Influence of charginos on the decay rate ofZ 0→Hγ with and withoutR-parity breaking

We present a detailed calculation for the decay rate ofZ ⁰→Hγ, when charginos are taken into account inside the relevant penguin diagram. One result is that supersymmetry can suppress the contribution of the graph withW-bosons in the loop. We also give a detailed calculation of the exact mass eigenstates of the charginos and the real parts of the Higgs particles, when the scalar tau neutrino gets a vacuum expectation valuev _τ. We show thatv _τ≠0 enhances the influence on the decay rateZ ⁰→Hγ.     

Radiative corrections toe + e −→Zh 0 andZ→γh 0 in the minimal supersymmetric theory

The Higgs sector of the minimal supersymmetric extension of the standard model (MSSM) becomes phenomenologically hard to distinguish from the standard model (SM) in the limit that the CP-odd scalar,A ⁰, is much heavier than theZ boson. If, in addition, all the superpartners lie outside the kinematic reach of present-day (or near-future) colliders the only experimental evidence for supersymmetry (SUSY) might be through internal loops involving superpartners. We have calculated sfermion-induced radiative corrections toe ⁺ e ^?→Zh ⁰ andZ→γh ⁰ in the MSSM, which are enhanced for highm _t .     

Search for neutral Higgs bosons in Z0 decays using the OPAL detector at LEP

A search for neutral Higgs bosons has been performed using the full sample of Z⁰ decays collected by the OPAL detector at LEP up to 1995. The data were taken at centre-of-mass energies between 88 GeV and 95 GeV and correspond to an integrated luminosity of approximately 160 pb^?1. The present search addresses the processes Z⁰→H⁰Z* and h⁰Z*, where H⁰ is the Higgs boson predicted by the Standard Model and h⁰ the lightest neutral scalar Higgs boson predicted in the framework of the Minimal Supersymmetric Standard Model. For the virtual Z⁰ boson, Z*, the following decay channels are considered: Z*→vv?, e⁺e^? and μ⁺μ^?. Two candidate events have been found in the vv?H⁰ channel and one in the μ⁺μ^?H⁰ channel. Combined with earlier searches, the present search excludes the SM Higgs boson, at the 95% confidence level (CL), from the mass range below 59.6 GeV. In the framework of the Minimal Supersymmetric Standard Model, allowing a wide range of variation for most relevant model parameters, a 95% CL lower limit of 44.3 GeV is obtained for the mass of the h⁰ boson. Combined with earlier direct searches for the Higgs boson pair production process Z⁰→h⁰A⁰ and with measurements of the Z⁰ line shape, a 95% CL lower limit of 23.5 GeV is obtained for the mass of the pseudoscalar Higgs boson A⁰, assuming tan β≥ 1.     

Improving the heavy-Higgs four-charged-lepton signature

Assuming the Higgs scalar boson H⁰ of the standard model has mass m_H>2M_Z, we show that the popular four-lepton decay signature H⁰→ZZ→ℓ₁⁺ℓ₁⁻ℓ₂⁺ℓ₂⁻ at the SSC can be significantly enhanced relative to the continuum ZZ background by requiring high transverse momentum for at least one Z. The calculated signal is sensitive to the top quark mass.     

Heavy SUSY Higgs bosons at e + e − linear colliders

The production mechanisms and decay modes of the heavy neutral and charged Higgs bosons in the Minimal Supersymmetric Standard Model are investigated at future e ⁺ e ^? colliders in the TeV energy regime. We generate supersymmetric particle spectra by requiring the MSSM Higgs potential to produce correct radiative electroweak symmetry breaking, and we assume a common scalar mass m₀, gaugino mass m_1/2 and trilinear coupling A, as well as gauge and Yukawa coupling unification at the Grand Unification scale. Particular emphasis is put on the low tan β solution in this scenario where decays of the Higgs bosons to Standard Model particles compete with decays to supersymmetric charginos/neutralinos as well as sfermions. In the high tan β case, the supersymmetric spectrum is either too heavy or the supersymmetric decay modes are suppressed, since the Higgs bosons decay almost exclusively into b and τ pairs. The main production mechanisms for the heavy Higgs particles are the associated AH production and H ⁺H^? pair production with cross sections of the order of a few fb.     

The intermediate-massL M Higgs boson at the NLC: reducible and irreducible backgrounds to the Bjorken production channel

Both the reducible and irreducible backgrounds to the Higgs production channele ⁺ e ^?→H ⁰ Z ⁰ at a Next Linear Collider (NLC) are studied, for the Standard Model (L M) Higgs boson in the intermediate-mass range. A phenomenological analysis that does not exploit any form of tagging on the Higgs decay products is assumed.     

Higgs boson physics in the s-channel at μ+μ− colliders

Techniques and strategies for discovering and measuring the properties of Higgs bosons via s-channel production at a μ⁺μ⁻ collider, and the associated requirements for the machine and detector, are discussed in detail. The unique feature of s-channel production is that, with good energy resolution, the mass, total width and partial widths of a Higgs boson can be directly measured with remarkable accuracy in most cases. For the expected machine parameters and luminosity the standard model (SM) Higgs boson h_SM, with mass ≲ 2m_W, the light h⁰ of the minimal supersymmetric standard model (MSSM), and the heavier MSSM Higgs bosons (the CP-odd A⁰ and the CP-even H⁰) can all be studied in the s-channel, with the heavier states accessible up to the maximal √s over a large fraction of the MSSM parameter space. In addition, it may be possible to discover the A⁰ and H⁰ by running the collider at full energy and observing excess events in the bremsstrahlung tail at lower energy. The integrated luminosity, beam resolution and machine/detector features required to distinguish between the h_SM and h⁰ are delineated.     

Production of neutral MSSM Higgs bosons ine + e − collisions: a complete 1-loop calculation

We present the first complete 1-loop diagrammatic calculation of the cross sections for the neutral Higgs production processese ⁺ e ^?→Z ⁰ h ⁰ ande ⁺ e ^?→A ⁰ h ⁰ in the minimal supersymmetric standard model. We compare the results from the diagrammatic calculation with the corresponding ones of the simpler and compact effective potential approximation and discuss the typical size of the differences.     

Radiative corrections with two Higgs doublets at LEP/SLC and HERA

Formulae for the radiative corrections toe ⁺ e ^?→f \(\bar f\) andep→eX,v _e X′ are given for two Higgs doublets inSU(2)×U(1). The magnitude of deviations from the minimal model is discussed for theM _W ?M _Z mass correlation, thee ⁺ e ^? forward-backward and polarization asymmetries and σ(e ⁺ e ^? → hadrons) at LEP/SLC, and for σ(NC)/σ(CC), charge and polarization asymmetries in deep inelasticep scattering at HERA. Precision experiments can restrict the mass splitting between neutral and charged Higgs bosons to ?100 GeV. In the supersymmetric Higgs model the additional corrections remain unobservably small.     

The CP violation of theB→Kϕ decay at γ(4s) in the non-standard models

The CP violating asymmetry in theB→K? decay is studied in both supersymmetric model and three Higgs doublets model. The absorptive part of the penguin amplitude leads to the CP violating asymmetry in theB _u ^+- →K ^±? decays, which is at most 3% due to the GIM mechanism in the standard model. However, the asymmetry could be relatively large if there exists a new source of the imaginary component in the decay amplitude. In the supersymmetric model, the contributions of the imaginary part of the super-penguin and the super-box processes rather suppress the magnitude of the asymmetry, whereas in the three Higgs doublets model, that of the charged Higgs-penguin process could significantly enhance it.     

Radiative corrections to the vector boson-charged-Higgs-boson vertex

Radiative corrections to the gauge bosoncharged Higgs vertex are calculated to one loop, and are applied to the decayZ→H ⁺ H ^?. Naively, one expects the corrections to be quite large due to the contribution from fermion loops. If this is the case one could hope to extract some information on the Yukawa couplings in the charged Higgs sector.     

Energy spectra and energy correlations in the decayH→ZZ→μ+μ−μ+μ−

It is shown that in the sequential decay $H \to ZZ \to (f_1 \bar f_1 ) + (f_2 \bar f_2 )$ , the energy distribution of the final state particles provides a simple and powerful test of theHZZ vertex. For a standard Higgs boson, the energy spectrum of any final fermion, in the rest frame ofH, is predicted to be dΓ/dx～1+β⁴?2(x?1)², with $\beta = \sqrt {1 - 4m_Z^2 /m_H^2 } $ and 1?β≤x=4E/m _H ≤1+β. By contrast, the spectrum for a pseudoscalar Higgs is dΓ/dx～β²+(x?1)². There are characteristic energy correlations betweenf ₁ andf ₂ and betweenf ₁ andf ₂. These considerations are applied to the “gold-plated” reactionH→ZZ→μ⁺μ^?μ⁺μ^?, including possible effects ofCP-violation in theHZZ coupling. Our formalism also yields the energy spectra and correlations of leptons in the decay $H \to W^ + W^ - \to l^ + v_l l^ - \bar v_l $ .     

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.     

Signatures for neutralinos at theZ 0

We calculate the visible and invisible width for production of neutralinos,Z ⁰→χ _i ⁰ χ _k ⁰ , and all possible decay rates of the neutralinos. We systematically study their dependence on the supersymmetry parameters and work out the domains where detectable signatures occur. These are the one-sided eventsZ ⁰→e ⁺ e ^?+p _miss,Z ⁰→jets+p _miss,Z ⁰→γ+p _miss andZ ⁰→H _2,3 ⁰ +p _miss, and the two-sided eventsZ ⁰→γ+γ+p _miss,Z ⁰→γ+l ⁺ l ^?+p _miss andZ ⁰→γ+jets+p _miss.     

On the Observation Effects of Higgs Particle in e+e—→bb Z0 and tt－Z0 at LEP Ⅱ and NLC

According to the Minimal Standard Model and the Extended Standard Model with two Higgs doublets, we calculated the total cross sections and differential cross sections for the e⁺e^—→bb Z⁰ and tt－Z⁰ at the LEP Ⅱ and NLC energies in order tO study the possible observational effects of the Higgs particle. We found that the observation for the e⁺e^—→bb Z⁰ can give the information on the Higgs particle as long as the mass of the Higgs particle M_H≤140GeV. However, for the e⁺e^—→tt－Z⁰ the effect from Higgs particle will be completely suppressed by the electrowead background and cannot be observed, no matter whether the Higgs particle is heavy or light.     

A search for monojet events produced by virtual Z0 bosons in e+e− annihalation at petra

A search was performed for the associated production of two different Higgs bosons via a virtual Z⁰ in e⁺e^? annihilation (e⁺e^? → h₁⁰h₂⁰) using the JADE detector at PETRA. This was motivated by the interpretation of the monojet events observed at the CERN p$\text{p}$ collider as anomalous Z⁰ decays into two neutral Higgs bosons (h₁⁰ and h₂⁰), where h₁⁰ is stable and escapes detection while h₂⁰ decays into hadrons. Single- or di-jet events with large momentum imbalance are then expected at PETRA energies. No evidence for such events was found in our data; this excludes h₂⁰ masses in the range of 1 to 21 GeV with 95% CL, if the branching fraction for Z⁰ → h₁⁰h₂⁰ is a larger than one half that for $\text{Z}{}^0\text{→ v}{}_{\mathrm{\mu }}\text{v}{}_{\mathrm{\mu }}$. The possibility that the monojets could originate from supersymmetric higgsino production from Z⁰ decay is also examined.     

On the role of the higgs mechanism in present electroweak precision tests

Based on the observablesM _W, Γ _l ,s _W ^?2 (M _Z ² ), we evaluate the parameters Δx, Δy and ε at one-loop level within an electroweak massive vector-boson theory, which does not employ the Higgs mechanism. The theoretical results are consistent with the experimental ones on Δx, Δy, ε. The theoretical prediction for Δy coincides with the standard-model one (apart from numerically irrelevant terms which vanish forM _H→∞). Nonrenormalizability only affects Δx and ε, which differ from the standard-model results by the replacement logM _H→log Λ for a heavy Higgs mass,M _H (where Λ denotes an effective UV cut-off).     

Anomalous interactions at theZ 0-pole

We study, with an effective lagrangian technique, the possible deviations from the standard model ine ⁺ e ^? collisions at theZ ⁰-pole. We consider dimension 6 anomalous interactions satisfying the fullSU(2)×U(1) gauge invariance, baryon and lepton number conservation, as well as full chiral and flavour symmetry. In this scheme, for reactions not involving the Higgs, the effects are all parametrized by modifications ofZ ⁰ partial widths into fermions. Significant anomalies, including CP violation, could only come out inZ ⁰→H ⁰γ and \(Z^0 \to H^0 f\bar f\) .