User:John R. Brews/Sample2

© Image: John R. Brews
Top: Electromagnetic scattering of two electromagnetic currents due to motion of charges e by exchange of a photon γ. Bottom: Weak scattering of a weak lepton current J ^ℓ and a weak hadron current J ^h by exchange of a W-boson.

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, or what is called generically the grand unified theory (GUT)).

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; MZ=91.1876±0.0021

Decay

The W-boson decay channels are listed below; W⁻ is the charge conjugate of the W⁺

Primary W⁺-boson decay channels

Products	Fraction
$e^{+}\ \nu _{e}$	(10.75 ± 0.13)%
$\mu ^{+}\ \nu _{\mu }$	(10.57 ± 0.15)%
$\tau ^{+}\ \nu _{\tau }$	(11.25 ± 0.20)%
$c\ {\overline {s}}$	(31−11+13)%
all types of hadron combined	(67.6 ± 0.27)%

 

The Z-boson decay channels are listed below:

Primary Z-boson decay channels

Products	Fraction
$e^{+}\ e^{-}$	(3.362 ± 0.0042)%
$\mu ^{+}\ \mu ^{-}$	(3.3662 ± 0.0066)%
$\tau ^{+}\ \tau ^{-}$	(3.3696 ± 0.0083)%
all types of hadron combined	(69.911 ± 0.056)%
$\nu _{e,\mu ,\tau }+{\overline {\nu }}_{e,\mu ,\tau }$ (undetectable)	(20.000 ± 0.055)%

These values compare well with theoretical estimates.

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. The weak isospin is to be distinguished from strong isospin that describes the hadrons as various mulitplets, for example, the proton and neutron as the spin states of a doublet, and the three pions as the three states of a triplet. The coupling constant analogous to the fine structure constant is:Eqs 11.19 - 11.21

${\frac {g_{w}^{2}}{\hbar c_{0}}}={\frac {1}{4\pi {\sqrt {2}}}}{\frac {1}{\hbar c_{0}}}\left({\frac {m_{W}c_{0}}{\hbar }}\right)^{2}G_{F}\approx {\frac {1}{240}}\ .$

Here m_W is the mass of the W-boson, and the Fermi coupling constant, symbol G_F, is defined in terms of the range of the Yukawa potential R_W, which is given by:

$f(r)={\frac {\exp(-r/R_{W})}{r}}\ .$

In terms of R_W:

$G_{F}=4\pi {\sqrt {2}}g_{w}^{2}R_{W}^{2}=4\pi {\sqrt {2}}\left({\frac {\hbar }{m_{W}c_{0}}}\right)^{2}g_{w}^{2}\ ,$

$G_{\mu }=1.16632(4)\times 10^{-5}\ \mathrm {GeV} ^{-2}\ .$ Paschos

with

$R_{W}={\frac {\hbar }{m_{W}c_{0}}}\approx 2.5\mathrm {am} \ .$

Importance

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

Beta decay

(from existing article weak force)

Some radioactive materials undergo a process called beta decay, in which a proton in the nucleus is converted in a neutron, or a neutron in a proton. However, a proton has a positive electric charge while a neutron has no charge. So that the law of conservation of electric charge is not violated, another particle with a positive charge (a positron) is created when a proton changes in a neutron. Positrons are also called beta particles, and hence the name beta decay. In fact, there is another particle which is created together with the positron; this is the elusive neutrino.

On a more fundamental level, both a proton and a neutron consist of three quarks. In the case of a proton, these are two up quarks and one down quark, while a neutron consists of two down quarks and one up quark. The weak force changes a proton in a neutron by changing an up quark to a down quark and creating another particle, the W boson. This W boson decays in the positron and neutrino which are emitted during beta decay.

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

NIST McParland isospin coupling