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     

Testing Higgs self-couplings at e^+e^- linear colliders

To establish the Higgs mechanism sui generis experimentally, the self-energy potential of the Higgs field must be reconstructed. This task requires the measurement of the trilinear and quadrilinear self-couplings, as predicted, for instance, in the Standard Model or in supersymmetric theories. The couplings can be probed in multiple Higgs production at high-luminosity linear colliders. Complementing earlier studies to develop a coherent picture of the trilinear couplings, we have analyzed the production of pairs of neutral Higgs bosons in all relevant channels of double Higgs-strahlung, associated multiple Higgs production and WW/ZZ fusion to Higgs pairs. Received: 3 March 1999 / Published online: 15 July 1999     

Prospects of measuring general Higgs couplings at e^+e^- linear colliders

We examine how accurately the general HZV couplings, with , may be determined by studying processes at future linear colliders. By using the optimal-observable method, which makes use of all available experimental information, we find out which combinations of the various HZV coupling terms may be constrained most efficiently with high luminosity. We also assess the benefits of measuring the tau-lepton helicities, identifying the bottom-hadron charges, polarizing the electron beam and running at two different collider energies. The HZZ couplings are generally found to be well constrained, even without these options, while the couplings are not. The constraints on the latter may be significantly improved by beam polarization. Received: 4 February 2000 / Published online: 6 April 2000     

Strong-interaction effects in stop pair production at e^+ e^- colliders

We discuss perturbative and non-perturbative strong-interaction effects in the pair production of stop squarks () at colliders. Events with an additional hard gluon allow to detect or exclude production even in scenarios with very small mass splitting between and an invisible lightest supersymmetric particle (LSP). Such events can also help to establish that transforms as a triplet under SU(3). We also carefully study non-perturbative fragmentation, which is currently not well understood: not only is the fragmentation function not known very well, but also there are ambiguities in the algorithm employed to model fragmentation. We present numerical results both for CERN LEP–183 and for a proposed future collider operating at center-of-mass energy GeV. Received: 19 February 1999 / Revised version: 12 March 1999 / Published online: 3 August 1999     

Heavy charged Higgs boson production at next generation e^\pm\gamma colliders

We assess the potential of future electron-positron linear colliders operating in the mode in detecting charged Higgs bosons with mass around and larger than the top quark mass, using Compton back-scattered photons from laser light. We compare the pair production mode, , to a variety of channels involving only one charged Higgs scalar in the final state, such as the tree-level processes ( and ) and ( and ) as well as the loop-induced channel . We show that, when the charged Higgs boson mass is smaller than or comparable to half the collider energy, , single production cross sections are of the same size as the pair production rate, whereas, for charged Higgs boson masses larger than , all processes are heavily suppressed. In general, production cross sections of charged Higgs bosons via scatterings are smaller than those induced at an collider and the latter represents a better option to produce and analyse such particles. Received: 29 August 2001 / Published online: 23 November 2001     

Estimates of threshold cross section for stoponium production at e^+e^- colliders

We estimate the total cross section near threshold for a Coulombic potential and compare it to the Born approximation. The effect of the beam energy spread for present and future colliders is discussed. Received: 27 September 2000 / Revised version: 22 February 2001 / Published online: 23 March 2001     

Supersymmetric hadronic bound state detection at e^+e^- colliders

We review the possibility of formation for a bound state with a stop quark and its antiparticle. The detection of a signal from its decay has been investigated for the case of an collider. Received: 9 July 1999 / Revised version: 30 March 2000 / Published online: 8 June 2000     

Reconstructing the chargino system at e^+e^- linear colliders

In most supersymmetric theories charginos, , belong to the class of the lightest supersymmetric particles. The chargino system can be reconstructed completely in collider experiments: . By measuring the total cross sections and the asymmetries with polarized beams, the chargino masses and the gaugino–higgsino mixing angles of these states can be determined accurately. If only the lightest charginos are kinematically accessible in a first phase of the machine, transverse beam polarization or the measurement of chargino polarization in the final state is needed to determine the mixing angles. From these observables the fundamental SUSY parameters can be derived: the SU(2) gaugino mass , the modulus and the cosine of the CP–violating phase of the higgsino mass parameter , and , the ratio of the vacuum expectation values of the two neutral Higgs doublet fields. The remaining two–fold ambiguity of the phase can be resolved by measuring the normal polarization of the charginos. Sum rules of the cross sections can be exploited to investigate the closure of the two–chargino system. Received: 2 February 2000 / Published online: 17 March 2000     

Higgs physics at future colliders: Recent theoretical developments

I review the physics of the Higgs sector in the standard model and its minimal supersymmetric extension, the MSSM. I will discuss the prospects for discovering the Higgs particles at the upgraded Tevatron, at the large hadron collider, and at a future high-energye ⁺ e ⁻ linear collider with centre-of-mass energy in the 350–800 GeV range, as well as the possibilities for studying their fundamental properties. Some emphasis will be put on the theoretical developments which occurred in the last two years.     

Exotic decays of the 125 GeV Higgs boson at future e~+e~- colliders

The discovery of unexpected properties of the Higgs boson would offer an intriguing opportunity to shed light on some of the most profound puzzles in particle physics. Beyond Standard Model(BSM) decays of the Higgs boson could reveal new physics in a direct manner. Future electron-positron lepton colliders operating as Higgs factories, including CEPC, FCC-ee and ILC, with the advantages of a clean collider environment and large statistics,could greatly enhance sensitivity in searching for these BSM decays. In this work, we perform a general study of Higgs exotic decays at future e~+e~-lepton colliders, focusing on the Higgs decays with hadronic final states and/or missing energy, which are very challenging for the High-Luminosity program of the Large Hadron Collider(HL-LHC).We show that with simple selection cuts,O(10~(-3)-10~(-5)) limits on the Higgs exotic decay branching fractions can be achieved using the leptonic decaying spectator Z boson in the associated production mode e~+e~-→ZH. We further discuss the interplay between detector performance and Higgs exotic decays, and other possibilities of exotic decays.Our work is a first step in a comprehensive study of Higgs exotic decays at future lepton colliders, which is a key area of Higgs physics that deserves further investigation.     

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%.     

Neutral MSSM Higgs-boson production at \mathrm{e^+e^-} colliders in the Feynman-diagrammatic approach

We calculate the cross sections for the neutral Higgs-boson production at colliders in the Minimal Supersymmetric Standard Model (MSSM) using the Feynman-diagrammatic approach and the on-shell renormalization scheme. We incorporate the Higgs-boson propagator corrections, evaluated up to two-loop order, into the prediction of the cross sections for the Higgs-boson production mechanism . The propagator corrections consist of the full one-loop contribution, including the effects of non-vanishing external momentum, and at the two-loop level of the dominant corrections of and further sub-dominant contributions. The results are supplemented with the complete set of one-loop vertex and box corrections. The effects of the two-loop propagator corrections are investigated in detail. We briefly discuss also the effect of the box contributions for high . We compare our results with the case where only the corrections to the effective mixing angle, evaluated within the renormalization-group-improved one-loop Effective Potential approach, are taken into account. We find agreement better than 10% for LEP2 energies and deviations larger than 20% for GeV. Received: 7 February 2001 / Published online: 23 March 2001     

Four-pion production in e^+e^- annihilation

Abstract. We calculate the processes and to in the low-energy expansion of the standard model. The chiral amplitudes of can be extended via resonance exchange to energies around 1 GeV. Higher-order effects have been included in the form of and double exchange and by performing a resummation of the pion form factor. The predicted cross sections and the branching ratios are in good agreement with the available data. Received: 8 March 2002 / Published online: 14 June 2002     

Supersymmetric Higgs boson pair production: Discovery prospects at hadron colliders

We study the potential of hadron colliders in the search for the pair production of neutral Higgs bosons in the framework of the minimal supersymmetric standard model. We perform a detailed signal and background analysis, working out efficient kinematical cuts for the extraction of the signal. The important role of squark loop contributions to the signal is re-emphasized. If the signal is sufficiently enhanced by these contributions, it could even be observable at the next run of the upgraded Tevatron collider in the near future. At the LHC the pair production of light and heavy Higgs bosons might be detectable simultaneously. Received: 23 February 2000 / Revised version: 18 April 2000 / Published online: 31 August 2000