The weak nuclear force is one of the four fundamental forces of physics through which particles interact with each other, together with the strong force, gravity, and electromagnetism. Compared to both electromagnetism and the strong nuclear force, the weak nuclear force has a much weaker intensity, which is why it has the name weak nuclear force. The theory of the weak force was first proposed by Enrico Fermi in 1933 and was known at that time as Fermi's interaction. The weak force is mediated by two types of gauge bosons: the Z boson and W boson.

### Weak Nuclear Force Examples

The weak interaction plays a key role in radioactive decay, the violation of both parity symmetry and CP symmetry, and changing the flavor of quarks (as in beta decay). The theory that describes the weak force is called quantum flavourdynamics (QFD), which is analogous to quantum chromodynamics (QCD) for the strong force and quantum electrodynamics (QFD) for the electromagnetic force. Electro-weak theory (EWT) is the more popular model of the nuclear force.

**Also Known As:** The weak nuclear force is also referred to as: the weak force, the weak nuclear interaction, and the weak interaction.

### Properties of the Weak Interaction

The weak force is different from the other forces:

- It is the only force that violates parity-symmetry (P).
- It is the only force that violates charge-parity symmetry (CP).
- It is the only interaction that can change one kind of quark into another or its flavor.
- The weak force is propagated by carrier particles that have significant masses (about 90 GeV/c).

The key quantum number for particles in the weak interaction is a physical property known as the weak isospin, which is equivalent to the role that electric spin plays in the electromagnetic force and color charge in the strong force. This is a conserved quantity, meaning that any weak interaction will have a total isospin sum at the end of the interaction as it had at the beginning of the interaction.

The following particles have a weak isospin of +1/2:

- electron neutrino
- muon neutrino
- tau neutrino
- up quark
- charm quark
- top quark

The following particles have a weak isospin of -1/2:

- electron
- muon
- tau
- down quark
- strange quark
- bottom quark

The Z boson and W boson are both much more massive than the other gauge bosons that mediate the other forces (the photon for electromagnetism and the gluon for the strong nuclear force). The particles are so massive that they decay very quickly in most circumstances.

The weak force has been unified together with the electromagnetic force as a single fundamental electroweak force, which manifests at high energy (such as those found within particle accelerators). This unification work received the 1979 Nobel Prize in Physics, and further work on proving that the mathematical foundations of the electroweak force were renormalizable received the 1999 Nobel Prize in Physics.

Edited by Anne Marie Helmenstine, Ph.D.