Fundamental interaction that underlies some forms of radioactivity and certain interactions between subatomic particles. It acts on all elementary particles that have a spin of
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Due to the large mass of the weak interaction's carrier particles (about 90 GeV/c2), their mean life is about 3×10−27 seconds. Even at the speed of light this effectively limits the range of the weak interaction to 10−18 meters, about 1000 times smaller than the diameter of an atomic nucleus.
Since the weak interaction is both very weak and very short range, its most noticeable effect is due to its other unique feature: flavour changing. Consider a neutron (quark content: UDD, or one up quark and two down quarks). Although the neutron is heavier than its sister nucleon, the proton (quark content UUD), it cannot decay into a proton without changing the flavour of one of its down quarks. Neither the strong interaction nor electromagnetism allow flavour changing, so this must proceed by weak decay. In this process, a down quark in the neutron changes into an up quark by emitting a W− boson, which then breaks up into a high-energy electron and an electron antineutrino. Since high-energy electrons are beta radiation, this is called a beta decay.
Due to the weakness of the weak interaction, weak decays are much slower than strong or electromagnetic decays. For example, an electromagnetically decaying neutral pion has a life of about 10−16 seconds; a weakly decaying charged pion lives about 10−8 seconds, a hundred million times longer. A free neutron lives about 15 minutes, making it the unstable subatomic particle with the longest known mean life.
Two charged-current interactions together are responsible for the beta decay phenomenon. The neutral current interaction was first observed in neutrino scattering experiments in 1974 and in collider experiments in 1983.
Although the weak interaction used to be described by Fermi's theory of a contact four-fermion interaction, the discovery of parity violation and renormalization theory suggested a new approach was needed. In 1957, Robert Marshak and George Sudarshan and, somewhat later, Richard Feynman and Murray Gell-Mann proposed a V−A (vector minus axial vector or left-handed) Lagrangian for weak interactions. In this theory, the weak interaction acts only on left-handed particles (and right-handed antiparticles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity.
However, this theory allowed a compound symmetry CP to be conserved. CP combines parity P (switching left to right) with charge conjugation C (switching particles with antiparticles). Physicists were again surprised when in 1964, James Cronin and Val Fitch provided clear evidence in kaon decays that CP symmetry could be broken too, winning them the 1980 Nobel Prize in Physics. Unlike parity violation, CP violation is a very small effect.
According to the electroweak theory, at very high energies, the universe has four identical massless gauge bosons similar to the photon and a scalar Higgs field. However, at low energies, the symmetry of the Higgs field is spontaneously broken by the Higgs mechanism. This symmetry breaking produces three massless Goldstone bosons which are "eaten" by three of the photon-like fields, giving them mass. These three fields become the W+, W– and Z bosons of the weak interaction, while the fourth field remains massless and is the photon of electromagnetism.
Although this theory has made a number of impressive predictions, including a prediction of the mass of the Z boson before its discovery, the Higgs boson itself has never been observed. Producing Higgs bosons will be a major goal of the Large Hadron Collider at CERN.