Searching for the Higgs at SLC and lep

We compute the cross section for the process e⁺e⁻ →Hff as a function of Higgs boson mass and of center-of-mass energies of M_z and beyond. We conclude that searches for a Higgs of mass less than 50 GeV are far more effective when carried out near the Z boson resonance that at any higher energy. However, a new window of Higgs boson masses extending from 50–107 GeV can be explored if and when e⁺e⁻ collisions can be studied with high luminosity at collision energies of 200 GeV. Collider energies at intermediate energies can play no useful role in the search for the Higgs.     

W pair production by Z0 pair fusion

Using the effective gauge boson approximation we calculate the invariant mass distribution and longitudinal polarization fraction for the production of W⁺W⁻ pairs by Z⁰ pair fusion at a collider energy of 40 TeV. The mass distribution is compared with the case where the production is by W⁺W₋ pair fusion. Results are presented for Higgs masses of 0.5 and 1 TeV.     

Higgs detection via Z0 polarisation

It is demonstrated that if one determines the polarisation of the Z⁰ final states from pp → Z⁰Z⁰ → charged leptons, then one can reliably detect the presence of a Higgs boson in the range 300 GeV < m_H < 1 TeV at the SSC. This is in contrast to other methods for Higgs detection which are susceptible to QCD backgrounds, severe cuts, or uncertainties in the Z⁰ continuum. For one year's running at the nominal SSC luminosity of 10³³ cm⁻² s⁻¹ and energy 40 TeV there are sufficient numbers of such events to perform the straightforward analysis.     

Higgs self-coupling in the fusion channel at the international linear collider

We investigate the Higgs pair production process at the international linear collider (ILC), focusing on the measurement of the trilinear self-coupling of the Higgs boson in the fusion channel. The sensitivity of this measurement is discussed in the Higgs mass range 140–200 GeV at a center-of-mass energy between 1 TeV and 1.5 TeV.      

Production on heavy Higgs bosons ate + e −-colliders

The production of Higgs bosons in the mass range of 200 GeV/c²<M _H <1TeV/c ² ate ⁺ e ^? super colliders is studied. We consider the possibilities to detect Higgs bosons via their decays into vector boson pairs. Single particle distributions of vektor bosons arising from this decay are compared with background reactions. We find, that the signal of Higgs bosons with masses up to 60- GeV/c² should be visible over the background already at \(\sqrt s = 1\) TeV, provided that the vektor bosons can be identified via their hadronic decay modes.     

On W±, Z0, γ self-interactions: High energy behaviour and tree unitarity bounds

We analyze the high-energy behaviour of vector boson scattering amplitudes within the framework of a recently suggested lagrangian model based on global weak isospin symmetry broken by electromagnetism. Requiring vanishing of the most strongly (as s²) rising contribution to vector boson scattering amplitudes leads to vector boson self-interactions dependent on a single parameter, for which the anomalous W^± magnetic moment, κ, can be chosen. Tree unitarity is violated at about 2 TeV for arbitrary κ as in the SU(2)_L × U(1)_Y theory for m_H → ∞. The model is well suited for significant tests of the vector boson sector of the SU(2)_L × U(1)_Y electroweak theory in processes such as e⁺e⁻ → W⁺W⁻.     

Bounds on new contact interactions from future e+e− colliders

A systematic study of the “reach” for contact interactions as a probe for new physics at future e⁺e⁻ colliders is performed. A large energy range is considered, from PETRA/PEP energies at the lower end, via the Z⁰ peak (SLC/LEP-I) and LEP-II, up to the TeV regime of a potential future linear e⁺e⁻ collider. Lower bounds on the compositeness scale Λ for leptons are calculated for Bhabha scattering and μ-pair production using realistic assumptions on the expected luminosities and detector performances. The impact of longitudinal and transverse polarization is studied in detail. The “reach” corresponding to high rates at the Z⁰ peak is compared to the one obtained from running at higher energies. At LEP-II energies, a sensitivity to (lepton) substructure at the level of Λ ∼ 10 TeV is to be expected. At a 2 TeV e⁺e⁻ collider the sensitivity can be as large as 100 TeV.     

Radiative corrections to Higgs production by e+e−→ZH in the Weinberg-Salam model

We present a complete analysis of the one-loop radiative corrections to the associated production of a Higgs boson (H) and a neutral vector boson (Z) in e⁺e^? annihilation up to energies and Higgs-boson masses of 1 TeV. In the region of interest for future experiments E ? 200 GeV (m_H ? 100 GeV) we find corrections of +25% which are essentially cancelled by soft-photon effects. The latter amount to ?31% for an electron counter resolution of 0.1.     

Baryogenesis at the weak phase transition

The weak phase transition of the hot big bang can produce quarks, leptons and weak bosons which are out of thermal equilibrium. In a simple extension of the standard model it is shown that the reactions following top quark decays can generate the cosmological baryon asymmetry. The top quark mass must be close to 80 GeV and the Higgs boson must be lighter than 1 GeV. This baryogenesis mechanism can be directly tested at e⁺e⁻ and hadron collider by searching for spectacular events containing six or more bottom quarks and a violation of baryon number at the decay vertex of a long lived neutral particle.     

Prospects for the measurement of the Higgs boson masswith a linear <Emphasis Type="Italic">e</Emphasis><Superscript> + </Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>-</Superscript> collider

The potential of a linear e ⁺ e^- collider operated at a centre-of-mass energy of 350 GeV is studied for the measurement of the Higgs boson mass. An integrated luminosity of 500 fb^-1 is assumed. For Higgs boson masses of 120, 150 and 180 GeV the uncertainty on the Higgs boson mass measurement is estimated to be 40, 65 and 70 MeV, respectively. The effects of beam related systematics, namely a bias in the beam energy measurement, the beam energy spread and the luminosity spectrum due to beamstrahlung, on the precision of the Higgs boson mass measurement are investigated. In order to keep the systematic uncertainty on the Higgs boson mass well below the level of the statistical error, the beam energy measurement must be controlled with a relative precision better than 10^-4. Received: 30 May 2005, Revised: 6 July 2005, Published online: 6 October 2005     

Bounds on a top quark with charged Higgs decay

If the top quark decays to a charged Higgs boson t→bH⁺, semileptonic decay signatures are suppressed and the mass bounds set by the UA1 top search are evaded. However the charged Higgs decay mode H⁺→τ⁺ν_τ gives a missing-ϱ_T signature; depending on the decay branching fraction and using the UA1 limits on new physics contributions to missing-ϱ_T, we set bounds on the top mass in this scenario. We also evaluate the feasibility of seeking visible-τ-jet signals for top at a pp̄ collider.     

Conclusions and perspectives

LEP1 precision measurements, combined with LEP2 searches for the Higgs boson, define the framework for future investigations in subatomic physics. In particular they define the energy and the luminosity which are needed at a future e⁺e⁻ collider to settle the issue of the origin of mass and to complement the LHC on the various scenarios proposed beyond the Standard Model. To cite this article: F. Richard, P. Zerwas, C. R. Physique 3 (2002) 1245–1253.     

Anomalous magnetic moment of the muon in the left-right model

The effect of Higgs bosons on the anomalous magnetic moment of the muon is considered within the model that is based on the SU(2)_L×SU(2)_R×U(1)_B–L gauge group and which involves a bidoublet and two triplets of Higgs fields (left-right model). For the Yukawa coupling constants and the masses of Higgs bosons, the regions are found where the model leads to agreement with experimental results obtained at the Brookhaven National Laboratory (BNL) for the anomalous magnetic moment of the muon. In order to explore corollaries from the constraints obtained for the parameters of the Higgs sector, the processes e⁺e^? → μ⁺μ^?, τ⁺τ^? and μ⁺μ^? → μ⁺μ^?, τ⁺τ^? are considered both within the left-right model and within the model involving two Higgs doublets (two-Higgs-doublet model). It is shown that, if the mass of the lightest neutral Higgs boson does indeed lie in the range 3.1–10 GeV, as is inferred from the condition requiring the consistency of the two-Higgs-doublet model with the data of the BNL experiment, this Higgs boson may be observed as a resonance peak at currently operating e⁺e^? colliders (VEPP-4M, CESR, KEKB, PEP-II, and SLC). In order to implement this program, however, it is necessary to reduce considerably the scatter of energy in the e⁺ and e^? beams used, since the decay width of the lightest neutral Higgs boson is extremely small at such mass values. It is demonstrated that, in the case of the left-right model, for which the mass of the lightest neutral Higgs boson is not less than 115 GeV, the resonance peak associated with it may be detected at a muon collider.     

Gaussian effective potential for the U(1) Higgs model

In order to investigate the Higgs mechanism nonperturbatively, we compute the Gaussian effective potential of the U(1) Higgs model (“scalar electrodynamics”). We show that the same simple result is obtained in three different formalisms. A general covariant gauge is used, with Landau gauge proving to be optimal. The renormalization generalizes the “autonomous” renormalization for λ?⁴ theory and requires a particular relationship between the bare gauge coupling e _B and the bare scalar self-coupling λ _B. When both couplings are small, then λ is proportional to e⁴ and the scalar/vector mass-squared ratio is of order e², as in the classic 1-loop analysis of Coleman and Weinberg. However, as λ increases, e reaches a maximum value and then decreases, and in this “nonperturbative” regime the Higgs scalar can be much heavier than the vector boson. We compare our results to the autonomously renormalized 1-loop effective potential, finding close agreement in the physical predictions. The main phenomenological implication is a Higgs mass of about 2 TeV.     

Solving the little hierarchy problem with a light singlet and supersymmetric mass terms

A generalization of the Next-to-Minimal Supersymmetric Model (NMSSM) is studied in which an explicit μ-term as well as a small supersymmetric mass term for the singlet superfield are incorporated. We study the possibility of raising the Standard Model-like Higgs mass at tree level through its mixing with a light, mostly-singlet, CP-even scalar. We are able to generate Higgs boson masses up to 145 GeV with top squarks below 1.1 TeV and without the need to fine tune parameters in the scalar potential. This model yields light singlet-like scalars and pseudoscalars passing all collider constraints.     

Top quark production in the reaction e+e− → e?tb at linear collider energies

The complete tree-level cross sections for the reaction e ⁺e^? →e?tb are computed for top masses of 160 to 200 GeV at center-of-mass energies between 0.2 and 2.0 TeV using the packages CompHEP and GRACE. It is demonstrated that tt? -pair production dominates around $sqrt s=0.5$ TeV, whereas soft photon t-channel exchange contributions grow with increasing energy such that above 1.5 TeV it dominates. Detailed cross section considerations close to the tt? threshold reveals some peculiar properties. It is shown that a precise top quark mass determination is not significantly hampered by the existence of non-tt? diagrams. With desirable assumptions on linear collider luminosities the CKM matrix element | V _tb | might be measured best at or close to $sqrt s=2$ TeV.     

The Higgs-photon-Z boson coupling revisited

We analyze the coupling of CP-even and CP-odd Higgs bosons to a photon and a Z boson in extensions, of the Standard Model. In particular, we study in detail the effect of charged Higgs bosons in two-Higgs doublet models;. and the contribution of SUSY particle loops in the minimal supersymmetric extension of the Standard Model: The Higgs-γZ coupling can be measured in the decayZ → γ+Higgs ate ⁺ e ^? colliders running on theZ resonance, or in the reverse process Higgs →Zγ with the Higgs boson produced at LHC. We show that a measurement of this coupling with a precision at the percent level, which could be the case at futuree ⁺ e ^? colliders, would allow to distinguish between the lightest SUSY and standard Higgs bosons in large areas of the parameter space.     

Vector boson scattering ate + e − colliders as a test ofW ±,Z 0, γ self-interactions

We investigate the potential of a high-energye ⁺ e ^? collider (e.g., CLIC,E _e ⁺=E _e ^?=1 TeV) for determining trilinear and quadrilinear vector boson self-interactions in various vector boson scattering processes which can be measured in reactions of the type \(e^ + e^ - \to (e^ + e^ - ,ve^ - ,\bar ve^ + ,v\bar v) + VV'\) . Our analysis is based upon a recently suggested single parameter Lagrangian model for vector boson self-interactions incorporating globalSU(2) weak isospin symmetry broken by electromagnetism and a minimal increase of vector boson scattering tree amplitudes as a function of the energy. The results are compared with theSU(2) _L ×U(1) _Y predictions for the cases of a light and a heavy Higgs boson. We find that the crosssection for the production of a vector boson pair,VV′, is very sensitively dependent on the magnitude of the single free parameter, κ, the anomalous magnetic dipole moment of theW ^±. The cross-section changes by approximately one order of magnitude, even near production threshold, if κ is varied by one unit around theSU(2) _L ×U(1) _Y value of κ=1.     

Pseudoscalar Higgs boson in experiments at <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> colliders

The potential of experiments at e ⁺ e ^? colliders to search for a signal of a new pseudoscalar Higgs boson and to study its CP properties is analyzed. A new Higgs boson coupling to the b quark and τ lepton, which is parameterized in a model-independent way as m _b /v(a + iγ₅ b), is chosen to be an indicator of its CP nature. We study the e ⁺ e ^? → τ⁺τ^?vv, e ⁺ e ^? → b \(\bar b\)vv, and e ⁺ e ^? → e ⁺ e ^? b \(\bar b\) processes highly sensitive to anomalous Higgs boson couplings due to the dominant contributions from the vector boson fusion processes W*W* → H and Z*Z* → H in the central region. It is shown that the study of polarization of particles in the final state could help to separate the contributions from the scalar and pseudoscalar Higgs bosons. For instance, the consideration of the cascade decays of τ leptons in the e ⁺ e ^? → τ⁺τ^?vv process allows one to determine reliably the CP state of the Higgs boson as well as the value and the sign of the b parameter. As a result of our analysis, we develop a search strategy for a signal of a new pseudoscalar Higgs boson in future experiments at linear e ⁺ e ^? colliders, which will allow one to understand the CP nature of the Higgs boson and set the bounds on its couplings to fermions.