Six-fermion production and Higgs boson physics at future e^+ e^- colliders

The six-fermion production processes , with all the lepton flavours and , relevant to the study of the intermediate-mass Higgs boson at future linear colliders are analysed. A Monte Carlo program, taking into account the whole set of tree-level scattering amplitudes and the relevant radiative effects, is developed to provide integrated cross-sections and generation of unweighted events. The complete calculation is compared with the available results of real Higgs production, and the opportunities of precision studies with event generation are discussed, demonstrating the relevance of a full six-fermion calculation. Numerical results for integrated cross-sections with various kinematical cuts and including radiative effects are given and commented. In the analysis of event samples, several distributions are studied and found to be sensitive to the presence and to the properties of the Higgs boson. Received: 20 December 1998 / Published online: 27 April 1999     

Vector boson production at colliders: A theoretical reappraisal

We study the production of vector bosons in hadron-hadron collisions via the Drell-Yan mechanism in QCD. Our treatment of the transverse momentum and rapidity distributions of the produced bosons takes all available theoretical principles and results into account in a systematic way. The resulting q_T distribution reduces to the perturbative limit for large q_T, includes the summation of soft gluons and reproduces the known results for the total cross section. A full numerical analysis of W and Z cross sections at collider and tevatron energies is made.     

Six-fermion calculation of intermediate-mass Higgs boson production at future {e^+ e^-} colliders

The production of an intermediate-mass Higgs boson in processes of the kind fermions at the energies of future linear colliders is studied. The recently developed and fully automatic algorithm/code ALPHA is used to compute the tree-level scattering amplitudes for the reactions . The code has been interfaced with the Monte Carlo program HIGGSPV/WWGENPV, properly adapted to 6-fermion production, in order to provide realistic results, both in the form of cross sections and event samples at the partonic level. Phenomenological results, that incorporate the effects of initial-state radiation and beamstrahlung, are shown and commented, emphasizing the potentials of full six-fermion calculations for precise background evaluation as well as for detailed studies of the fundamental properties of the Higgs particle. Received: 22 May 1997 / Published online: 20 February 1998     

On MSSM charged Higgs-boson production in association with an electroweak W boson at electron–positron colliders

We present a calculation of the cross section for the process e⁺e^-→W^±H^∓ in the minimal supersymmetric standard model (MSSM) and the two Higgs doublet model (THDM). We study the basic features of the MSSM prediction for some distinctive parameter scenarios. We find large effects from virtual third-generation squarks for scenarios with large mixing, which can lead to a cross section vastly different from a THDM with identical Higgs sector parameters. We investigate this interesting behaviour in more detail by thoroughly scanning the MSSM parameter space for regions of large cross section. For a charged Higgs boson too heavy to be pair produced at a future high-energy electron–positron collider it turns out that a large MSSM cross section with a good chance of observation is linked to a squark mass scale below 600 GeV and a considerable amount of mixing in either the stop and sbottom sector.     

NLO electroweak corrections to Higgs boson production at hadron colliders

Results for the complete NLO electroweak corrections to Standard Model Higgs production via gluon fusion are included in the total cross section for hadronic collisions. Artificially large threshold effects are avoided working in the complex-mass scheme. The numerical impact at LHC (Tevatron) energies is explored for Higgs mass values up to 500 GeV (200 GeV). Assuming a complete factorization of the electroweak corrections, one finds a +5% shift with respect to the NNLO QCD cross section for a Higgs mass of 120 GeV both at the LHC and the Tevatron. Adopting two different factorization schemes for the electroweak effects, an estimate of the corresponding total theoretical uncertainty is computed.     

Soft-gluon resummation for pseudoscalar Higgs boson production at hadron colliders

We compute the threshold-resummed cross section for pseudo-scalar MSSM Higgs boson production by gluon fusion at hadron colliders. The calculation is performed at next-to-next-to leading logarithmic accuracy. We present results for both the LHC and Tevatron Run II. We analyze the factorization and renormalization scale dependence of the results, finding that after performing the resummation the corresponding cross section can be computed with an accuracy better than 10%.     

Z boson pair production and compositeness at large hadron colliders

Z boson pair production at LHC and SSC provides an opportunity to study possible effects due to compositeness in a very high energy domain. We discuss several sources of anomalous effects and give the corresponding bounds on the appropriate compositeness scale. The best bounds are obtained for contact interactions implying longitudinal components of theZ's and for the anomalous couplings of three neutral vector bosons.