Perspectives on Higgs Physics II

The following sections are included:

• CONTENTS

• INTRODUCTION AND OVERVIEW

Using recent electroweak precision experiments, I try to assess what exactly we have learned about electroweak symmetry breaking. First, the ρ parameter is very close to one. Second, all radiative corrections-sensitive observables are consistent with the observed value of the top quark mass, and leave space for essentially nothing else. Therefore the mechanism for electroweak symmetry breaking must be very discrete in its effect on today's observables. If one assumes the validity of the Minimal Standard Model, with a single elementary Higgs boson, and nothing else, then its mass is constrained to be M_H< 375 GeV/c² at 95% C.L.

The Higgs sector of the Minimal Supersymmetric Model (MSSM) is a CP-conserving two-Higgs doublet model that depends, at tree-level, on two Higgs sector parameters. In order to accurately determine the phenomenological implications of this model, one must include the effects of radiative corrections. The leading contributions to the one-loop radiative corrections are exhibited; large logarithms are resummed by the renormalization group method. Implications for Higgs phenomenology are briefly discussed.

We consider the phenomenology of the Standard Model intermediate mass Higgs boson, 71 GeV < M_h < 2M_w The motivation for a Higgs boson in this mass region is emphasized. The branching ratios for the Higgs boson, including electroweak and QCD radiative corrections, are presented, along with production cross sections for e⁺e−,/μ+/μ−,γγ, and hadronic interactions. Search strategies are surveyed briefly.

Higgs boson searches started in the early eighties, but it is only with the achievement of LEP that the first significant and unambiguous bounds were set. The situation after six years of LEP running at centre-of-mass energies at and around the Z peak (LEP 1) is reviewed in detail, in the framework of both the Standard Model and its Minimal Supersymmetric extension (MSSM). The first results obtained after the increase of the LEP centre-of-mass energy above the W pair production threshold (LEP 2) are also given, together with the expected sensitivity after three years of running at the highest energy.

A light mass Higgs boson h can be discovered at the Tevatron in the associated production modes and → Z^°h with enough integrated luminosity. For M_h = 120 GeV, a signal can be established independently in the

channel, where ℓ^± = e^±,μ^±, or in the channel with 20 fb⁻¹. Much less data (≃ 10 fb⁻¹) is needed if these channels are combined, even less if the h(→ r⁺ r⁻) decays can be used. The presence or absence of such a signal is a test of weak scale Supersymmetry, which predicts a lightest Higgs boson with mass M_h ≲ 130 GeV. For some regions of Supersymmetric parameter space, signals containing several b quarks or τ leptons in the final state may be more important.

We review the constraints on Higgs boson properties from effective potential methods. In the Standard Model, the requirement of stability (or metastability) of the standard electroweak minimum puts an upper bound on the scale of new physics as a function of the Higgs mass. This upper bound is below the Planck scale if the Higgs weights ≲ 130 GeV. In supersymmetric extensions of the Standard Model the former methods are useful to compute the Higgs mass spectrum, and couplings, after resumming higher loop effects. In particular, if the Higgs mass weights ≳ 130 GeV, the minimal supersymmetric extension of the Standard Model will be ruled out, while its non-minimal supersymmetric extension containing singlets is only marginally allowed.

After reviewing briefly the upper bounds on the mass of the lightest Higgs boson in the most general unconstrained minimal supersymmetric extension of the Standard Model, we discuss various arguments which reduce the parameter space of the model and give stronger predictions for M_ho. First, the constraints from the presently available experimental data are summarized. Next, the role of several additional theoretical assumptions is studied, after extrapolating the model to high energy scales. The most important ones are: perturbative validity up to the GUT scale and the electroweak symmetry breaking. A Higgs boson with M_ho <100 GeV is predicted in several scenarios. Its absence in that mass range will have important implications for the parameter space of the model.

We discuss the mechanism for electroweak symmetry breaking in supersymmetric versions of the standard model. After briefly reviewing the possible sources of supersymmetry breaking, we show how the required pattern of symmetry breaking can automatically result from the structure of quantum corrections in the theory. We demonstrate that this radiative breaking mechanism works well for a heavy top quark and can be combined in unified versions of the theory with excellent predictions for the running couplings of the model.

We review the ability of the LHC (large hadron collider), NLC (next linear lepton collider) and FMC (first muon collider) to detect and study Higgs bosons, with emphasis on the Higgs bosons of extended Higgs sectors, especially those of the minimal supersymmetric standard model (MSSM). Particular attention is given to means for distinguishing the lightest neutral CP-even Higgs boson of the MSSM from the single Higgs boson of the minimal Standard Model (SM).

Electroweak Baryogenesis is a particularly attractive theoretical scenario, since it relies on physics which can be tested at present high energy collider facilities. Within the Standard Model, it has been shown that the requirement of preserving the baryon number generated at the weak scale leads to strong bounds on the Higgs mass, which are already inconsistent with the present experimental limits. In the Minimal Supersymmetric extension of the Standard Model we demonstrate that light stop effects can render the electroweak phase transition sufficiently strongly first order, opening the possibility of electroweak baryogenesis for values of the Higgs mass at the LEP2 reach. The generation of the observed baryon asymmetry also requires small chargino masses and new CP-violating phases associated with the stop and Higgsino mass parameters. We discuss the direct experimental tests of this scenario and other relevant phenomenological issues related to it.

We present a general overview of models where CP violation enters through the Higgs sector of the theory. These models can be divided into two categories with very distinct phenomenologies, namely models with and without flavour changing neutral currents. The former could saturate ∈_K they constitute a dynamical realization of the ‘superweak’ scenario; they could – possibly as their only other phenomenological consequence – affect significantly the pattern of CP asymmetries in beauty and charm decays. Models without flavour changing neutral currents on the other hand are quite unlikely to provide the major source of CP violation in K → λλ decays. Yet even so they could quite naturally induce an electric dipole moment for neutrons, electrons and atoms relatively close to present experimental upper bounds. Such models could also produce muons in K → μνλ decays with a transverse polarization of O(10⁻⁴) or more and top decays might exhibit CP asymmetries that could reach the 0.1 % level.

I explain the difference between the Higgs mechanism and the Higgs, discuss various options for spontaneous SU(2) × U(l) symmetry breaking and quark and lepton mass generation, and speculate about chiral gauge theories.

We discuss a class of dynamical models in which top condensation occurs at the weak scale, giving rise to the large top quark mass and other phenomena. This typically requires a color embedding, SU(3)_C → SU(3)1×SU(3)2, ergo “Topcolor.” Topcolor suggests a novel route to technicolor models in which sequential quarks condense under the Topcolor interaction to break electroweak symmetries.

We discuss how to experimentally study the electroweak symmetry breaking (EWSB) sector by observing W_LW_L → W_LW_L interactions in the TeV region. We discuss some general features of the event structure in the signal and the background events. Various techniques to enhance the signal-to-background ratio are also presented. We show how to detect longitudinal W-bosons either in the central rapidity region of the detector or in the beam pipe direction. Finally, we globally classify the sensitivities of the Large Hadron Collider and the Linear Colliders to probing the next-to-leading order bosonic operators of the electroweak chiral lagrangian used to parametrize models of strongly coupled EWSB sector.

Signals and backgrounds for strong WW scattering at the SSC and LHC are considered. The Higgs mechanism without Higgs bosons is reviewed from the perspective of what it implies about the possibility of strong WW scattering. Complementarity of resonant signals in the I = 1 WZ channel and nonresonant signals in the I = 2 W⁺W⁺ channel is illustrated using a chiral lagrangian with a J = 1 WW “ρ” resonance. Results are presented for purely leptonic final states in the WZ, W⁺W⁺, and ZZ channels. With a few exceptions, the signals are cleanly observable at the SSC with 10 fb⁻¹, and each model provides an observable signal in at least one gauge boson pair channel. Luminosity requirements are given for the LHC signals to meet the minimum significance criterion.