Electroweak
Unification

Beta Decay and the Weak Interaction

The weak nuclear force is responsible for beta decay — the process by which a neutron decays into a proton, an electron, and an antineutrino. This happens because a down quark inside the neutron is converted into an up quark via a W⁻ boson. The weak force is the only force that breaks the flavour symmetry of quarks — meaning it can transform one quark type into another.

The range of the weak force is extremely short — only about 10⁻¹⁸ meters, far smaller than a proton. This is due to the large mass of its carrier bosons: W⁺, W⁻, and Z⁰. This mass is the direct fingerprint of Higgs symmetry breaking. At low energy, the weak force therefore acts as if it were point-like — Fermi described it this way in his 1934 theory of beta decay, without knowing that massive bosons were behind it.

Another peculiarity of the weak force: it is maximally parity-violating. It couples exclusively to left-handed fermions and right-handed antifermions. Right-handed quarks and electrons do not participate in the weak interaction. This maximal parity violation was one of the biggest surprises in 20th-century particle physics.

W and Z Bosons

The W and Z bosons were discovered in 1983 at CERN's Proton-Antiproton Collider (SppS) — at exactly the masses predicted by electroweak theory. Carlo Rubbia and Simon van der Meer received the Nobel Prize for this in 1984. It was a triumphant confirmation of the theory's predictive power.

Glashow, Salam, Weinberg — One Force from Two

In the 1960s, Sheldon Glashow, Abdus Salam, and Steven Weinberg independently developed a theory that describes electromagnetism and the weak force within a common framework. The core idea: both forces are manifestations of a single SU(2)×U(1) gauge symmetry that was broken at the Big Bang.

Above an energy scale of about 100 GeV — attainable in modern accelerators — the W and Z bosons behave as if massless and become comparable to photons. Electromagnetism and the weak force then become indistinguishable. At low energies (in everyday life), Higgs symmetry breaking splits this unity apart: the W and Z gain mass, the range of the weak force collapses, and the two forces appear radically different.

The Puzzle of Neutrino Mass

In the original electroweak Standard Model, neutrinos are massless — they exist only in a left-handed form and couple exclusively to the weak force. But neutrino oscillation experiments (SNO, Super-Kamiokande, Nobel Prize 2015) showed that neutrinos switch between their three flavour types — which is only possible if they have mass.

This neutrino mass is not included in the Standard Model. It points toward an extension — possibly right-handed "sterile" neutrinos that do not couple to the weak force, or a so-called seesaw mechanism that explains the very small neutrino masses via a very large new mass scale. The absolute mass of individual neutrinos is still not precisely known today: direct measurements (KATRIN, 2025) set an upper limit of 0.45 eV/c² for the effective electron-neutrino mass, while cosmological observations (Planck, DESI) constrain the sum of the three neutrino masses much more tightly to below roughly 0.07–0.12 eV/c². Both are upper limits — that the masses are greater than zero follows from the oscillation experiments, not from a direct mass measurement.