W Boson is one of the massive gauge bosons responsible for the weak interaction. There are two electrically charged forms called the W plus and W minus bosons. Each has spin one and a mass of approximately 80 GeV. The W bosons are elementary particles in the Standard Model and are associated with the charged current component of the electroweak interaction. Their large mass makes the weak interaction short ranged because the exchange of a massive mediator introduces a characteristic length scale inversely related to the mediator mass.
W Bosons interact with quarks and leptons and can change particle flavour within the allowed weak interaction structure. In beta decay, a neutron can transform into a proton through a down quark changing into an up quark while emitting a W minus boson at the underlying interaction level, with the W subsequently producing an electron and electron antineutrino in the effective decay description. The corresponding reverse charge processes involve W plus interactions. These processes are responsible for many forms of radioactive beta decay.
The W Boson is a gauge boson arising from the electroweak theory. Before electroweak symmetry breaking, the relevant gauge fields are associated with the underlying electroweak gauge structure and are not simply the physical W and Z particles observed at low energies. The Higgs field and spontaneous symmetry breaking give the W bosons their mass while leaving the photon massless. This mechanism is a central feature of the Standard Model and connects the properties of the weak and electromagnetic interactions.
W Bosons have very short lifetimes and decay before they can travel macroscopic distances. Their allowed decay channels include leptonic modes involving a charged lepton and its associated neutrino, as well as hadronic modes involving quarks. The exact observed decay probabilities are constrained by the electroweak theory, available phase space and the quark mixing structure. Because neutrinos are not directly detected in ordinary collider detectors, W decays involving neutrinos often appear through missing transverse momentum together with a charged lepton.
W Bosons were discovered at the CERN Super Proton Synchrotron in the early 1980s through proton antiproton collision experiments. Their discovery provided direct evidence for the electroweak gauge bosons predicted by the developing Standard Model. The Z Boson was discovered in the same experimental programme. These discoveries represented major tests of electroweak theory because the observed masses and interaction properties were broadly consistent with theoretical predictions.
W Boson measurements are important precision tests of the Standard Model. Experiments measure its mass, width, production rates and decay properties. The W mass is especially significant because it is related within the electroweak theory to other fundamental parameters and receives quantum corrections from other particles. Precise measurements therefore provide indirect sensitivity to consistency among Standard Model parameters. Different experimental and theoretical determinations must account carefully for calibration, detector effects, parton distributions and higher order quantum corrections.
W Bosons are produced in high energy particle collisions and in some high energy astrophysical processes. At colliders they can be created directly or through decays of heavier particles. The top quark commonly decays into a bottom quark and a W Boson, making W identification important for top quark measurements. W bosons are also central to studies of Higgs boson production and decay channels because electroweak processes can contain W particles as intermediate or final states.
W Boson should not be confused with the photon or Z Boson. The W Bosons carry electric charge and mediate charged current interactions, whereas the photon is electrically neutral and mediates electromagnetic interactions and the Z Boson is neutral and mediates neutral current weak interactions. The W Bosons are also much heavier than the photon. Their mass and charge produce distinctive production and decay signatures. Their short lifetime means that experiments generally reconstruct them from their decay products rather than observing stable tracks corresponding to W particles themselves.
