weak force

weak force
Name	Weak interaction
Caption	A Feynman diagram depicting beta decay, a classic process mediated by the weak interaction.
Theorized	Enrico Fermi (1933)
Discovered	Experiments on beta decay (late 19th–mid 20th century)
Types	Charged current, Neutral current
Mediators	W and Z bosons
Affected particles	Quarks, leptons
Strength	~10−6 at 10−18 m

weak force. It is one of the four known fundamental interactions in physics, alongside gravity, electromagnetism, and the strong interaction. This force is responsible for processes such as beta decay in atomic nuclei and is essential for nuclear fusion reactions that power the Sun. Unlike other forces, it is the only one capable of changing the flavor of quarks and leptons, and it violates the symmetry of parity.

Overview

The weak interaction operates at an extremely short range, approximately 10⁻¹⁸ meters, and is mediated by massive gauge bosons known as the W and Z bosons. Its discovery emerged from studies of radioactivity, particularly the work of Ernest Rutherford and Frederick Soddy on atomic transmutation. A pivotal theoretical formulation was provided by Enrico Fermi in 1933, who described beta decay through a contact interaction. The modern understanding was solidified within the framework of the Standard Model, developed by theorists including Sheldon Glashow, Abdus Salam, and Steven Weinberg, which unified it with electromagnetism into the electroweak interaction. This unification was experimentally confirmed at CERN through the discovery of the W and Z bosons by the UA1 and UA2 collaborations.

Fundamental properties

A defining characteristic is its violation of several symmetries. It maximally violates parity, as demonstrated by the Wu experiment conducted by Chien-Shiung Wu following the proposal of Tsung-Dao Lee and Chen Ning Yang. It also violates charge conjugation symmetry and, through the Cabibbo–Kobayashi–Maskawa matrix, CP violation. The interaction couples to all quarks and leptons, but not to force carriers like the photon or gluon. Its strength is characterized by the Fermi coupling constant, and it is unique in allowing interactions that change electric charge, facilitated by the charged W boson.

Role in particle physics

It is crucial for processes that change particle identity. In beta decay, a down quark transforms into an up quark inside a neutron, emitting a W boson which decays into an electron and an electron antineutrino. This process is governed by the V-A theory. It enables the fusion of protons into deuterium in the proton–proton chain within stellar nucleosynthesis. Furthermore, all leptons experience its effects, with interactions described by the Pontecorvo–Maki–Nakagawa–Sakata matrix leading to phenomena like neutrino oscillation, observed in experiments such as Super-Kamiokande and the Sudbury Neutrino Observatory.

Mediating particles

The force carriers are the massive W and Z bosons, discovered in 1983 at CERN's Super Proton Synchrotron. The W boson comes in charged variants, W⁺ and W⁻, and mediates processes that change particle charge, such as muon decay. The neutral Z boson mediates processes like elastic scattering and was crucial in confirming the electroweak theory. Their large masses, around 80–90 GeV, result from the Higgs mechanism via interaction with the Higgs field, which breaks the electroweak symmetry. The discovery of the Higgs boson at the Large Hadron Collider completed this picture.

Phenomenology and effects

Observable effects are widespread. In cosmology, it governs the neutron decay that determined the baryon asymmetry of the early universe. In medicine, positron emission tomography relies on beta plus decay. The interaction is studied in massive detectors like IceCube and the Kamioka Observatory. It also underlies the operation of nuclear reactors and the synthesis of heavy elements in supernovae via the r-process. The precise measurement of its parameters, such as the Weinberg angle, is a key goal of facilities like the SLAC National Accelerator Laboratory and Fermilab.

History and discovery

The history traces to the study of radioactivity by Henri Becquerel and Marie Curie. The continuous energy spectrum of beta decay puzzled scientists until Wolfgang Pauli postulated the neutrino in 1930. Enrico Fermi's 1933 theory named the interaction. The concept of weak isospin and the V-A theory were advanced by Richard Feynman, Murray Gell-Mann, and George Sudarshan. The proposal of neutral currents by Gerard 't Hooft and Martinus Veltman and their observation in the Gargamelle bubble chamber at CERN were milestones. The awarding of the Nobel Prize in Physics to Sheldon Glashow, Abdus Salam, and Steven Weinberg in 1979, and to Carlo Rubbia and Simon van der Meer in 1984, recognized the theoretical and experimental achievements.

Category:Fundamental interactions Category:Particle physics Category:Quantum field theory