Weak Interaction - Properties

Properties

The weak interaction is unique in a number of respects:

1. It is the only interaction capable of changing the flavor of quarks (i.e., of changing one type 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. It is propagated by carrier particles that have significant masses (particles called gauge bosons), an unusual feature which is explained in the Standard Model by the Higgs mechanism.

Due to their large mass (approximately 90 GeV/c2) these carrier particles, termed the W and Z bosons, are short-lived: they have a lifetime of under 1×10−24 seconds. The weak interaction has a coupling constant (an indicator of interaction strength) of between 10−7 and 10−6, compared to the strong interaction's coupling constant of about 1 and the electromagnetic coupling constant of about 10-2; consequently the weak interaction is weak in terms of strength. The weak interaction has a very short range (around 10−17–10−16 m). At distances around 10−18 meters, the weak interaction has a strength of a similar magnitude to the electromagnetic force; but at distances of around 3×10−17 m the weak interaction is 10,000 times weaker than the electromagnetic.

The weak interaction affects all the fermions of the Standard Model, as well as the Higgs boson; neutrinos interact through gravity and the weak interaction only, and neutrinos were the original reason for the name weak force. The weak interaction does not produce bound states (nor does it involve binding energy) – something that gravity does on an astronomical scale, that the electromagnetic force does at the atomic level, and that the strong nuclear force does inside nuclei.

Its most noticeable effect is due to its first unique feature: flavor changing. A neutron, for example, is heavier than a proton (its sister nucleon), but it cannot decay into a proton without changing the flavor (type) of one of its two down quarks to up. Neither the strong interaction nor electromagnetism permit flavour changing, so this must proceed by weak decay; without weak decay, quark properties such as strangeness and charm (associated with the quarks of the same name) would also be conserved across all interactions. All mesons 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 W− boson which is then converted into an electron and an electron antineutrino.

Due to the large mass of a boson, weak decay is much more unlikely than strong or electromagnetic decay, and hence occurs less rapidly. For example, a neutral pion (which decays electromagnetically) has a life of about 10−16 seconds, while a charged pion (which decays through the weak interaction) lives about 10−8 seconds, a hundred million times longer. In contrast, a free neutron (which also decays through the weak interaction) lives about 15 minutes.