At about the same time, the LEP at CERN became the first e + e − collider to operate above the e + e − → W + W − threshold. Shortly thereafter the Collider Detector at Fermilab (CDF) and DØ experiments at the Fermilab Tevatron collider ( at TeV) pushed the precision below 100 MeV ( 9– 12) using data from Run I (1992–1995) of the Tevatron, achieving a combined precision of 59 MeV ( 13). UA2 performed the first measurement with a precision better than 1 GeV ( 8). More precise measurements of the W boson mass were performed by UA1 ( 7) and UA2 ( 8) with upgraded detectors and much larger data sets delivered by the upgraded operating at GeV. In 1983 this central prediction was confirmed by the discovery ( 3, 4) of the W boson (with a mass of 81 ± 5 GeV) and the discovery of the Z boson just months later ( 5, 6) by the UA1 and UA2 experiments at the European Organization for Nuclear Research (CERN) collider ( at GeV). In this theory the weak force is mediated by the massive W and Z bosons. The electroweak SU( 2) × U( 1) gauge theory, which unifies the weak and EM forces, was proposed in 1967 ( 2). As of now the Tevatron is operating again, and the results from Run II will drive the precision on M W for at least the next few years. The analyses of the Tevatron Run I data were then being finalized and LEP was about to finish collecting data. At that time the most precise measurements available were from Run I of the Tevatron and from the Large Electron-Positron Collider (LEP). Even more precise measurements of M W are needed to test the SM at the loop level and to fully exploit this window on physics beyond the SM.Ī previous review of direct experimental determinations of M W was published in 2000 ( 1). Extensions to the SM (e.g., supersymmetry) predict additional loops that can result in sizeable corrections. In the SM, the quantum loop corrections to M W are dominated by the top quark and Higgs boson loops, aside from the running of the electromagnetic coupling. The precision of the direct measurements of M W and M Z has increased dramatically over the past 25 years, and the predictions are now being tested at the quantum loop level. 1 The agreement of these predictions with the early measurements is one of SM's successes.
Given the precise measurements of the latter quantities, plus experimental determinations of the weak mixing angle from scattering data available at the time, the masses of both the W boson and the Z boson can be predicted to within a few gigaelectronvolts (GeV). In the SM, the mass of the W boson, M W, is related at tree level to the mass of the Z boson, M Z, and the electromagnetic (EM) and weak coupling constants.
The massive W and Z bosons that it predicted have since been discovered. In its four decades of existence, the standard model (SM) of the electroweak interactions has been an impressive success.