Boson 2: W and Z bosons responsible for weak force

wo kinds of W bosons exist with +1 and −1 elementary units of electric charge; the W+ is the antiparticle of the W−. The Z boson (or Z) is electrically neutral and is its own antiparticle. All three particles are very short-lived with a mean life of about 3×10−25  s.

These bosons are heavyweights among the elementary particles. With a mass of 80.4 GeV/c2  and 91.2 GeV/c2 , respectively, the W and Z particles are almost 100 times as massive as the proton—heavier than entire atoms of iron. The masses of these bosons are significant because they limit the range of the weak nuclear force. The electromagnetic force, by contrast, has an infinite range because its boson (the photon) is massless.

All three types have a spin of 1.

The emission of a W+ or W− boson can either raise or lower electric charge of the emitting particle by 1 unit, and alter the spin by 1 unit. At the same time a W boson can change the generation of the particle, for example changing a strange quark to an up quark. The Z boson cannot change either electric charge nor any other charges (like strangeness, charm, etc.), only spin and momentum, so it never changes the generation or flavor of the particle emitting it (see weak neutral current).

[edit]The weak nuclear force

The Feynman diagram for beta decay of a neutroninto a proton, electron, and electron antineutrino via an intermediate heavy W boson

The W and Z bosons are carrier particles that mediate the weak nuclear force, much like the photon is the carrier particle for the electromagnetic force. The W boson is best known for its role in nuclear decay. Consider, for example, the beta decay of cobalt-60, an important process in supernova explosions.

6027Co → 6028Ni + e− + νe

This reaction does not involve the whole cobalt-60 nucleus, but affects only one of its 33 neutrons. The neutron is converted into aproton while also emitting an electron (called a beta particle in this context) and an antineutrino:

n0 → p+ + e− + νe

Again, the neutron is not an elementary particle but a composite of an up quark and two down quarks (udd). It is in fact one of the down quarks that interacts in beta decay, turning into an up quark to form a proton (uud). At the most fundamental level, then, the weak force changes the flavor of a single quark:

d → u + W−

which is immediately followed by decay of the W⁻ itself:

W− → e− + νe

Being its own antiparticle, the Z boson has all zero quantum numbers. The exchange of a Z boson between particles, called a neutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of momentum. Unlike beta decay, the observation of neutral current interactions requires huge investments in particle accelerators and detectors, such as are available in only a few high-energy physics laboratories in the world.