Weak interaction
The weak interaction reaches no farther than a distance smaller than the diameter of a single proton. Yet inside that vanishing span, it does something no other force in nature can. It lets one kind of matter become another. A neutron quietly turns into a proton. A heavier particle sheds its identity. Carbon-14 ages into nitrogen-14, the slow tick that makes radiocarbon dating possible. The weak interaction is one of the four known fundamental interactions, sitting alongside electromagnetism, the strong interaction, and gravitation. It is the mechanism behind the radioactive decay of atoms, and it takes part in both nuclear fission and nuclear fusion. Why is a force this short-ranged and this faint responsible for change at the deepest level of matter? Why does it alone refuse to look the same in a mirror? And how did three physicists come to argue it was never a separate force at all, but one half of something larger?
Field strength is the reason for the name. Over any fixed distance, the weak interaction is typically several orders of magnitude weaker than the electromagnetic force, which is itself orders of magnitude weaker than the strong nuclear force. Its coupling constant, the indicator of how often interactions occur, is tiny next to the strong interaction's value of about 1. So the word weak describes intensity, not importance.
Distance changes the picture sharply. At separations of around a thousandth of a femtometer, the weak interaction matches the electromagnetic force in intensity, but from there it falls off exponentially as particles move apart. Scaled up by just one and a half orders of magnitude, at distances of around three femtometers, it becomes ten thousand times weaker. Its effective range stays confined to roughly a hundredth to a tenth of a femtometer.
Mass explains that confinement. The force is carried by bosons whose masses sit at approximately 90 GeV/c squared, far greater than the mass of a proton or neutron. Heavy carriers cannot travel far, and so the range stays short. Those same carriers are short-lived, decaying rapidly once they appear. The Standard Model accounts for their unusual masses through the Higgs mechanism, a point that would later move from theory to confirmed prediction.
Quarks come in six flavours: up, down, charm, strange, top, and bottom. These flavours give composite particles like protons and neutrons their properties. The weak interaction is the only interaction that can change one flavour into another, and the force-carrier bosons mediate the swap. Neither the strong interaction nor electromagnetism permits this. Without weak decay, properties such as strangeness and charm would be conserved across every interaction.
Beta-minus decay is the textbook case. A down quark inside a neutron turns into an up quark, converting the neutron into a proton, and out comes an electron and an electron antineutrino. A common variant runs the other way. In electron capture, a proton and an electron inside an atom combine into a neutron, an up quark becoming a down quark, with an electron neutrino emitted.
Stars depend on this conversion. Hydrogen fuses into helium because the weak interaction can turn a proton into a neutron, which then fuses with another proton to form deuterium. That deuterium carries fusion forward toward helium, and the steady accumulation of neutrons helps build heavier nuclei. The same flavour-changing power explains instability elsewhere: all mesons are unstable because of weak decay.
Time is the signature the weak interaction leaves behind. Because decays governed by it depend on the large masses of the W bosons, they unfold far more slowly than processes driven by the strong or electromagnetic forces. Weak decays can only be observed at all when faster competing decays are absent.
Two pions show the gap. A neutral pion decays electromagnetically and survives only a fleeting instant. A charged pion can only decay through the weak interaction, so it lives about a hundred million times longer than its neutral counterpart. The contrast comes purely from which force is allowed to act.
The free neutron is the extreme case. Left on its own, it takes about fifteen minutes to decay by the weak force, an eternity by the standards of particle physics. Inside that decay, a down quark emits a virtual W boson and becomes an up quark. The limited energy available, set by the small mass difference between the quarks, means that boson can only produce the two lightest possible products: an electron and an electron antineutrino. That same decay is what powers tritium luminescence and the related field of betavoltaics.
Currents define the two types of weak interaction. The charged-current interaction is named because the fermions involved form a current with nonzero total electric charge. The neutral-current interaction is named because that current carries zero total charge. The labels predate the discovery of the mediator bosons, so they describe the charge of the fermion current, not the bosons themselves.
The charged current rearranges identities. A charged lepton can absorb a W boson and become the matching neutrino, keeping the same flavour family of electron, muon, or tau. A down-type quark can turn into an up-type quark by emitting or absorbing a W boson, becoming a quantum superposition of the three up-type quarks. The probabilities for which one it becomes are set out in the CKM matrix tables.
The neutral current is more permissive. Here a quark or lepton emits or absorbs a neutral Z boson, and unlike the charged current, it can deflect any two fermions in the Standard Model. Particles or antiparticles, any electric charge, both left and right chirality all participate, though with differing strength. This is the interaction responsible for the rare deflection of neutrinos, which otherwise feel only gravity and the weak force.
Around 1968, Sheldon Glashow, Abdus Salam, and Steven Weinberg described electromagnetism and the weak interaction as two aspects of a single electroweak interaction. Their work earned the 1979 Nobel Prize in Physics. The theory explained why there are three massive weak carriers alongside the massless photon of electromagnetism.
The Higgs field is the hinge of the argument. At very high energies, four components of the Higgs field and four massless electroweak vector bosons exist, each boson resembling the photon. At low energies the gauge symmetry breaks down to the symmetry of electromagnetism, because one Higgs field acquires a vacuum expectation value. Three would-be massless Higgs bosons are absorbed into the weak bosons, which gain mass through the Higgs mechanism. The fourth gauge boson, the photon, couples to no Higgs field and stays massless.
Prediction turned into detection. The theory forecast the masses of the W and Z bosons before they were found, and their existence was directly confirmed in 1983. On the 4th of July 2012, the CMS and ATLAS teams at the Large Hadron Collider independently announced a previously unknown boson of mass between 125 and 127, behaviour consistent with a Higgs boson. By the 14th of March 2013, a Higgs boson was tentatively confirmed to exist.
Parity conservation was once assumed to be a universal law. An experiment and its mirror image were expected to give identical results, and classical gravitation, electromagnetism, and the strong interaction all respect this symmetry. In the mid-1950s, Chen-Ning Yang and Tsung-Dao Lee suggested the weak interaction might not. In 1957, Chien Shiung Wu and her collaborators discovered that it indeed violates parity, and the Wu experiment earned Yang and Lee the 1957 Nobel Prize in Physics.
A new framework followed the surprise. In 1957, Robert Marshak and George Sudarshan, and somewhat later Richard Feynman and Murray Gell-Mann, proposed a V minus A, or left-handed, Lagrangian for weak interactions. In this theory the weak interaction acts only on left-handed particles and right-handed antiparticles. Because the mirror image of a left-handed particle is right-handed, the violation of parity is maximal. The theory predated the Z boson, so it left out the right-handed fields of the neutral current.
A deeper symmetry then fell as well. The V minus A theory still allowed the combined symmetry CP, which pairs parity with charge conjugation, the swap of particles and antiparticles. In 1964, James Cronin and Val Fitch found clear evidence in kaon decays that CP could be broken too, winning the 1980 Nobel Prize in Physics. In 1973, Makoto Kobayashi and Toshihide Maskawa showed that CP violation required more than two generations of particles, effectively predicting an unknown third generation, work that earned them half of the 2008 Nobel Prize in Physics. CP violation is rare, yet it is widely believed to explain why the universe holds far more matter than antimatter, forming one of Andrei Sakharov's three conditions for baryogenesis.
Common questions
What is the weak interaction in particle physics?
The weak interaction, also called the weak force or weak nuclear force, is one of the four known fundamental interactions, alongside electromagnetism, the strong interaction, and gravitation. It is the mechanism responsible for the radioactive decay of atoms and takes part in nuclear fission and nuclear fusion.
Why is the weak interaction called weak?
The weak interaction is called weak because its field strength over any set distance is typically several orders of magnitude less than that of the electromagnetic force, which is itself orders of magnitude weaker than the strong nuclear force. Its coupling constant is tiny compared with the strong interaction's value of about 1.
How does the weak interaction change quark flavour?
The weak interaction is the only interaction that lets quarks swap their flavour, mediated by force-carrier bosons. In beta-minus decay, a down quark in a neutron changes into an up quark, converting the neutron into a proton and emitting an electron and an electron antineutrino.
Who developed the electroweak theory of the weak interaction?
Sheldon Glashow, Abdus Salam, and Steven Weinberg developed the electroweak theory around 1968, showing electromagnetism and the weak interaction to be two aspects of a single force. They were awarded the 1979 Nobel Prize in Physics for the work.
Why does the weak interaction violate parity symmetry?
The weak interaction violates parity because it acts only on left-handed particles and right-handed antiparticles, as described by the V minus A theory. Chien Shiung Wu and collaborators discovered this parity violation in 1957, and Chen-Ning Yang and Tsung-Dao Lee received the 1957 Nobel Prize in Physics for proposing it.
When were the W and Z bosons of the weak interaction confirmed?
The existence of the W and Z bosons was directly confirmed in 1983, matching the masses the electroweak theory had predicted beforehand. These carrier particles have masses of approximately 90 GeV/c squared and are short-lived.
How is the weak interaction connected to radiocarbon dating?
The weak interaction makes radiocarbon dating possible because carbon-14 decays through the weak interaction into nitrogen-14. Most fermions decay by a weak interaction over time, and the same process underlies tritium luminescence and betavoltaics.
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