Weak interaction

 ( Weak Interaction ) 

Its most noticeable effect is due to its first unique feature: The charged weak interaction causes flavour change. For example, a neutron is heavier than a proton (its partner nucleon) and can decay into a proton by changing the flavour (type) of one of its two down quarks to an up quark. Neither the strong interaction nor electromagnetism permit flavour changing, so this can only proceed by weak decay; without weak decay, quark properties such as strangeness and charm (associated with the strange quark and charm quark, respectively) would also be conserved across all interactions.

All are unstable because of weak decay. In the process known as beta decay, a down quark in the neutron can change into an up quark by emitting a Virtual particle  boson, which then decays into an electron and an electron antineutrino. Another example is electron capture a common variant of radioactive decay wherein a proton and an electron within an atom interact and are changed to a neutron (an up quark is changed to a down quark), and an electron neutrino is emitted.

Due to the large masses of the W bosons, particle transformations or decays (e.g., flavour change) that depend on the weak interaction typically occur much more slowly than transformations or decays that depend only on the strong or electromagnetic forces. For example, a neutral pion decays electromagnetically, and so has a life of only about  seconds. In contrast, a charged pion can only decay through the weak interaction, and so lives about  seconds, or a hundred million times longer than a neutral pion. A particularly extreme example is the weak-force decay of a free neutron, which takes about 15 minutes.

All particles have a property called weak isospin (symbol ), which serves as an additive quantum number that restricts how the particle can interact with the of the weak force. Weak isospin plays the same role in the weak interaction with as electric charge does in electromagnetism, and color charge in the strong interaction; a different number with a similar name, weak charge, discussed below, is used for interactions with the . All left-handed have a weak isospin value of either or ; all right-handed fermions have 0 isospin. For example, the up quark has and the down quark has . A quark never decays through the weak interaction into a quark of the same : Quarks with a of only decay into quarks with a of and conversely.

In any given strong, electromagnetic, or weak interaction, weak isospin is conserved: The sum of the weak isospin numbers of the particles entering the interaction equals the sum of the weak isospin numbers of the particles exiting that interaction. For example, a (left-handed) with a weak isospin of +1 normally decays into a (with ) and a (as a right-handed antiparticle, ).

For the development of the electroweak theory, another property, weak hypercharge, was invented, defined as

$Y\_\backslash text\{W\}\; =\; 2\backslash ,(Q\; -\; T\_3),$

$\backslash mu^-\; +\; \backslash mathrm\{W\}^+\; \backslash to\; \backslash nu\_\backslash mu$

$\backslash begin\{align\}$

                \mathrm{d} &\to \mathrm{u} + \mathrm{W}^- \\
 \mathrm{d} + \mathrm{W}^+ &\to \mathrm{u} \\
                \mathrm{c} &\to \mathrm{s} + \mathrm{W}^+ \\
 \mathrm{c} + \mathrm{W}^- &\to \mathrm{s}
     

The W boson is unstable so will rapidly decay, with a very short lifetime. For example:

$\backslash begin\{align\}$

 \mathrm{W}^- &\to \mathrm{e}^- + \bar\nu_\mathrm{e} ~ \\
 \mathrm{W}^+ &\to \mathrm{e}^+ + \nu_\mathrm{e} ~
     

Decay of a W boson to other products can happen, with varying probabilities.

In the so-called beta decay of a neutron (see picture, above), a down quark within the neutron emits a Virtual particle boson and is thereby converted into an up quark, converting the neutron into a proton. Because of the limited energy involved in the process (i.e., the mass difference between the down quark and the up quark), the virtual boson can only carry sufficient energy to produce an electron and an electron-antineutrino – the two lowest-possible masses among its prospective decay products.

At the quark level, the process can be represented as:

$\backslash mathrm\{d\}\; \backslash to\; \backslash mathrm\{u\}\; +\; \backslash mathrm\{e\}^-\; +\; \backslash bar\backslash nu\_\backslash mathrm\{e\}\; ~$

$\backslash mathrm\{e\}^-\; \backslash to\; \backslash mathrm\{e\}^-\; +\; \backslash mathrm\{Z\}^0$

Like the  bosons, the  boson also decays rapidly, for example:

$\backslash mathrm\{Z\}^0\; \backslash to\; \backslash mathrm\{b\}\; +\; \backslash bar\; \backslash mathrm\{b\}$

Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, weak isospin, the neutral-current interaction can cause any two fermions in the standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs.

The quantum number weak charge () serves the same role in the neutral current interaction with the that electric charge (, with no subscript) does in the electromagnetic interaction: It quantifies the vector part of the interaction. Its value is given by:

$Q\_\backslash mathsf\{w\}\; =\; 2\; \backslash ,\; T\_3\; -\; 4\; \backslash ,\; Q\; \backslash ,\; \backslash sin^2\backslash theta\_\backslash mathsf\{w\}\; =\; 2\; \backslash ,\; T\_3\; -\; Q\; +\; (1\; -\; 4\; \backslash ,\; \backslash sin^2\backslash theta\_\backslash mathsf\{w\})\; \backslash ,\; Q\; ~.$

$\backslash \; Q\_\backslash mathsf\{w\}\; \backslash approx\; 2\; \backslash \; T\_3\; -\; Q\; =\; \backslash sgn(Q)\backslash \; \backslash big(1\; -\; |Q|\backslash big)\backslash \; ,$