W and Z Boson Physics

Core Idea

The W+/- and Z bosons are the massive gauge bosons of the weak interaction, discovered at CERN's SppS collider in 1983. Their masses (~80.4 and 91.2 GeV), widths, and couplings to fermions are precisely predicted by the electroweak theory. The W boson mediates charged-current interactions (changing quark and lepton flavor) while the Z mediates neutral-current interactions, and their detailed study tests the SU(2)_L x U(1)_Y gauge structure at the quantum level.

Explainer

The W and Z bosons were discovered at CERN in 1983 by the UA1 and UA2 experiments at the SppS proton-antiproton collider, confirming the electroweak theory of Glashow, Weinberg, and Salam (Nobel Prize 1979). The W boson (mass 80.4 GeV, width 2.1 GeV) mediates all charged-current weak processes: nuclear beta decay, muon decay, quark flavor changes. The Z boson (mass 91.2 GeV, width 2.5 GeV) mediates neutral-current processes. Their masses arise from the Higgs mechanism and are predicted by the gauge couplings and the Higgs vacuum expectation value.

W boson physics at the LHC involves production rates of tens of nanobars (billions of events per year at high luminosity), making the W a precision tool. The charge asymmetry constrains PDFs; the transverse mass distribution measures M_W with ~10 MeV precision; the W polarization tests the V-A structure of the charged current; and W+jets production is a major background to top quark and new physics searches. The helicity structure of W decays is maximally parity-violating: W+ preferentially emits the positively charged lepton in its spin direction, and the negatively charged lepton opposite. This polarization is directly observable in the lepton angular distribution.

The Z boson has been the most precisely studied particle in history, thanks to the LEP and SLD programs. At LEP, approximately 17 million Z decays were recorded across four experiments (ALEPH, DELPHI, L3, OPAL), enabling measurements of M_Z, Gamma_Z, and the Z couplings to individual fermion species with permille precision. The forward-backward asymmetries A_FB measure the product of initial- and final-state Z couplings, directly testing the electroweak mixing angle. The left-right asymmetry A_LR at SLD (using polarized electron beams) provides the single most precise determination of sin^2(theta_eff). Together, these measurements form the foundation of the electroweak precision program.

Vector boson scattering (VBS) -- processes like WW -> WW, WZ -> WZ, and ZZ -> ZZ -- probes the mechanism of electroweak symmetry breaking at the highest energies. Without the Higgs boson, the scattering amplitude for longitudinal W pairs grows as E^2 and violates unitarity at approximately 1.2 TeV. The Higgs boson restores unitarity through cancellation between s-channel Higgs exchange and the gauge boson self-coupling diagrams. The LHC has observed VBS processes and confirmed the expected energy behavior, but precision tests of the WWWW quartic coupling and searches for anomalous couplings continue to probe whether the Higgs sector is exactly as the Standard Model predicts.