Theoretical implications of the possible observation of Higgs bosons at LEP

Theoretical implications of the possible observation of a Higgs boson with a mass of about 115 GeV at LEP are discussed. Within the Standard Model a Higgs boson in this mass range agrees well with the indirect constraints from electroweak precision data. However, it would nevertheless point towards physics beyond the Standard Model, in particular to Supersymmetric extensions. The interpretation of the LEP excess as production of the light or the heavy CP-even Higgs boson is discussed within the unconstrained MSSM and the mSUGRA, GMSB and AMSB scenarios. Prospects for Higgs physics at future colliders are briefly summarized.Comment: 11 pages, Contribution to the proceedings of the XXXVIth Rencontres de Moriond, Les Arcs, March 200

Feynman-diagram evaluation in the electroweak theory with computer algebra

The evaluation of quantum corrections in the theory of the electroweak and strong interactions via higher-order Feynman diagrams requires complicated and laborious calculations, which however can be structured in a strictly algorithmic way. These calculations are ideally suited for the application of computer algebra systems, and computer algebra has proven to be a very valuable tool in this field already over several decades. It is sketched how computer algebra is presently applied in evaluating the predictions of the electroweak theory with high precision, and some recent results obtained in this way are summarized.Comment: 7 pages, updated version of proceedings contribution to ACAT 2000, Fermilab, October 200

Higgs Production and Decay in the MSSM: Status and Perspectives

The theoretical predictions in the MSSM for Higgs-boson production at a future e^+e^- Linear Collider and Higgs-boson decay processes are discussed focusing in particular on recent diagrammatic two-loop results in the MSSM Higgs sector. The present status of the theoretical predictions is briefly summarized, and it is emphasized that considerable improvements will be necessary in order to match the high experimental accuracy achievable at a future Linear Collider.Comment: 6 pages, 2 figures, to appear in the proceedings of LCWS 200

Electroweak Precision Tests with GigaZ

By running the prospective high-energy e^+e^- collider TESLA in the GigaZ mode on the Z resonance, experiments can be performed on the basis of more than 10^9 Z events. This will allow the measurement of the effective electroweak mixing angle to an accuracy of \delta sin^2(theta_W,eff) \approx \pm 10^-5. The W boson mass is likewise expected to be measurable with an error of \delta M_W \approx \pm 6 MeV near the W^+W^ threshold. We review the electroweak precision tests that can be performed with these high precision measurements within the Standard Model (SM) and its minimal Supersymmetric extension (MSSM). The complementarity of direct measurements at a prospective linear e^+e^- collider and indirect constraints following from measurements performed at GigaZ is emphasized.Comment: 5 pages, 3 figures, talk given by S. Heinemeyer at the 5th International Linear Collider Workshop (LCWS 2000), Fermilab, Batavia, Illinois, 24-28 Oct 200

The MSSM in the Light of Precision Data

The potential of present and anticipated future electroweak precision data, including the Higgs boson and top quark masses, for testing quantum effects of the electroweak theory is investigated in the context of the Minimal Supersymmetric Standard Model (MSSM). The present status of the theoretical predictions is analyzed. The impact of the parametric uncertainties from the experimental errors of the input parameters is studied, and an estimate for the remaining uncertainties from unknown higher-order corrections is given both in the Standard Model (SM) and the MSSM. Examples of electroweak precision tests in the mSUGRA scenario and the unconstrained MSSM are analyzed, and the status of the global fit to all data is discussed.Comment: 16 pages, 4 figures, to appear in the proceedings of the workshop ``Electroweak precision data and the Higgs mass'', DESY Zeuthen, February 200

Prospects for direct searches for light Higgs bosons at the ILC with 250 GeV

The particle discovered in the Higgs boson searches at the LHC with a mass of about 125 GeV is compatible within the present uncertainties with the Higgs boson predicted in the Standard Model (SM), but it could also be identified with one of the neutral Higgs bosons in a variety of Beyond the SM (BSM) theories with an extended Higgs sector. The possibility that an additional Higgs boson (or even more than one) could be lighter than the state that has been detected at 125 GeV occurs generically in many BSM models and has some support from slight excesses that were observed above the background expectations in Higgs searches at LEP and at the LHC. The couplings between additional Higgs fields and the electroweak gauge bosons in BSM theories could be probed by model-independent Higgs searches at lepton colliders. We present a generator-level extrapolation of the limits obtained at LEP to the case of a future $e^+e^-$ collider, both for the search where the light Higgs boson decays into a pair of bottom quarks and for the decay-mode-independent search utilising the recoil method. We find that at the ILC with a c.m. energy of 250 GeV, an integrated luminosity of 500 fb^{-1} and polarised beams, the sensitivity to a light Higgs boson with reduced couplings to gauge bosons is improved by more than an order of magnitude compared to the LEP limits and goes much beyond the projected indirect sensitivity of the HL-LHC with 3000 fb^{-1} from the rate measurements of the detected state at 125 GeV.Comment: Minor changes, version to appear in EPJC, 13 pages, 4 figure

FeynHiggsFast: a program for a fast calculation of masses and mixing angles in the Higgs Sector of the MSSM

FeynHiggsFast is a Fortran code for the calculation of the masses and the mixing angle of the neutral CP-even Higgs bosons in the MSSM up to two-loop order. It is based on a compact analytical approximation formula of the complete diagrammatic one-loop and the dominant two-loop contributions. At the one-loop level a leading logarithmic result is used, taking into account all sectors of the MSSM. At the two-loop level at O(alpha alpha_s) the leading logarithmic and non-logarithmic contributions are taken into account. The approximation formula is valid for arbitrary choices of the parameters in the Higgs sector of the model. Comparing its quality to the full diagrammatic result, we find agreement better than 2 GeV for most parts of the MSSM parameter space.Comment: 10 pages, 6 figures. Extended version of the contribution to the Report of the HIGGS working group for the Workshop ``Physics at TeV Colliders'' Les Houches, France, June 199