W. Hollik

Reconciling the two loop diagrammatic and effective field theory computations of the mass of the lightest CP - even Higgs boson in the MSSM

The mass of the lightest CP-even Higgs boson of the minimal supersymmetric extension of the Standard Model (MSSM) has previously been computed including O(ααs) twoloop contributions by an on-shell diagrammatic method, while approximate analytic results have also been obtained via renormalization-group-improved effective potential and effective field theory techniques. Initial comparisons of the corresponding two-loop results revealed an apparent discrepancy between terms that depend logarithmically on the supersymmetry-breaking scale, and different dependences of the non-logarithmic terms on the squark mixing parameter, Xt. In this paper, we determine the origin of these differences as a consequence of different renormalization schemes in which both calculations are performed. By re-expressing the on-shell result in terms of MS parameters, the logarithmic two-loop contributions obtained by the different approaches are shown to coincide. The remaining difference, arising from genuine non-logarithmic two-loop contributions, is identified, and its effect on the maximal value of the lightest CP-even Higgs boson mass is discussed. Finally, we show that in a simple analytic approximation to the Higgs mass, the leading two-loop radiative corrections can be absorbed to a large extent into an effective one-loop expression by evaluating the running top quark mass at appropriately chosen energy scales.

QCD corrections to the masses of the neutralCP-even Higgs bosons in the minimal supersymmetric standard model

We perform a diagrammatic calculation of the leading two-loop QCD corrections to the masses of the neutral CP-even Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM). The two-loop corrections are found to reduce the mass of the lightest Higgs boson considerably compared to its one-loop value. Consequences for the discovery potential of LEP2 in the GUT favored regions of small and large tan β are discussed.

Precise prediction for the mass of the lightest Higgs boson in the MSSM

Abstract The leading diagrammatic two-loop corrections are incorporated into the prediction for the mass of the lightest Higgs boson, mh, in the Minimal Supersymmetric Standard Model (MSSM). The results, containing the complete diagrammatic one-loop corrections, the new two-loop result and refinement terms incorporating leading electroweak two-loop and higher-order QCD contributions, are discussed and compared with results obtained by renormalization group calculations. Good agreement is found in the case of vanishing mixing in the scalar quark sector, while sizable deviations occur if squark mixing is taken into account.

Supersymmetric contributions to electroweak precision observables: QCD corrections

We calculate the two-loop QCD correction to the scalar quark contributions to the electroweak gauge-boson self-energies at zero momentum transfer in the supersymmetric extension of the standard model (SM) and derive the associated O({alpha}{sub s}) correction to the {rho} parameter. The two-loop corrections modify the one-loop contribution by up to 30{percent} the gluino decouples for large masses. Contrary to the SM case where the QCD corrections are negative and screen the one-loop value, the corresponding corrections in the supersymmetric case are in general positive, increasing the sensitivity in the search for scalar quarks through their virtual effects in high-precision electroweak observables. {copyright} {ital 1997} {ital The American Physical Society}

Leading QCD corrections to scalar quark contributions to electroweak precision observables

In the supersymmetric extension of the Standard Model we derive the two–loop QCD corrections to the scalar quark contributions to the electroweak precision observables entering via the ρ parameter. A very compact expression is derived for the gluon–exchange contribution. The complete analytic result for the gluino–exchange contribution is very lengthy; we give expressions for several limiting cases that were derived from the general result. The two–loop corrections, generally of the order of 10 to 30% of the one-loop contributions, can be very significant. Contrary to the Standard Model case, where the QCD corrections are negative and screen the one–loop value, the corresponding corrections in the supersymmetric case are in general positive, therefore increasing the sensitivity in the search for scalar quarks through their virtual effects in high–precision electroweak observables.

Physics Impact of 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. They will allow the measurement of the effective electroweak mixing angle to an accu- racy ofsin 2 �eff � ±1×10 5 . Likewise the W boson mass is expected to be measurable with an error ofMW � ±6 MeV near the W + W threshold. In this note, we study the accuracy with which the Higgs boson mass can be determined from loop correc- tions to these observables in the Standard Model. The comparison with a directly observed Higgs boson may be used to constrain new physics scales affecting the virtual loops. We also study constraints on the heavy Higgs particles predicted in the Minimal Supersymmetric Standard Model, which are very difficult to observe directly for large masses. Similarly, it is possible to constrain the mass of the heavy scalar top particle.

The coupling of the lightest SUSY Higgs boson to two photons in the decoupling regime

We analyze the contribution of the SUSY particles to the coupling of the lightest Higgs boson to two photons in supersymmetric theories. We discuss to what extent these contributions can be large enough to allow for a discrimination between the lightest SUSY and the standard Higgs particles in the decoupling limit where all other Higgs bosons are very heavy and no supersymmetric particle has been discovered at future colliders. We find that only chargino and top squark loops can generate a sizeable difference between the standard and the SUSY Higgs-photon couplings. For masses above 250 GeV, the effect of chargino loops on the two-photon width is however smaller than ≈ 10% in the entire SUSY parameter space. Top squarks heavier than 250 GeV can induce deviations larger than 10% only if their couplings to the Higgs boson are large. Since top squark contributions can be sizeable, we derive the two-loop QCD correction to squark loops and show that they are well under control.

The Higgs-photon-Z boson coupling revisited

We analyze the coupling of CP-even and CP-odd Higgs bosons to a photon and a Z boson in extensions, of the Standard Model. In particular, we study in detail the effect of charged Higgs bosons in two-Higgs doublet models;. and the contribution of SUSY particle loops in the minimal supersymmetric extension of the Standard Model: The Higgs-γZ coupling can be measured in the decayZ → γ+Higgs ate+e− colliders running on theZ resonance, or in the reverse process Higgs →Zγ with the Higgs boson produced at LHC. We show that a measurement of this coupling with a precision at the percent level, which could be the case at futuree+e− colliders, would allow to distinguish between the lightest SUSY and standard Higgs bosons in large areas of the parameter space.

Regularization and supersymmetry-restoring counterterms in supersymmetric QCD

Abstract. We calculate symmetry-restoring counterterms in supersymmetric QCD at the one-loop level. First we determine loop corrections to the supersymmetry and gauge transformations and find counterterms in such a way that the symmetry algebra holds at the one-loop level. Then these results are used to derive the symmetry-restoring counterterms to all trilinear interactions. In order to obtain unique results it is crucial to use the Slavnov-Taylor identity, which does not only contain supersymmetric and gauge Ward identities but also describes the symmetry algebra. In dimensional regularization this procedure yields unique non-zero values for the counterterms. In contrast, in dimensional reduction we find that no non-symmetric counterterms are needed, neither for the symmetry transformations nor for the physical interactions. For the considered cases this result constitutes a definite test of the supersymmetry and gauge invariance of the scheme.

QCD Corrections to Scalar Quark Decays

In supersymmetric theories, the main decay modes of scalar quarks are decays into quarks plus charginos or neutralinos, if the gluinos are heavy enough. We calculate the