8.1 Standard

The Standard Model of electroweak interactions is currently in very good agreement with high-precision experimental data, as detailed in Chaps. 3 and 4. Cornerstones are the precise determination of mass and width of the Z and W bosons, which currently yield:

$$\begin{aligned} {M_{\textrm Z}}&=91.1875\pm 0.0021 {{\textrm GeV}},\\ \Gamma _Z&= 2.4952\pm 0.0023 {{\textrm GeV}},\\ {M_{\textrm W}}&= 80.399\pm 0.023 {{\textrm GeV}},\\ \Gamma _W&= 2.098\pm 0.048 {\textrm GeV}, \end{aligned}$$

combining LEP and Tevatron results. By studying in detail the Z decays to fermions, the corresponding coupling constants and partial decay widths were measured and the electroweak mixing angle together with the \(\rho\)-parameter were extracted as

$$\begin{aligned} {\sin^2{\theta_{eff}^\ell}}& = 0.23153\pm 0.00012, \\ {\rho_\ell}&= 1.0050\pm 0.0010. \end{aligned}$$

These results differ significantly from the tree level predictions of the Standard Model, showing clearly the necessity to include radiative corrections. Their analysis indicates that the Standard Model Higgs boson should be lighter than \(186 {\textrm GeV}\) when evaluating a large set of electroweak data.

At LEP energies above the Z pole, the electroweak gauge bosons were produced singly or in pairs which allowed a detailed study of the triple and quadruple gauge boson couplings. In both, the neutral and charged sector, they were found to be in good agreement with the non-abelian \(SU(2)\times U(1)\) gauge structure of the Standard Model. In particular, the W boson couplings to the photon and Z boson were determined as:

$$\begin{aligned} g_1^Z&=&\phantom{-}0.991^{+0.022}_{-0.021}\;,\\ \kappa_\gamma&=&\phantom{-}0.984^{+0.042}_{-0.047}\;,\\ \lambda_\gamma&=&-0.016^{+0.021}_{-0.023}\;, \end{aligned}$$

in perfect agreement with the theoretical prediction.

The measurement of the W mass at LEP and the control of the corresponding systematic uncertainties would not have been possible without the detailed understanding of Final State Interactions in hadronic W-pair decays. The effects of Bose-Einstein correlations between the decay products of two hadronically decaying W bosons were found to be small or absent. However, there is a 51% probability for Colour Reconnection between these decay products. The LEP W-mass analyses were therefore optimised to reduce the sensitivity of the mass measurement to Colour Reconnection effects.

The large collection of electroweak data, including top mass and low-\(Q^2\) measurements, provides constraints to the last missing piece in the Standard Model, the mass of the Higgs boson, though not proving its existence. This will only be achieved by direct searches which are currently ongoing at the Tevatron and which will be intensified once the LHC will start collecting data.

The LHC collider and the experiments ATLAS and CMS are now completed and ready to take data. Subdetectors for measuring electrons, muons, taus and jets with good precision were built. They will be able to trigger on and measure the different particles up to the design luminosity of \(10^{34} {cm}^{-2}{s}^{-1}\) at centre-of-mass energies between 7 and 14 TeV. The road-map to achieve the necessary detector performance is laid out, and analysis frameworks to measure the detector parameters from data are being prepared.

ATLAS and CMS are expected to improve the precision measurements of the masses of the W boson and of the top quark, with uncertainties below \(10 {\textrm MeV}\) and \(1 {\textrm GeV}\) respectively, once the detectors are understood and their performances optimised. By studying Z boson decays to electron-positron pairs the electroweak mixing angle, \({\sin^2{\theta_{eff}^\ell}}\), can be determined with an accuracy of about \(2\times 10^{-4}\), close to the LEP and SLC results. The sensitivity to triple gauge boson couplings is enhanced at the LHC due to the high centre-of-mass energy that will be reached. Anomalous contributions to the TGCs will be tested at the per cent level. Constraints from electroweak data for theoretical models will therefore be narrowed further.

Once the LHC running starts and enough data is collected, the ATLAS and CMS experiments will eventually clarify if the Standard Model Higgs boson exists. In the complete theoretically and experimentally possible mass range of \(114.4 {\textrm GeV} < {\textrm M}_{{\textrm H}} < 1 {\textrm TeV}\) the combined analyses of ATLAS and CMS will be able to discover the Higgs boson with about \(2-20 \mathrm{fb^{-1}}\) of data depending on the Higgs mass. With some more luminosity collected, the properties of the Higgs boson, like mass, width, couplings, spin and CP structure can be measured.

However, there are arguments, of which some are mentioned in Chaps. 1 and 4, that the theory may need to be extended beyond the well-working Standard Model. Electroweak symmetry breaking, which necessarily appears at the \({\textrm TeV}\) scale, may be induced by super-symmetry, composite Higgs models, or theories with strongly interacting vector-bosons. These signatures may appear even before a Higgs boson is seen, for example as heavy Z’ or W’ bosons or light super-symmetric particles.

At the advent of the LHC start, the LHC experiments are well prepared to further test the Standard Model and to search for new physics beyond it. Very rare processes and detailed studies of the findings will be possible at the upgraded LHC, the sLHC, with a factor of 10 increase in instantaneous luminosity. The ATLAS and CMS detector communities are already now investigating further improvements of their tracking and forward detectors, and their trigger and detector readout systems to be prepared for an even more challenging background environment.

Complementary measurements to the LHC in the Higgs, electroweak, and super-symmetric sector will be possible with a future International Linear Collider [1,2,3]. In \(\{{e^+e^-}\}\) collisions up to 500 TeV, the Standard Model Higgs and top quark masses can be measured with about 50 MeV precision. The HVV couplings can be derived with a few percent uncertainty from the Higgs-strahlung and boson-fusion production cross-sections. The top Yukawa coupling to the Higgs boson is measurable in \(\{{textrm t} \overline{{t}}\}\)H production, and the ratios of the other \(\{{f\bar{f}}\}\)H couplings are accessible through the different branching fractions \(Br(H\to \{{f\bar{f}}\})\). If spin and CP parameters of the Higgs boson will not have been determined by the LHC, the ILC experiments will be able to measure those, e.g., by scanning the threshold region of \(\{{e^+e^-}\}\to ZH\) production and by studying the ZH production and decay kinematics. The ILC will also allow a more detailed measurement of the properties of possible super-symmetric particles that may be discovered at the LHC. The tandem of ILC precision measurements and LHC discovery potential in Higgs and SUSY physics can therefore fulfil a similar task as the Sp\({\bar{p}}\)S and LEP experiments in the past in case of W and Z discovery and precision physics.

In the very near future it is however guaranteed that the LHC experiments, once having analysed the first few \(100 pb^{-1}\) to \(fb^{-1}\) of data and especially when running at LHC design luminosity and beyond, will certainly change the landscape in particle physics. They will provide interesting insights into physics at the TeV energy scale and will eventually shed light on the mechanism of electroweak symmetry breaking.