The Case for New Physics

There is strong evidence that new physical degrees of freedom and new phenomena exist and may be revealed in future collider experiments. The best hints of what this new physics might be are provided by electroweak symmetry breaking. I briefly review certain theories for physics beyond the standard model, including the top-quark seesaw model and universal extra dimensions. A common feature of these models is the presence of vector-like quarks at the TeV scale. Then I discuss the role of a linear ee collider in disentangling this new physics. THE CASE FOR NEW PHYSICS The observed fundamental particles, namely the SU(3)C × SU(2)W × U(1)Y gauge bosons, the longitudinal degrees of freedom of the W and Z, and three generations of quarks and leptons, may explain in principle all observed physical phenomena. However, there is strong evidence for the existence of new phenomena at higher energy scales than the ones probed so far. The most robust argument is provided by the perturbative violation of unitarity in the WW scattering, at a scale of order 1 TeV [1]. Therefore, either the W and Z have strongly coupled selfinteractions at the TeV scale, or new fundamental degrees of freedom exist. The scale of these new phenomena is within the reach of future collider experiments, and exploring them is at the heart of high-energy physics. The standard model accomodates rather well all available data, especially when the Higgs boson is light [2]. More generally, any model with a decoupling limit [3] given by the standard model is viable, at least in that limit. These are models in 1) Plenary talk at the 5th International Linear Collider Workshop (LCWS 2000), Fermilab, October 24-28, 2000. 2) This work was supported by DOE under contract DE-FG02-92ER-40704. which all the particles beyond the standard model may be given large electroweaksymmetric masses. I will refer to them as “decoupling models”. In practice, the decoupling may be only partial, so that new particles with electroweak-symmetric masses of order 1 TeV give rise at the electroweak scale to higher-dimensional operators which may change the fit of the standard model to the data. In particular, the Higgs boson mass may be larger in this case, even close to the triviality bound [4]. In what follows I will concentrate on decoupling models, but one should keep in mind that it is not currently possible to rule out all models without a Higgs boson, because the physical quantities relevant for comparing with the data typically cannot be computed when the interactions are non-perturbative. Even if a standard-model-like Higgs boson will be discovered, and maybe for a while no experimental data will hint at other new physics, there are robust theoretical reasons to expect physics beyond the standard model. First, the Higgs self-coupling increases with the energy and the theory breaks down at some scale. Second, the U(1)Y gauge coupling is also ill-behaved at high energy. Third, the standard model does not include gravity, while the measured gravitational effects are produced by matter composed of standard model particles. Additional reasons to expect new physics are given by the large number of parameters in the standard model, the large hierarchies between their values, and the rather strange field content of the standard model (for example, the Higgs doublet is the only scalar field, and does not appear to be theoretically motivated.) It would be highly desirable that a theory which includes both the standard model and gravity explains why the electroweak interactions are so much stronger than the gravitational interactions. Currently, the only possible explanations for this hierarchy between the electroweak and Planck scales require new particles with mass of order 1 TeV. The decoupling models may be classified according to the solution for the hierarchy problem into supersymmetric extensions of the standard model, theories with a composite Higgs doublet, and theories with extra dimensions. Although convenient, this classification is not clear-cut given that there are supersymmetric models in extra dimensions [5,6], models of extra dimensions that generate composite Higgs doublets [7–9], as well as supersymmetric models of Higgs compositeness [10]. The supersymmetric extensions of the standard model and theories with extra dimensions accessible only to gravity have been covered in other talks at this con-