |author1=M. E. Peskin
 |author2=D. V. Schroeder
 |title=An Introduction to Quantum Field Theory
 |url=https://archive.org/details/introductiontoqu0000pesk
 |url-access=registration
 |publisher=[[Addison–Wesley]]
 |year=1995
 |isbn=978-0-201-50397-5
}}</ref> Just as the laws of physics are independent of which directions in space are designated ''x'', ''y'', and ''z'', and remain unchanged if the coordinate axes are rotated to a new orientation, the physics of quantum chromodynamics is independent of which directions in three-dimensional color space are identified as blue, red, and green. SU(3)<sub>c</sub> color transformations correspond to "rotations" in color space (which, mathematically speaking, is a [[complex vector space|complex space]]). Every quark flavor ''f'', each with subtypes ''f''<sub>B</sub>, ''f''<sub>G</sub>, ''f''<sub>R</sub> corresponding to the quark colors,<ref>
{{cite book
 |author=V. Icke
 |title=The Force of Symmetry
 |url=https://archive.org/details/forceofsymmetry0000icke
 |url-access=registration
 |page=[https://archive.org/details/forceofsymmetry0000icke/page/216 216]
 |publisher=[[Cambridge University Press]]
 |year=1995
 |isbn=978-0-521-45591-6
}}</ref> forms a triplet: a three-component [[quantum field]] that transforms under the fundamental [[representation theory|representation]] of SU(3)<sub>c</sub>.<ref>
{{cite book
 |author=M. Y. Han
 |title=A Story of Light
 |url=https://archive.org/details/storylightshorti00hanm_264
 |url-access=limited
 |page=[https://archive.org/details/storylightshorti00hanm_264/page/n86 78]
 |publisher=[[World Scientific]]
 |year=2004
 |isbn=978-981-256-034-6
}}</ref> The requirement that SU(3)<sub>c</sub> should be [[local symmetry|local]] – that is, that its transformations be allowed to vary with space and time – determines the properties of the strong interaction. In particular, it implies the existence of [[Gluon#Eight colors|eight gluon types]] to act as its force carriers.<ref name="PeskinSchroeder"/><ref>
{{cite encyclopedia
 |author=C. Sutton
 |title=Quantum Chromodynamics (physics)
 |url=http://www.britannica.com/EBchecked/topic/486191/quantum-chromodynamics#ref=ref892183
 |encyclopedia=[[Encyclopædia Britannica Online]]
 |access-date=2009-05-12
}}</ref>

=== Mass ===
[[Image:Quark masses as balls.svg|thumb|Current quark masses for all six flavors in comparison, as [[w:ball (mathematics)|balls]] of proportional volumes. [[Proton]] (gray) and [[electron]]&nbsp;(red) are shown in bottom left corner for scale.]]
{{See also|Invariant mass}}
Two terms are used in referring to a quark's mass: ''[[current quark]] mass'' refers to the mass of a quark by itself, while ''[[constituent quark]] mass'' refers to the current quark mass plus the mass of the [[gluon]] [[quantum field theory|particle field]] surrounding the quark.<ref>
{{cite book
 |author=A. Watson
 |title=The Quantum Quark
 |pages=285–286
 |publisher=[[Cambridge University Press]]
 |year=2004
 |isbn=978-0-521-82907-6
}}</ref> These masses typically have very different values. Most of a hadron's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy – more specifically, [[quantum chromodynamics binding energy]] (QCBE) – and it is this that contributes so greatly to the overall mass of the hadron (see [[mass in special relativity]]). For example, a proton has a mass of approximately {{val|938|ul=MeV/c2}}, of which the rest mass of its three valence quarks only contributes about {{val|9|u=MeV/c2}}; much of the remainder can be attributed to the field energy of the gluons<ref name=PDGQuarks/><ref>
{{cite book
 |author1=W. Weise
 |author2=A. M. Green
 |title=Quarks and Nuclei
 |pages=65–66
 |publisher=[[World Scientific]]
 |year=1984
 |isbn=978-9971-966-61-4
}}</ref> (see [[chiral symmetry breaking]]). The Standard Model posits that elementary particles derive their masses from the [[Higgs mechanism]], which is associated to the [[Higgs boson]]. It is hoped that further research into the reasons for the top quark's large mass of ~{{val|173|u=GeV/c2}}, almost the mass of a gold atom,<ref name=PDGQuarks/><ref>
{{cite book
 |author=D. McMahon
 |title=Quantum Field Theory Demystified
 |url=https://archive.org/details/quantumfieldtheo00mcma_095
 |url-access=limited
 |page=[https://archive.org/details/quantumfieldtheo00mcma_095/page/n35 17]
 |publisher=[[McGraw–Hill]]
 |year=2008
 |isbn=978-0-07-154382-8
}}</ref> might reveal more about the origin of the mass of quarks and other elementary particles.<ref>
{{cite book
 |author=S. G. Roth
 |title=Precision Electroweak Physics at Electron–Positron Colliders
 |page=VI
 |publisher=[[Springer Science+Business Media|Springer]]
 |year=2007
 |isbn=978-3-540-35164-1
}}</ref>

=== Size ===

In QCD, quarks are considered to be point-like entities, with zero size. As of 2014, experimental evidence indicates they are no bigger than 10<sup>−4</sup> times the size of a proton, i.e. less than 10<sup>−19</sup> metres.<ref>{{cite web| url = http://www.pbs.org/wgbh/nova/blogs/physics/2014/10/smaller-than-small/| title = Smaller than Small: Looking for Something New With the LHC by Don Lincoln ''PBS Nova'' blog 28 October 2014| website = [[PBS]]| date = 28 October 2014}}</ref>

=== Table of properties ===
{{See also|Flavour (particle physics)}}
The following table summarizes the key properties of the six quarks. [[Flavour quantum numbers|Flavor quantum numbers]] ([[isospin]] (''I''<sub>3</sub>), [[Charm (quantum number)|charm]] (''C''), [[strangeness]] (''S'', not to be confused with spin), [[topness]] (''T''), and [[bottomness]] (''B''′)) are assigned to certain quark flavors, and denote qualities of quark-based systems and hadrons. The [[baryon number]] (''B'') is +{{sfrac|1|3}} for all quarks, as baryons are made of three quarks. For antiquarks, the electric charge (''Q'') and all flavor quantum numbers (''B'', ''I''<sub>3</sub>, ''C'', ''S'', ''T'', and ''B''′) are of opposite sign. Mass and [[total angular momentum]] (''J''; equal to spin for point particles) do not change sign for the antiquarks.

{| class="wikitable" style="margin: 0 auto; text-align:center"
|+'''Quark flavor properties'''<ref name=PDGQuarks>
{{cite journal
 |author1=K. A. Olive
 |display-authors=etal
 |collaboration=[[Particle Data Group]]
 |title=Review of Particle Physics
 |journal=[[Chinese Physics C]]
 |volume=38 |issue=9 |pages=1–708
 |year=2014
 |bibcode=2014ChPhC..38i0001O
 |doi=10.1088/1674-1137/38/9/090001 |pmid=10020536
 |doi-access=free
 |arxiv=1412.1408
}}</ref>
! colspan="2" | Particle
! rowspan="2" | Mass<sup>*</sup> {{br}}{{bracket|{{val|ul=MeV/c2}}}}
! rowspan="2" width="50"| ''J'' {{br}}{{bracket|[[elementary charge|''ħ'']]}}
! rowspan="2" width="50"| ''B''
! rowspan="2" width="50"| ''Q'' {{br}}{{bracket|[[elementary charge|''e'']]}}
! rowspan="2" width="50"| ''I''<sub>3</sub>
! rowspan="2" width="50"| ''C''
! rowspan="2" width="50"| ''S''
! rowspan="2" width="50"| ''T''
! rowspan="2" width="50"| ''B′''
! colspan="2" | Antiparticle
|-
! Name
! Symbol
! Name
! Symbol
|-
|colspan="13"|'''''First generation'''''
|-
| up
| {{SubatomicParticle|Up quark}}
| {{val|2.3|0.7}}&nbsp;±&nbsp;0.5
| {{sfrac|1|2}}
| +{{sfrac|1|3}}
| +{{sfrac|2|3}}
| +{{sfrac|1|2}}
| 0
| 0
| 0
| 0
| antiup
| {{SubatomicParticle|Up antiquark}}
|-
| down
| {{SubatomicParticle|Down quark}}
| {{val|4.8|0.5}}&nbsp;±&nbsp;0.3
| {{sfrac|1|2}}
| +{{sfrac|1|3}}
| −{{sfrac|1|3}}
| −{{sfrac|1|2}}
| 0
| 0
| 0
| 0
| antidown
| {{SubatomicParticle|Down antiquark}}
|-
|colspan="13"|'''''Second generation'''''
|-
| charm
| {{SubatomicParticle|Charm quark}}
| {{val|1275|25}}
| {{sfrac|1|2}}
| +{{sfrac|1|3}}
| +{{sfrac|2|3}}
| 0
| +1
| 0
| 0
| 0
| anticharm
| {{SubatomicParticle|Charm antiquark}}
|-
| strange
| {{SubatomicParticle|Strange quark}}
| {{val|95|5}}
| {{sfrac|1|2}}
| +{{sfrac|1|3}}
| −{{sfrac|1|3}}
| 0
| 0
| −1
| 0
| 0
| antistrange
| {{SubatomicParticle|Strange antiquark}}
|-
|colspan="13"|'''''Third generation'''''
|-
| top
| {{SubatomicParticle|Top quark}}
| {{val|173210|510}} ±&nbsp;710&nbsp;*
| {{sfrac|1|2}}
| +{{sfrac|1|3}}
| +{{sfrac|2|3}}
| 0
| 0
| 0
| +1
| 0
| antitop
| {{SubatomicParticle|Top antiquark}}
|-
| bottom
| {{SubatomicParticle|Bottom quark}}
| {{val|4180|30}}
| {{sfrac|1|2}}
| +{{sfrac|1|3}}
| −{{sfrac|1|3}}
| 0
| 0
| 0
| 0
| −1
| antibottom
| {{SubatomicParticle|Bottom antiquark}}
|}
{{center|1=<small><br/>''J'': [[total angular momentum]], ''B'': [[baryon number]], ''Q'': [[electric charge]], ''I''<sub>3</sub>: [[isospin]], ''C'': [[Charm (quantum number)|charm]], ''S'': [[strangeness]], ''T'': [[topness]], ''B''′: [[bottomness]]. <br/>* Notation such as {{val|173210|510}}&nbsp;±&nbsp;710, in the case of the top quark, denotes two types of [[measurement uncertainty|measurement uncertainty]]: The first uncertainty is [[statistical error|statistical]] in nature, and the second is [[systematic error|systematic]].</small>}}