Weak interaction

The weak interaction, which is also often called the weak force or weak nuclear force, is responsible for some nuclear phenomena such as beta decay of the atomic nuclei associated with radioactivity and acts between leptons and quarks (semileptonic interactions), between only leptons (leptonic interactions) and between only quarks (non-leptonic interactions) through the exchange of massive vector bosons called the W+, W, and Z bosons. It is the only force that in experiments acts on neutrinos, for which gravity is negligible. A vector boson is a boson with the spin equal to 1.

The masses of W+, W, and Z bosons are far greater than the mass of a proton or neutron, which is consistent with the short range of the weak force. In fact, the force is termed weak because its field strength over a given distance is typically several orders of magnitude less than that of the strong nuclear force or electromagnetic force.

The weak interaction has a finite range of action comparable to the subatomic length scale, therefore particularly small, when compared with human scales. The weak interaction is the only known fundamental interaction that does not conserve parity-symmetry, and similarly, the only one to break charge parity symmetry; it is left-right asymmetric. The weak interaction even violates CP symmetry but does conserve CPT.

According to the electroweak theory, at certain levels of energy, the weak interaction is unified to the electromagnetic interaction into a single interaction called electroweak. Electromagnetism and the weak force are now understood to be two aspects of a unified electroweak interaction — this discovery was the first step toward the unified theory known as the Standard Model. The Standard Model provides a uniform framework for understanding the electromagnetic, weak, and strong interactions.

For energies of the order of 100 GeV, the electromagnetic force and the weak force appear as unified in the electroweak interaction. The range and intensity of a force are the two most important characteristics of fundamental interactions. Intensity provides a measure of the power relationships between the interactions of different nature. Simplifying, the range can be thought of as the maximum distance at which interaction is influential. For example, gravitational interaction has an infinite range of action; for this reason, the Sun exerts its gravitational force even on very distant objects such as Pluto.