Search for Higgs Bosons in e+e− Colliders

Introduction In spite of theoretical and experimental outstanding achievements, particle physics has not been able to reach a conclusion on the simple question: from which mechanism does mass originate ? It is a common belief that such a mechanism will be characterized by the observation of at least a scalar particle. Whether this object is elementary (SM or MSSM scenario), composite (technicolor scenario) or too heavy to be observed as a particle (cf. M. Chanowitz'talk at this school) remains uncertain. In this talk, I will discuss the first scenario and argue that a leptonic collider provides the best tool to study the properties of the Higgs boson(s). In the first chapter, I will recall the theoretical and experimental arguments in favour of this scenario and the predicted properties of the neutral Higgs boson within the SM scheme and its supersymmetric extensions (for a thorough discussion of the theoretical aspects, I refer the reader to the talk given by Z. Kunszt at this school). In the second part of my talk, I will describe the strategy followed at LEP2 for the neutral Higgs boson searches, discuss the present results and future prospects. In the third part, I will go through the future lepton colliders under study and give some examples of what could be achieved with such machines. Theory The SM scenario In this scenario one simply assumes that there is a single scalar complex iso-doublet[1] Φ " which does everything " :-It gives masses to W ± and Z bosons (and leaves the photon massless) through the Higgs mechanism. The masses are given in terms of the vacuum expectation of Φ, v, and of the gauge couplings. 3 components of Φ are " eaten " to create the longitudinal components of W ± and Z, while the last degree of freedom left is identified to the Higgs boson.-It generates the fermion masses (with the exception of neutrinos) through the Yukawa couplings. As a consequence, the Higgs boson couples to the fermions proportionally to their masses. This scenario is clearly minimal in the sense that it implies a common source for all masses. Nature could clearly be less " economical " and use different fields for both purposes. As we shall see in describing the detectability of the Higgs boson through b-tagging, this would have serious consequences on the ongoing experimental searches. The scalar potential of Φ can be written: V …