W and Z Bosons

W and Z bosons are elementary particles that mediate the weak interaction, one of the fundamental interactions described by the Standard Model. They comprise the positively charged W⁺, the negatively charged W⁻, and the electrically neutral Z⁰, usually called simply Z. Together, they enable processes that change the identities of matter particles or scatter them through weak interactions. Their discovery at CERN in 1983 provided direct experimental confirmation of central predictions of electroweak theory. (home.cern)

Physical properties

W and Z particles are bosons with spin quantum number 1, making them vector bosons. W⁺ and W⁻ carry opposite electric charges, each equal in magnitude to the elementary charge, and are one another’s antiparticles. The Z has no electric charge and is its own antiparticle. Neither carries the color charge associated with the strong interaction. Their approximate masses are 80.4 GeV/c² for the W and 91.2 GeV/c² for the Z, making both much heavier than ordinary atomic constituents. (pdgweb.lbl.gov)

Both are unstable. Their decay widths are approximately 2.1 GeV and 2.5 GeV, respectively, corresponding to mean lifetimes of roughly $3\times10^{-25}$ seconds. Here, a decay width expresses the inverse lifetime in energy units through $\tau=\hbar/\Gamma$. Their large masses also explain the weak interaction’s short characteristic range: the scale $\hbar/(mc)$ is only a few $10^{-18}$ metres. This is distinct from the distance an energetic, real boson travels before decaying. (pdgweb.lbl.gov)

Charged and neutral weak currents

W exchange produces charged-current interactions. At a matter-particle vertex, the exchanged W transfers one unit of electric charge, connecting a charged lepton with a neutrino, or an up-type quark with a down-type quark. For example, a neutrino can scatter from matter and produce an electron or another charged lepton. Electric charge remains conserved for the complete process. (cds.cern.ch)

In beta decay, a down quark inside a neutron changes into an up quark through a virtual W⁻, which connects to an electron and an electron antineutrino. At the composite-particle level, the neutron becomes a proton. The W is not produced as a freely propagating, on-shell particle: the decay energy is far below its rest energy. Its role is represented by an internal propagator in the quantum calculation. (cds.cern.ch)

Z exchange produces neutral-current interactions, without transferring electric charge. A neutrino may therefore scatter from an electron or quark while remaining a neutrino. Unlike a photon, the Z interacts with electrically neutral neutrinos as well as charged matter particles. W interactions couple to left-chiral fermion fields and their conjugate antifermion fields; Z interactions have different couplings to left- and right-chiral charged fermions. This structure makes weak interactions violate parity, the symmetry relating a process to its spatial mirror image. (cds.cern.ch)

Electroweak theory and mass generation

The electroweak interaction describes weak interactions and electromagnetism within a single gauge theory, based on the symmetry group $SU(2)_L\times U(1)_Y$. Before symmetry breaking, its fields consist of three weak-isospin gauge fields and one hypercharge field. The physical W particles arise from combinations of two weak-isospin fields; the Z and photon arise from mixtures of the remaining neutral fields. (pdg.lbl.gov)

Through the Higgs mechanism, the Higgs field acquires a nonzero vacuum value. The W and Z become massive, while the photon remains massless. Each massive vector boson has three physical polarization states, including a longitudinal state. At tree level, using units with $c=\hbar=1$,

where $g$ and $g'$ are gauge couplings and $v$ is the Higgs vacuum value. These imply $m_W=m_Z\cos\theta_W$, with quantum corrections modifying the relationship between measured quantities. (pdg.lbl.gov)

Prediction and discovery

Electroweak theory developed during the 1960s predicted the charged W and neutral Z particles. In 1973, CERN’s Gargamelle experiment observed weak neutral-current events, establishing evidence for the predicted interaction before direct Z production became possible. These events involved neutrino scattering without the accompanying charged lepton characteristic of charged-current reactions. (home.cern)

Direct discovery followed after CERN converted its Super Proton Synchrotron into a proton–antiproton collider. The UA1 and UA2 experiments identified W candidates through energetic charged leptons accompanied by missing momentum from neutrinos. CERN announced the W discovery on January 25, 1983; the Z discovery followed in June that year. Carlo Rubbia and Simon van der Meer received the 1984 Nobel Prize in Physics for contributions that enabled the discoveries, including the collider project and stochastic cooling of antiproton beams. (home.cern)

Decays and precision measurements

W bosons predominantly decay into a quark–antiquark pair or a charged lepton with its associated neutrino or antineutrino. Z bosons decay into quark–antiquark, charged-lepton–antilepton, and neutrino–antineutrino pairs. Approximately 70% of Z decays are hadronic, 20% are invisible neutrino decays, and 10% produce charged-lepton pairs. (pdgweb.lbl.gov)

Measurements at the Large Electron–Positron Collider determined the Z resonance’s shape and invisible width. These established that three light neutrino species have the ordinary weak coupling and are accessible in Z decay; they do not exclude heavier neutrinos or hypothetical neutrinos without that coupling. At the Large Hadron Collider, W and Z production, masses, decay rates, and interactions provide precision tests of the theory. Their masses are especially sensitive to quantum corrections involving other particles, allowing comparisons between direct measurements and predictions from electroweak fits. (home.web.cern.ch)