E. Lisi

Bounds on MH from electroweak radiative corrections

We report the results of a global fit to mt and MH within the standard model using the latest LEP and other precision electroweak data. We find 92 < mt < 147 GeV and 1.4 < MH < 160 GeV (68% CL) if all data are included, although the upper limit on MH would be relaxed if (more conservatively) the troublesome measurements of the hadron-to-lepton ratio on the Z0 peak are omitted from the fit. These ranges of mt and MH are consistent with the limits from direct searches, which is an impressive and nontrivial check on the consistency of the standard model beyond the tree level. Our analysis of radiative corrections disfavours a strongly-interacting Higgs system, as well as naive tt condensate versions of the standard model. The top quark and the Higgs boson are probably both close to our present experimental reach.

Updated prediction of the top quark mass and implications of its possible confirmation

Abstract Using the latest LEP and other precision data, we make what could be the last prediction of the mass of the top quark before its detection during the current Fermilab Collider run: mt=124−28+26 GeV within the context of the standard model. We also predict that the standard model Higgs boson should weigh 25−19+275 GeV. We present the implications for MH and the parameters of the minimal supersymmetric extension of the standard model if indeed mt=130 GeV, and comment on the implications for the status of other proposed extensions of the standard model such as technicolour.

The Top quark and Higgs boson masses in the standard model and the MSSM

Abstract We update the precision electroweak prediction of m t , making a global analysis including low-energy data as well as the latest LEP and SLC data, and compare it with the mass indicated by CDF measurements: theory and experiments match within errors. In view of the excellent agreement, we combine the information on m t to sharpen indications on M H , finding a central value around 100 GeV, in agreement with the prediction of the Minimal Supersymmetric extension of the Standard Model (MSSM). Our most complete fit gives m t = 162±9GeV, with M H free. We also present bounds from precision electroweak data on the sfermion and gaugino masses in the MSSM, which are competitive with direct limits from CDF, and exclude charginos close to the present mass limits.

Global analysis of precision electroweak data within the minimal supersymmetric extension of the standard model

We present detailed results of a global analysis of precision electroweak data within the context of the Minimal Supersymmetric Extension of the Standard Model (MSSM). We present constraints on the parameters of the MSSM in 3 planes that are commonly used in phenomenological analyses, namely the (mA, tan β) plane describing the supersymmetric Higgs sector, the (μ, mg) plane describing the chargino/neutralino sector, and the (m0, mg) plane describing sfermions. In each case, we compare the indirect limits that we derive with the results of direct searches and the prospects for future experiments. We find that the indirect constraints are not yet very significant in the supersymmetric Higgs sector, although they may have the potential to become so, particularly if the top-quark mass is relatively light. The indirect constraints also give only modest extensions of direct bounds on charginos and neutralinos. However, the indirect constraints already give interesting improvements of existing direct searches for sfermions, and may even pre-empt some of the physics reach of LEP II and the FNAL Tevatron Collider.

Technicolour and precision electroweak data revisited

Abstract We examine the present status of technicolour (TC) models in the light of recent improvements both in theory and experiment. First we present an updated model-independent fit of high-energy precision electroweak data, and emphasize their compatibility with the Standard Model. We then compare the model-independent fit with a one-generation TC model with N tc = 2, degenerate techniquarks and either a Dirac or a Majorana technineutrino, not necessarily degenerate with the technielectron. The results are charted in a way which clearly shows how data disfavour simple TC models, unless judicious combinations of additional effects (induced by the dynamics and/or the spectrum) are allowed. Techniquark mass splitting is shown to be fatal to the Dirac case. Even allowing for theoretical uncertainties, a pattern of stringent constraints on TC models is seen to emerge.

Precision electroweak data, supersymmetry and GUTs

Abstract Following an update of indirect constraints on mt, MH and α s (M Z ) ∥ MS from precision electroweak measurements in the Standard Model, we analyze the data within the context of the Minimal Supersymmetric extension of the Standard Model (MSSM), including the constraint from supersymmetric grand unification. Within the Standard Model, we find somewhat larger values of mt and MH than previously, and a somewhat lower value of α s (M Z ) ∥ MS . The favoured value of mt can be decreased (increased) by O (10 to 20) GeV if the scalar particle masses (lightest chargino mass) are close to the direct experimental limits. We find for the first time a significant lower limit on supersymmetric Higgs masses. The supersymmetric GUT constraint gives a coupled upper limit on the scalar and gaugino masses that depends sensitively on the other MSSM parameters.

Constraints on extensions of the standard model from precise electroweak data

Abstract We present an analysis of the constraints on one-loop “oblique” (vacuum polarization) electroweak radiative corrections imposed by precise data from LEP and other experiments. We present these constraints in full three-dimensional S , T , U space, using a complete set of low-energy data, and LEP data on heavy flavour productionas well as leptons and hadrons. We take into account the uncertain top quark and Higgs boson contributions to non-oblique radiative corrections and discuss quantitatively variations associated with higher-order effects. We compare our bounds with recent estimates in a number of different technicolour scenarios.

Supersymmetric Higgses and electroweak data

Abstract Higgs-related radiative corrections are re-evaluated within the minimal supersymmetric extension of the standard model (MSSM), and incorporated into an analysis of the available precision electroweak data. The datas preferences within the standard model, noted previously, for a light Higgs boson and m t ∼(100–150) GeV, are maintained in the MSSM, with the optimal value of the lightest CP -even Higgs boson mass coming closer to the direct experimental lower limit.

Indirect bounds on the Higgs boson mass from precision electroweak data

We discuss the masses of the top quark and of the Higgs boson in the light of the available precision electroweak data, considering the latest LEP data as well as the latest CCFR measurements of νN and the SLC measurement of the left-right polarization asymmetry. The high precision reached by the experiments makes now possible to observe for the first time some discrepancies between different sets of data. In particular, the values of mt indicated by different subsets of data differ by about 1.3 standard deviations, and the present direct experimental limit on the Higgs mass is somewhat higher than the indirect indications about its mass from the radiative correction effects. Ignoring these discrepancies, we find mt < 155 GeV, MH < 250 GeV, each at the 95% confidence level, from the precision electroweak data alone.

Indications from precision electroweak physics confront theoretical bounds on the mass of the Higgs boson

Abstract An updated fit to the precision electroweak data and to the direct measurement of the top quark mass mt provides significant constraints on mt and on the Higgs boson mass M H : m t GeV = 172 ± 6 and log 10 ( M H GeV ) = 2.16 ± 0.33 , with an error correlation ϱ = 0.5. We integrate the (MH, mt) probability distribution found in this analysis over various zones of the (MH, mt) plane defined by one-sided experimental and theoretical bounds on the Higgs boson mass, both in the Standard Model and in its minimal supersymmetric extension. The comparison of the cumulative probabilities gives interesting information on the likelihood that the true value of MH is compatible with different theoretical scenarios.