Electroweak Theory

This is a theory that combines two of the four fundamental forces, the electromagnetic force and the weak nuclear force into a single, electroweak force. It is also known as the Glashow-Weinberg–Salam theory.

In simple terms, the \(Z^0\) boson can mediate any reaction mediated by virtual photons. This means it can interact with electromagnetic-like particles. But this contribution is very low at low energies. But at high energies, the \(Z^0\) boson dominates the photon-photon interactions.

Description

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

However, at low energies, the gauge symmetry is spontaneously broken down (Spontaneous Symmetry Breaking) due to the \(U(1)\) symmetry of electromagnetism, since one of the Higgs fields acquires a vacuum expectation value of 246 GeV.

Naively, the symmetry breaking would be expected to produce three massless bosons, but instead those three extra bosons get incorporated into the weak bosons, which then acquire mass through the Higgs mechanism. These three bosons are the \(W^+\), \(W^-\) and \(Z^0\) bosons actually observed in the weak interaction. The fourth electroweak gauge boson is the photon (\(\lambda\)), which does not couple to any of the Higgs fields and so remains massless.