6 Experimental Precision Tests for the Electroweak Standard Model

Abstract

This paper contains a review of recent precision measurements of electroweak observables and resulting tests of the electroweak Standard Model. Invited Chapter for a Handbook of Particle Physics IntroductionThe last twenty years have seen enormous progress in experimental precision measurements in particle physics and tests of the electroweak Standard Model. New generations of experiments using advanced detectors at high-energy particle colliders perform measurements with a precision unprecedented in high-energy particle physics. This review summarises the major exciting experimental results measured at the highest energies and pertaining to the electroweak interaction.Comparisons with the theory, the Standard Model of particle physics (SM) [1], are used to test the theory and to constrain its free parameters. The data sets analysed for the precision measurements presented here have been accumulated at the world's highest-energy particle colliders over the last two decades. They verify the SM as a renormalisable field theory correctly describing nature. Electron-positron collisions at 91 GeV centre-of-mass energy were studied by the SLD detector [8], operating at Stanford's Linear Collider (SLC) [12] (1989-1998), and by the experiments ALEPH [13], DELPHI [15], L3 [17] and OPAL [21], taking data at the Large Electron Positron collider (LEP) [25] (1989-1995) at the European Laboratory for Particle Physics, CERN, Geneva, Switzerland. This centre-of-mass energy corresponds to the mass of the Z boson, the heavy neutral exchange particle of the electroweak interaction, which is thus produced in resonance, e + e − → Z. While SLC provided interactions at a fixed centre-of-mass energy corresponding to the maximum of the Z resonance cross section, the LEP-I collider provided collisions at several centre-of-mass energy points, thereby scanning the Z resonance lineshape in the range from 88 GeV to 94 GeV. By increasing the LEP center-of-mass energy up to 209 GeV (LEP-II, 1996, the precision measurements of Z-boson properties at LEP-I were complemented by measurements of the properties of the W-boson, the charged carrier of the electroweak interaction, in the reaction e + e − → W + W − at LEP-II. Proton-antiproton collisions are provided by the Tevatron collider operating at the Fermi Na-2 tional Accelerator Laboratory close to Chicago in the USA, at centre-of-mass energies of 1.8 TeV (Run-I, 1992(Run-I, -1996 and 2.0 TeV (Run-II, since 2001), and are studied by the experiments CDF [29] and DØ [30]. Results from the Tevatron experiments important for the electroweak interaction include measurements of the mass of the W boson, as well as the discovery of the sixth and heaviest quark known today, the top quark, in 1995 and the measurements of its properties, in particular its mass. In addition, key measurements were performed in dedicated experiments at lower interaction energies, notably the measurement of the anomalous magnetic moment of the muon [32], as well as the measurements of parity violation effects in a...

LHC Expectations (Machine, Detectors and Physics)

“…The electro-weak fits in the context of the SM, combined with the limits from the direct LEP searches, indicate that we can expect a Higgs boson with 114 < m H < 219 GeV/c 2 [33], with a preference for a very light Higgs, m H ≈ 120 GeV/c 2 . In this mass range the dominant production mode is gluon fusion, but additional channels such as vector boson fusion (VBF) and associated Higgs production (WH, ZH, t tH) are also considered.…”

Asymmetries in Processes of Electron–Positron Annihilation

“…where σ L and σ R are the cross sections with left and right handed helicities of the initial electrons. ) stand for the expression (12) with polarization degrees (13).…”

“…Among these collider projects, the FCC-ee one has the most advanced program of highprecision measurements of SM processes at the Z peak. Such tests have been performed at LEP and SLC and they have confirmed the validity of the SM at the electroweak (EW) energy scale of about 100 GeV [12,13]. During the LEP era, extensive experimental and theoretical studies of asymmetries made an important contribution to the overall verification of the SM, see review [14] and references therein.…”

“…energy in the Z peak region. The constants C(P 1,2 ) stand for the expression(12) with polarization degrees(13).…”