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[en] We study the Higgs-boson decays h⁰→bb-bar, h⁰→γγ and h⁰→γZ within the framework of the two Higgs doublet model (THDM) in the context of the decoupling regime, together with tree level unitarity constraints. We show that when the light CP-even Higgs boson of the THDM mimics the Standard Model Higgs boson, not only the one-loop effects to h⁰→{γγ,γZ} but also the one-loop contribution to h⁰→bb-bar can be used to distinguish between THDM and SM. The size of the quantum effects in h⁰→bb-bar are of the same order as in h⁰→{γγ,γZ} and can reach 25% in both cases

[en] One-loop radiative corrections to e⁺e^- → H⁺H^- are considered at future linear collider energies, in the general type II Two Higgs Doublet Model (THDM) and in the Minimal Supersymmetric Standard Model-like (MSSM) Higgs sector. To make the comparison between THDM and MSSM tractable, we have introduced a quasi-SUSY parameterization which preserves all the tree-level Higgs mass-sum-rules of the MSSM, and involves just 3 free parameters in the Higgs sector (instead of 7 in the general THDM) and comprises the MSSM as a particular case. The model-independent soft photon contribution is isolated and shown to be substantial. Important effects come also from the contribution of the model dependent h⁰H⁺H^- and H⁰H⁺H^- vertices to the final state. In the MSSM, the contribution of the Higgs sector is moderate (a few percent) while in the THDM and both for small and large tan β important effects (∼ + 30%) can be found

[en] We study the associated production of charged Higgs bosons with W gauge bosons in high energy e⁺e^- collisions at the one loop level. We present the analytical results and give a detailed discussion for the total cross section predicted in the context of a general Two Higgs Doublet Model (THDM)

[en] We study various production mechanisms of neutral Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM) of relevance for high-energy e⁺e^- colliders. Firstly we consider the associated production of the A⁰ neutral CP-odd Higgs boson with a neutral gauge boson Z at the one loop level. We present a detailed discussion for the total cross-section predicted in the context of the MSSM without supersymmetric (SUSY) CP violating phases, and make a comparison with the non-SUSY Two Higgs Doublet Model (THDM). We show that the MSSM cross-section may be enhanced compared to that for the THDM. Then we study the production processes e⁺e^- → H_i⁰Z, H_i⁰H_j⁰ and H_i⁰ν_eν-bar_e in the context of the MSSM with explicit SUSY CP violating phases. In a given channel we show that the cross-section for all i (=1,2,3) can be above 0.1 fb provided M_H2,3 < or approx. 300 GeV. This should be detectable at a Next Generation Linear Collider and would provide evidence for scalar-pseudoscalar mixing. (author)

[en] Full one-loop electroweak corrections to the on-shell decay H⁺ →W⁺A⁰ are computed in the framework of models with two Higgs doublets (THDM). Such a decay may be dominant for H^± over a wide range of parameter space relevant at present and future colliders. We show that the corrections may approach 40% and in particular are sensitive to λ₅, which parameterizes the discrete symmetry breaking term. We suggest that a measurement of the branching ratio of H⁺ →W⁺A⁰ may offer a possibility of measuring the magnitude of λ₅. (orig.)

[en] We study the effect of a sequential fourth generation on b→sγ, B_s mixing, B→K^(*)l⁺l^-, X_sl⁺l^- and φK_S, taking into account the presence of a new CP phase in quark mixing. We find that the effects via electromagnetic and strong penguins, such as in b→sγ or CP-violation in B→φK_S, are rather mild, but the impact via boxes or Z penguins is quite significant. Not only can one have a B_s mixing much larger than expected, even if it is measured near the present limit, mixing-dependent CP-violation could become maximal. For electroweak penguin modes, the fourth generation can provide the enhancement that perhaps may be needed, while the m_l⁺l^- dependence of the forward-backward asymmetry in B→K^*l⁺l^- can further probe for the fourth generation. These effects can be studied in detail at the B factories and the Tevatron. (orig.)

[en] We calculate the QCD rediative corrections to the production of the supersymmetric scalar partners of quarks in e⁺e^- annihilation. We include both the standard gluonic corrections and the genuine supersymmetric QCD corrections due to quark-gluino loops, and allow for mixing between left- and right-handed scalar quarks which leads to the possibility that the two final state particles have different masses. The corrections are found to be much larger than the ones affecting the production of spin-1/2 particles

[en] We consider the production of a single Standard Model (SM) Higgs boson (h${}^0$) in association with a photon at the future Linear collider (LC) in the context of a type-II seesaw model. The type-II seesaw model extends the SM particle content by adding one triple Higgs where its Higgs coupling is a key parameter of the scalar potential triggering the electroweak symmetry-breaking mechanism in the SM. It is a well motivated model since it provides neutrino masses and mixing. The LC presents a particularly interesting possibility to probe Higgs couplings due to the clean beams in the initial state. We study the one loop processes e${}^{+}$e${}^{-<!--\; ???\; -->}$→h${}^0$γ and e${}^{-<!--\; ???\; -->}$γ→e${}^{-<!--\; ???\; -->}$h${}^0$, we show that it is possible to correlate the total cross section predictions with the couplings h${}^0$γγ and h${}^0$γZ which are sensitive to the presence of singly and doubly charged Higgs. For parameter points allowed by the current experimental constraints, our numerical results show that the effect of H${}^{\pm <!--\; ??\; -->}$ and H${}^{\pm <!--\; ??\; -->\pm <!--\; ??\; -->}$ can be as large as ± 25% with respect to the SM predictions. For both processes, we also present differential cross section for different center of mass energy.

[en] The Higgs boson recently discovered at the Large Hadron Collider has shown to have couplings to the remaining particles well within what is predicted by the Standard Model. The search for other new heavy scalar states has so far revealed to be fruitless, imposing constraints on the existence of new scalar particles. However, it is still possible that any existing heavy scalars would preferentially decay to final states involving the light Higgs boson thus evading the current LHC bounds on heavy scalar states. Moreover, decays of the heavy scalars could increase the number of light Higgs bosons being produced. Since the number of light Higgs bosons decaying to Standard Model particles is within the predicted range, this could mean that part of the light Higgs bosons could have their origin in heavy scalar decays. This situation would occur if the light Higgs couplings to Standard Model particles were reduced by a concomitant amount. Using a very simple extension of the SM — the two-Higgs doublet model — we show that in fact we could already be observing the effect of the heavy scalar states even if all results related to the Higgs are in excellent agreement with the Standard Model predictions