User:John R. Brews/Sample2

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:

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

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 mW is the mass of the W-boson, and the Fermi coupling constant, symbol GF, is defined in terms of the range of the Yukawa potential RW as:


 * $$G_F = 4\pi \sqrt 2 g_w^2 R_W^2 = 4 \pi \sqrt2 \left(\frac{\hbar} {m_W c_0}\right)^2 g_w^2 \ ,$$
 * $$G_{\mu} = 1.166 32(4) \times 10^{-5} \ \mathrm{GeV}^{-2} \ . $$ 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.

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.