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The '''Standard Model''' of particle physics is the mathematical theory that describes the [[weak force|weak]], [[Maxwell's equations|electromagnetic]] and [[Strong force|strong]] interactions between [[lepton]]s and [[quark]]s, the basic particles of particle physics. This model is very strongly supported by experimental observations, and is considered to be a major achievement (perhaps the most outstanding achievement) of theoretical physics. It does not, however, treat the [[Gravitation|gravitational force]], inclusion of which remains an elusive goal of the ultimate "theory of everything". The Standard Model is accordingly not consistent with [[general relativity]]. The theory is consistent with [[special relativity]].
In the [[Standard Model]] of particle physics, the '''weak interaction''' or '''weak force''' is one of three fundamental interactions, the other two being the ''strong interaction'' (also called the ''color force'') and the [[Maxwell equations|''electromagnetic interaction'']]. [[Gravitation]], the fourth fundamental interaction, is not included in the Standard Model, and its inclusion remains an outstanding issue (for example, an aspect of [[string theory]] and of [[quantum gravity]]).


The model is only qualitatively described in this article, and mathematical details are not provided. To begin, the basic particles in the Standard Model and their interactions are introduced.
The weak interaction is viewed as an ''exchange force'' mediated by three ''messenger particles'', the [[boson]]s: ''W<sup>+</sup>, W<sup>−</sup>'' and ''Z'', with properties listed below:
 
==Particles and interactions==
The interactions between the particles of the Standard Model are well known experimentally, and transcend the Standard Model. However, the particles of the Standard Model are introduced with the ways that they use these interactions to assemble a complete theory of the interactions between various manifestations of matter. The fundamental particles are spin 1/2 [[fermion]]s of two ''types'': [[lepton]]s and [[quark]]s. Their interactions are viewed as ''exchange forces'', which is to say the forces are introduced by the trading back and forth of ''force carriers'', different kinds of particle that represent [[quantum theory|quanta]] of the underlying force fields. So, for example, the quanta of the electromagnetic field are ''[[photon]]s''. The strength of an electromagnetic field is dictated by the number of photons that make it up,  and the exchange of photons between particles with [[electric charge]] is the mechanism underlying the field's ability to exert an electromagnetic force upon these bodies.
===Leptons===
[[Lepton]]s are one ''type'' of fundamental particle. They have spin 1/2 and are ''not'' subject to the strong force. The known leptons are said to be of three ''families'' or ''generations'' (labeled in the table as ''1, 2, 3'') and of six ''flavors'', a generic term for the particle names. They are listed in the table below. Their antiparticles also are leptons with opposite [[electric charge]] ''Q'' and opposite Lepton number ''L<sub>e,&mu;,&tau;</sub>''.
{| class="wikitable" style="margin: 0 auto; text-align:center"
|+Lepton flavor properties
|-
! Particle name
! Symbol
! Family/Generation
! [[Electric charge|Q]] ([[elementary charge|e]])
! L<sub>e</sub>
! L<sub>μ</sub>
! L<sub>τ</sub>
! Mass (MeV)
! Lifetime ([[second|s]])
 
|-
|style="text-align:left"| [[Electron]]
| ''e<sup>−</sup>''
| 1
| −1
| +1
| 0
| 0
| 0.510 998 928(11)<ref name=NISTe/>
| Stable
 
|-
|style="text-align:left"| [[Muon]]
| ''&mu;<sup>−</sup>''
| 2
|−1
| 0
| +1
| 0
| 105.658 3715(35)<ref name=NISTmu/>
| 2.197019(21) × 10<sup>−6</sup>
 
|-
|style="text-align:left"| [[Tau]]
| ''&tau;<sup>−</sup>
| 3
| −1
| 0
| 0
| +1
| 1776.82(16)<ref name=NISTtau/>
| 2.906(10) × 10<sup>-13</sup>
 
|-
|style="text-align:left"| [[Electron neutrino]]
| ''&nu;<sub>e</sub>''
| 1
| 0
| +1
| 0
| 0
| < 225 × 10<sup>−6</sup> <ref name=nuM/>
| Unknown
 
|-
|style="text-align:left"| [[Muon neutrino]]
| ''&nu;<sub>&mu;</sub>''
| 2
| 0
| 0
| +1
| 0
| < 0.19 <ref name=nuM/>
| Unknown
 
|-
|style="text-align:left"| [[Tau neutrino]]
| ''&nu;<sub>&tau;</sub>''
| 3
| 0
| 0
| 0
| +1
| < 18.2 <ref name=nuM/>
| Unknown
 
|}
 
===Quarks===
[[Quark]]s are a ''type'' of particle with spin 1/2 that are subject to strong, weak and electromagnetic forces. The known quarks are listed in the table below. The kinds of quark (''u, d, c, s, t, d'') are referred to as the ''flavor index'' of the quark, and besides a flavor index, each quark has a ''color index'', which may be any of three colors: red, green and blue (''r, g, b''). Their antiparticles also are quarks, but carry ''anti''-colors: anti-red, anti-green, anti-blue. Unlike a particle's electric charge, which can be any multiple of the elementary charge ''e'', a quark can carry only ''one'' unit of color.
 
{| class="wikitable" style="margin: 0 auto; text-align:center"
|+'''Quark flavor properties'''
! Name
! Symbol
! Family/Generation
!width="50"|''B''
!width="50"|''[[Electric charge|Q]]''([[elementary charge|''e'']])
!width="50"|''I''
!width="50"|''C''
!width="50"|''S''
!width="50"|''T''
!width="50"|''B&prime;''
! Mass (MeV)
! Antiparticle
! Antiparticle symbol
 
|-
| Up
| ''u''
| 1
| +1/3
| +2/3
| +1/2
| 0
| 0
| 0
| 0
| 2.34 ± 0.19 <ref name=quark/>
| Antiup
| ''ū''
|-
| Down
| ''d''
| 1
| +1/3
| −1/3
| −1/2
| 0
| 0
| 0
| 0
| 4.78 ± 0.11 <ref name=quark/>
| Antidown
| <math>\overline d</math>
|-
| Charm
| ''c''
| 2
| +1/3
| +2/3
| 0
| +1
| 0
| 0
| 0
| 1.294 ± 0.004 × 10<sup>3</sup> <ref name=quark/>
| Anticharm
| <math>\overline c</math>
|-
| Strange
| ''s''
| 2
| +1/3
| −1/3
| 0
| 0
| −1
| 0
| 0
| 100.2 ± 2.4<ref name=quark/>
| Antistrange
| <math>\overline s</math>
|-
| Top
| ''t''
| 3
| +1/3
| +2/3
| 0
| 0
| 0
| +1
| 0
| 172.9 ±0.6 ±0.9 × 10<sup>3</sup> <ref name=quark/>
| Antitop
| <math>\overline t</math>
|-
| Bottom
| ''b''
| 3
| +1/3
| −1/3
| 0
| 0
| 0
| 0
| −1
| 4.19 (+0.18) (−0.06) × 10<sup>3</sup> <ref name=quark/>
| Antibottom
| <math>\overline b</math>
|}
<small><center>''B'' = [[baryon number]], ''Q'' = [[electric charge]], ''I'' = [[isospin]], ''C'' = [[Charm (quantum number)|charm]], ''S'' = [[strangeness]], ''T'' = [[topness]], ''B''&prime; = [[bottomness]]. <br />* Notation such as ±''xxx'' denotes [[measurement uncertainty]]. In the case of the top quark, the first uncertainty is [[statistical error|statistical]] in nature, and the second is [[systematic error|systematic]].</center></small>
 
The quarks carry fractional electric charge. However, no quark has been observed in isolation, so a "free" fractional electric charge has not been seen. Another result of failure to observe an isolated quark is that the unit of color cannot be measured.
 
===Quanta===
Because gravitation is not included in the standard model, there are three type of interaction included. Each type of interaction is mediated by exchange of quanta that are [[boson]]s, sometimes called ''messenger particles''.<ref name=Britannica/>
{| class="wikitable" style="margin: 0 auto; text-align:center"
{| class="wikitable" style="margin: 0 auto; text-align:center"
|+Messenger particles
|+Messenger particles
Line 208: Line 11:
! Spin
! Spin
! Range ([[metre (unit)|m]])
! Range ([[metre (unit)|m]])
! Mass(GeV)
! Mass(GeV/''c<sub>0</sub>''<sup>2</sup>)
|-
 
|style="text-align:left"| [[Electromagnetism|Electromagnetic field]]
| Photon
| <span style="font-family: Times New Roman; font-size:120%; font-style:italic; font-weight:bold;"> γ </span>
| 1
| &infin;
| < 10<sup>−27</sup> <ref name=gammaM/>
|-  
|-  
|style="text-align:left"| [[Weak field]]
|style="text-align:left"| Weak field  
| Weak bosons
| Weak bosons
| ''W<sup>+</sup>, W<sup>−</sup>, Z''
| ''W<sup>+</sup>, W<sup>−</sup>, Z''
Line 223: Line 20:
|  &asymp; 10<sup>−17</sup>
|  &asymp; 10<sup>−17</sup>
| ''M<sub>W</sub>''=80.399±0.023;<ref name=Wboson/> ''M<sub>Z</sub>''=91.1876±0.0021<ref name=Zboson/>
| ''M<sub>W</sub>''=80.399±0.023;<ref name=Wboson/> ''M<sub>Z</sub>''=91.1876±0.0021<ref name=Zboson/>
|-
|style="text-align:left"| [[Strong field]]
| Gluons (8)
| ''g''
| 1
| &asymp; 10<sup>−15</sup>
| 0<ref name=gluonM/>
|-
|}
====Photons====
The photon mediates the electromagnetic force and has a very long history. It has neither electric charge nor mass.
====Weak gauge bosons====
The massive ''W<sup>+</sup>, W<sup>−</sup>, Z'', weak bosons are the messenger particles for the weak force. Their nonzero mass is expected from the Standard Model because of the introduction of the ''[[Higgs boson]]'', a massive particle yet to be seen experimentally. The ''W<sup>+</sup>, W<sup>−</sup>'' are antiparticles of each other and the  ''Z'' boson is its own antiparticle.
====Gluons====
Gluons mediate the strong force as an exchange force due to ''color'', one property of quarks. A gluon carries both a color and an anti-color. When a quark emits a gluon, its color changes in a way dependent upon the color/anti-color of the emitted gluon. For example, a red quark can emit a red-antiblue gluon, becoming a blue quark. There are nine possible color-anticolor combinations of ''r, g, b'', which leads to only eight possible gluons because emission by a quark of one color-anticolor combination <math>(r\overline r + g \overline g + b \overline b)/\sqrt{3} </math> doesn't change the state of a quark and cannot act as a messenger.<ref name=Boyarkin/> The remaining eight gluon color combinations are shown below.<ref name=Watson/>
{| class="wikitable" style="margin: 0 auto; text-align:center"
|+Gluon color combinations
|-
|style="text-align:left"| <math>r\overline g </math>
| style="text-align:left"|<math>g\overline b </math>
| style="text-align:left"|<math>b\overline r </math>
| <math>\frac{1}{\sqrt{6}}\left(r\overline r+g\overline g -2 b\overline b \right) </math>
|-
| style="text-align:left"|<math>g\overline r </math>
| style="text-align:left"|<math>b\overline g </math>
| style="text-align:left"|<math>r\overline b </math>
| <math>\frac{1}{\sqrt{2}}\left(r\overline r-g\overline g \right) </math>
|-
|-


|}
|}
==Weak isospin==
As with electromagnetism where [[electric charge]] serves to couple matter to the field, and with the strong interaction where [[color]] couples matter to the interaction, with the weak interaction it is the ''weak isospin'' that couples matter to the weak interaction.


====Nuclear forces and mesons====
==Importance==
In 1935 [http://www.nobelprize.org/nobel_prizes/physics/laureates/1949/yukawa-bio.html Yukawa] invented the [[meson]] theory for explaining the forces holding atomic nucleii together, an assemblage of neutrons and protons. The theory led to the experimental observation of the [[pion]] or &pi;-meson and the [[muon]] or &mu;-meson. The behavior of nuclear forces was explained as an exchange of mesons. Today, mesons are considered to be quark-antiquark pairs, and a more refined theory of nuclear interactions is based upon the Standard Model. Nuclear forces are not considered fundamental today, but are a consequence of the underlying ''strong forces'' between quarks, also called chromodynamic forces or color forces. On that basis, nuclear forces are an exchange force fundamentally based upon color, and only approximated by the Yukawa theory.
The weak interaction is responsible for the radioactive decay of subatomic particles and initiates hydrogen fusion in stars.  


==Peculiarities==
The weak interaction is unique in a number of respects:
# It is the only interaction capable of changing the flavor of [[quark]]s (that is, the changing of one species of quark into another).
# It is the only interaction which violates [[parity (physics)|'''P''' or parity-symmetry]]. It is also the only one which violates [[CP-symmetry|'''CP''' symmetry]].
# Its messenger particles have large masses, a feature explained in the [[Standard Model]] by introduction of the [[Higgs boson]], a massive particle yet to be observed. By contrast, the strong force is mediated by zero mass [[gluon]]s, while the electromagnetic force is mediated by the very small (possibly zero) mass [[photon]]s.
==References==
==References==
{{Reflist|refs=
{{Reflist|refs=
<ref name=Boyarkin>
{{cite book |title=Introduction to Physics of Elementary Particles |author=O. M. Boyarkin, Alfred L. Heinzerton |url=http://books.google.com/books?id=WFDs_SJgILQC&pg=PA2 |pages=p. 2 |isbn=160021200X |year=2007 |publisher=Nova Publishers}}
</ref>
<ref name=Britannica>
{{cite book |author=Britannica Educational Publishing |title=The Britannica Guide to Particle Physics |isbn=1615303820 |publisher=The Rosen Publishing Group |year=2011 |pages=p. 15 |url=http://books.google.com/books?id=NJP22nPwnRoC&pg=PA15 |chapter=The basic forces and their messenger particles |editor=Erik Gregersen, ed}}
</ref>
<ref name=gammaM>
{{cite web |url=http://pdg.lbl.gov/2011/listings/rpp2011-list-photon.pdf |title=&gamma; |work=PDG Particle listings  |author=K. Nakamura ''et al.'' |publisher=Particle Data Group |date=June 16, 2011 }}
</ref>
<ref name=gluonM>
{{cite web |url=http://pdg.lbl.gov/2011/listings/rpp2011-list-gluon.pdf |title=g or gluon |work=PDG Particle listings  |author=K. Nakamura ''et al.'' |publisher=Particle Data Group |date=June 16, 2011 }}
</ref>
<ref name=NISTe>
{{cite web |url=http://physics.nist.gov/cgi-bin/cuu/Value?mec2|search_for=atomnuc! |title=Electron mass energy equivalent in MeV ''m<sub>e</sub>c<sub>0</sub><sup>2</sup>'' |publisher=NIST |accessdate=2011-08-26}}
</ref>
<ref name=NISTmu>
{{cite web |url=http://physics.nist.gov/cgi-bin/cuu/Value?mec2|search_for=atomnuc! |title=Muon mass energy equivalent in MeV ''m<sub>&mu;</sub>c<sub>0</sub><sup>2</sup>''|publisher=NIST|accessdate=2011-08-26}}
</ref>
<ref name=NISTtau>
{{cite web |url=http://physics.nist.gov/cgi-bin/cuu/Value?mec2|search_for=atomnuc!|title=Tau mass energy equivalent in MeV ''m<sub>&tau;</sub>c<sub>0</sub><sup>2</sup>''|publisher=NIST|accessdate=2011-08-26}}
</ref>
<ref name=nuM>
{{cite web |url=http://pdg.lbl.gov/2011/listings/rpp2011-list-neutrino-prop.pdf |title=Neutrino properties |work=PDG Particle listings  |author=K. Nakamura ''et al.'' |publisher=Particle Data Group |date=June 16, 2011 }}
</ref>
<ref name=quark>
{{cite web |url=http://pdg.lbl.gov/2011/listings/contents_listings.html |title=QUARKS |work=PDG Particle listings  |author=K. Nakamura ''et al.'' |publisher=Particle Data Group |date=January 15, 2011 }}
</ref>
<ref name=Watson>
{{cite book |title=The quantum quark |author=Andrew Watson |chapter=Table 4.4: The eight gluon color configurations |pages=p. 175 |url=http://books.google.com/books?id=ip50x8IOfnEC&pg=PA175 |isbn=0521829070 |publisher=Cambridge University Press |year=2004}}
</ref>


<ref name=Wboson>
<ref name=Wboson>
Line 310: Line 45:
{{cite web |url=http://pdg.lbl.gov/2011/listings/rpp2011-list-z-boson.pdf |title=Z |work=PDG Particle listings  |author=K. Nakamura ''et al.'' |publisher=Particle Data Group |date=June 16, 2011 }}
{{cite web |url=http://pdg.lbl.gov/2011/listings/rpp2011-list-z-boson.pdf |title=Z |work=PDG Particle listings  |author=K. Nakamura ''et al.'' |publisher=Particle Data Group |date=June 16, 2011 }}
</ref>
</ref>




Revision as of 08:20, 5 September 2011

In the Standard Model of particle physics, the weak interaction or weak force is one of three fundamental interactions, the other two being the strong interaction (also called the color force) and the electromagnetic interaction. Gravitation, the fourth fundamental interaction, is not included in the Standard Model, and its inclusion remains an outstanding issue (for example, an aspect of string theory and of quantum gravity).

The weak interaction is viewed as an exchange force mediated by three messenger particles, the bosons: W+, W− and Z, with properties listed below:

Messenger particles
Interaction field Particle name Symbol Spin Range (m) Mass(GeV/c02)
Weak field Weak bosons W+, W−, Z 1 ≈ 10−17 MW=80.399±0.023;[1] MZ=91.1876±0.0021[2]

Weak isospin

As with electromagnetism where electric charge serves to couple matter to the field, and with the strong interaction where color couples matter to the interaction, with the weak interaction it is the weak isospin that couples matter to the weak interaction.

Importance

The weak interaction is responsible for the radioactive decay of subatomic particles and initiates hydrogen fusion in stars.

Peculiarities

The weak interaction is unique in a number of respects:

  1. It is the only interaction capable of changing the flavor of quarks (that is, the changing of one species of quark into another).
  2. It is the only interaction which violates P or parity-symmetry. It is also the only one which violates CP symmetry.
  3. Its messenger particles have large masses, a feature explained in the Standard Model by introduction of the Higgs boson, a massive particle yet to be observed. By contrast, the strong force is mediated by zero mass gluons, while the electromagnetic force is mediated by the very small (possibly zero) mass photons.

References

  1. ↑ K. Nakamura et al. (June 16, 2011). W. PDG Particle listings. Particle Data Group.
  2. ↑ K. Nakamura et al. (June 16, 2011). Z. PDG Particle listings. Particle Data Group.

NIST McParland

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