Weak interaction

The theory describing its behaviour and effects is sometimes called quantum flavordynamics (QFD); however, the term QFD is rarely used, because the weak force is better understood by electroweak theory (EWT).

[1] The effective range of the weak force is limited to subatomic distances and is less than the diameter of a proton.

[2] The Standard Model of particle physics provides a uniform framework for understanding electromagnetic, weak, and strong interactions.

An interaction occurs when two particles (typically, but not necessarily, half-integer spin fermions) exchange integer-spin, force-carrying bosons.

In the weak interaction, fermions can exchange three types of force carriers, namely W+, W−, and Z bosons.

He suggested that beta decay could be explained by a four-fermion interaction, involving a contact force with no range.

[5][6] In the mid-1950s, Chen-Ning Yang and Tsung-Dao Lee first suggested that the handedness of the spins of particles in weak interaction might violate the conservation law or symmetry.

[10](p8) The electrically charged weak interaction is unique in a number of respects: Due to their large mass (approximately 90 GeV/c2[11]) these carrier particles, called the W and Z bosons, are short-lived with a lifetime of under 10−24 seconds.

Scaled up by just one and a half orders of magnitude, at distances of around 3×10−17 m, the weak interaction becomes 10,000 times weaker.

The weak interaction does not produce bound states, nor does it involve binding energy – something that gravity does on an astronomical scale, the electromagnetic force does at the molecular and atomic levels, and the strong nuclear force does only at the subatomic level, inside of nuclei.

[16] Its most noticeable effect is due to its first unique feature: The charged weak interaction causes flavour change.

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.

Because of the limited energy involved in the process (i.e., the mass difference between the down quark and the up quark), the virtual W− boson can only carry sufficient energy to produce an electron and an electron-antineutrino – the two lowest-possible masses among its prospective decay products.

For example: Like the W± bosons, the Z0 boson also decays rapidly,[18] for example: Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, and / or weak isospin, the neutral-current Z0 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.

⁠, with its value varying slightly with the momentum difference (called "running") between the particles involved.

This theory was developed around 1968 by Sheldon Glashow, Abdus Salam, and Steven Weinberg, and they were awarded the 1979 Nobel Prize in Physics for their work.

[22] The Higgs mechanism provides an explanation for the presence of three massive gauge bosons (W+, W−, Z0, the three carriers of the weak interaction), and the photon (γ, the massless gauge boson that carries the electromagnetic interaction).

[23] According to the electroweak theory, at very high energies, the universe has four components of the Higgs field whose interactions are carried by four massless scalar bosons forming a complex scalar Higgs field doublet.

The fourth electroweak gauge boson is the photon (γ) of electromagnetism, which does not couple to any of the Higgs fields and so remains massless.

[24] In a speculative case where the electroweak symmetry breaking scale were lowered, the unbroken SU(2) interaction would eventually become confining.

[26] However, in the mid-1950s Chen-Ning Yang and Tsung-Dao Lee suggested that the weak interaction might violate this law.

Chien Shiung Wu and collaborators in 1957 discovered that the weak interaction violates parity, earning Yang and Lee the 1957 Nobel Prize in Physics.

In this theory, the weak interaction acts only on left-handed particles (and right-handed antiparticles).

Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity.

The V − A theory was developed before the discovery of the Z boson, so it did not include the right-handed fields that enter in the neutral current interaction.

Physicists were again surprised when in 1964, James Cronin and Val Fitch provided clear evidence in kaon decays that CP symmetry could be broken too, winning them the 1980 Nobel Prize in Physics.