Combining the electromagnetic and weak force
For a long time, electromagnetic and weak interactions were thought to be distinct, because their strength, at the low energies normally encountered in daily life, is very different. In a breakthrough in our understanding of nature it was realized that electromagnetic and weak interactions can in reality be traced back to a common origin (Noble prize for S.Weinberg and A.Salam and S.L.Glashow).

S.L.Glashow
Abdus Salam
S.Weinberg

S.L.Glashow, Abdus Salam and S.Weinberg, Nobel Prize 1979, for work on the combination of the electromagnetic and weak interaction
© The Nobel Foundation

A consequence of this new understanding is that at HERA energies both interactions should become equally strong. Measurements at HERA show this directly for the first time. The figure shows the comparison of the two reactions ep --> eX and ep --> Neutrino X, where X stands for the remaining produces particles. The first reaction is electromagnetic (in the quark picture it is electromagnetic elastic electron-quark scattering); the second reaction happens through the weak interaction, as signaled by the outgoing neutrino (in the quark picture the reaction is eq --> Neutrino q). The latter reaction was observed at HERA for the first time (see figure).

Reaction e q -> neutrino q

Reaction Electron Proton --> Neutrino X in the ZEUS detector. The sketch at left shows the reaction mechanism in the quark picture: The electron hits a quark inside the proton and transforms itself into an (invisible) neutrino via weak interaction. The quark is knocked out of the proton and is visible as a jet of particles.

The figure shows that indeed both reactions occur with the same cross section, i.e. the same probability at the highest energy, as expressed by the variable Q2. This means that both, the electromagnetic and weak forces become equally strong at high energies.

Comparison of cross sections of neutral and charged current
Comparison of the cross sections of the reactions electron proton --> electron X (neutral current, NC) and electron proton --> neutrino X (charged current, CC). Both reactions are very similar, expect that the first one occurs via the electromagnetic interaction and the second one via the weak interaction. At high energy, expressed by Q2, both reaction occur with equal probability.

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