One of the most bizarre phenomena in radioactivity that had been observed in the early part of the 20th century was beta decay. This process of radioactive decay involves an atomic nucleus emitting an electron, and atomic number increases by 1 as a result, presumably due to a neutron becoming converted into a proton. A similar process known as positron emission also occurs, where a proton emits a positron and turns into a neutron (a related process is electron capture).

Beta decay was highly problematic because observing the momenta of the emitted electrons showed that there was an entire spectrum of results. If beta decay was the emission of a single particle from another one that transformed as a result, then there should have been a fixed energy for the emitted electrons in order to conserve momentum and energy. It also failed to conserve angular momentum, as the neutron is a spin-1/2 particle, so is the electron and so is the new neutron, so we have 1/2ħ units of angular momentum that seemingly evaporated into midair!

Wolfgang Pauli came up with a "desperate remedy", which involved postulating the existence of another particle with no mass or a very, very small mass and spin 1/2 that had not yet been detected that carries away part of the extra momentum. Enrico Fermi picked up on this idea and fleshed out the theory, calling the new unidentified particle the neutrino, and subsequent experiments have since proven its existence. An approximate Feynman diagram of Fermi's theory of beta decay:

 \ p  | _
  \   | ν
   \  |          -
    \ |         e
/ n

(the ν is the neutrino).

However, there were problems with this explanation, as it involved a point interaction between four particles, which is very unlikely (like getting two couples to meet at a rendezvous at the same time). A more correct view, which is part of the theory of the weak nuclear force, discovered by Steven Weinberg and Abdus Salam in the 1960's, was that an intermediate vector boson, known as the W particle, is exchanged between the pair of quarks in the nucleon and the lepton pair, or equivalently, the neutron (to be more precise a down quark in the neutron) emits a virtual W- boson, turning into a proton (or the down quark turns into an up quark), which then decays almost instantly into an electron and an electron anti-neutrino. The Feynman diagram then becomes:

              / u
d            /
            \    -     -
                W   / e
              \    /
                  \  _
                   \ ν

Positron emission is similar, except that an up quark turns into a down quark, the W boson is positive, which decays into a positron and an electron neutrino. Because the down quark is heavier than the up quark, there has to be extra energy present in the nucleus (possibly from the nucleus' extra binding energy) to allow this type of reaction to occur.

In experiments conducted to validate the Weinberg-Salam theory performed by Simon van der Meer and Carlo Rubbia at CERN, the W particle was found to be very massive, with a mass of about 80.410 GeV/c2, or roughly the same as the mass of a bromine atom. This great mass means that the distance at which this type of interaction takes place is very, very small (about ħ/mWc ≤ 3 × 10-17 m, less than the diameter of the neutron!), so the point interaction in Fermi's theory is almost correct for beta decay. For higher energies, or interactions involving more massive particles (at relativistic energies for instance), the interaction distance becomes greater, and a real W particle may be emitted.

Another example of a reaction involving a W boson is the decay of the Σ - particle, consists of one strange quark and two down quarks. The decay happens with strange quark emitting a W+ boson. The strange quark as a result turns into an up quark, so the Σ - transforms into a neutron. The boson decays very rapidly into a muon and a muon antineutrino, because the W+ boson has enough energy to turn into the "fat electron".

The W particle in general explains processes that involve quark flavor changes, and is responsible for the fact that almost all of the ordinary matter in the universe today consists of up and down quarks only. Other, more massive quark types are unstable thanks to the existence of the force carried by this particle. Any quark type can change to one of the opposite electric charge by emitting or absorbing a W particle. Decay processes always proceed from a more massive quark to a less massive one, as the opposite would violate conservation of energy. A scattering process, such as occurs in particle accelerators like CERN or Fermilab, can reverse the process, producing more massive quarks, provided sufficient energy is available.

The W boson carrying the weak nuclear force can be either positively or negatively charged (a single elementary charge), and the two types are antiparticles of each other. The particle has spin 1, as the weak force produces a vector field. They have a very short lifespan, decaying into a lepton (such as an electron, muon or tau), depending on the amount of energy it has, and the corresponding neutrino in less than 10-25 second. Nearly all elementary particles, with the exception of the other field bosons (i.e. photons, gluons, and gravitons), are subject to interactions by this particle.


Rubin H. Landau, Quantum Mechanics II

Arthur Beiser, Concepts of Modern Physics