We study constraints on the supersymmetric standard model from the updated electroweak precision measurements --- the Z-pole experiments and the $W$-boson mass measurements. The supersymmetric-particle contributions to the universal gauge-boson-propagator corrections are parametrized by the three oblique parameters Sz, Tz and mw. The oblique corrections, the Zqq and Zll vertex corrections, and the vertex and box corrections to the \mu-decay width are separately studied in detail. We first study individual contribution from the four sectors of the model, the squarks, the sleptons, the supersymmetric fermions (charginos and neutralinos), and the supersymmetric Higgs bosons, to the universal oblique parameters, where the sum of individual contributions gives the total correction. We find that the light squarks or sleptons, whose masses just above the present direct search limits, always make the fit worse than that of the Standard Model (SM), whereas the light charginos and neutralinos generally make the fit slightly better. The contribution from the supersymmetric Higgs sector is found small. We then study the vertex/box corrections carefully when both the supersymmetric fermions (-inos) and the supersymmetric scalars (squarks and sleptons) are light, and find that no significant improvement over the SM fit is achieved. The best overall fit to the precision measurements are found when charginos of mass $\sim 100\gev$ with a dominant wino-component are present and the doublet squarks and sleptons are all much heavier. The improvement over the SM is marginal, however, where the total \chi^2 of the fit to the 22 data points decreases by about one unit, due mainly to a slightly better fit to the $Z$-boson total width.Comment: 61 pages, 16 figures. References added. version to appear in Nuclear Physics

We update the electroweak study of the predictions of the Minimal Supersymmetric Standard Model (MSSM) including the recent results on the muon anomalous magnetic moment, the weak boson masses, and the final precision data on the Z boson parameters from LEP and SLC. We find that the region of the parameter space where the slepton masses are a few hundred GeV is favored from the muon g − 2 for tan β < ∼ 10, whereas for tan β ≃ 50 heavier slepton mass up to ∼ 1000 GeV can account for the reported 3.2 σ difference between its experimental value and the Standard Model (SM) prediction. As for the electroweak measurements, the SM gives a good description, and the sfermions lighter than 200 GeV tend to make the fit worse. We find, however, that sleptons as light as 100 to 200 GeV are favored also from the electroweak data, if we leave out the jet asymmetry data that do not agree with the leptonic asymmetry data. We extend the survey of the preferred MSSM parameters by including the constraints from the b → sγ transition, and find favorable scenarios in the minimal supergravity, gauge-, and mirage-mediation models of supersymmetry breaking.

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We calculate the contributions of the vectorlike quark model to B s B s mixing, taking into account the constraints from the decay B→X s ␥. In this model the neutral bosons mediate flavor-changing interactions at the tree level. However, B s B s mixing is dominated by contributions from the box diagrams with the top quark and the extra up-type quark. In sizable ranges of the model parameters, the mixing parameter x s is much different from the standard model prediction.The standard model ͑SM͒ may have to be extended to describe physics around or above the electroweak scale. Various works have thus discussed phenomena involving the B meson which are sensitive to new physics ͓1͔. For instance, the radiative B-meson decay B→X s ␥ and BB mixing could receive nontrivial contributions from supersymmetry ͓2,3͔. The vectorlike quark model ͑VQM͒ could also affect these processes of flavor-changing neutral currents ͓4,5͔.In this Brief Report we discuss B s B s mixing within the framework of the VQM which is one of the minimal extensions of the SM. The mixing parameter x s is evaluated and its dependencies on the model parameters are analyzed. These model parameters are constrained by the branching ratio of the decay B→X s ␥. Even under these constraints, the value of x s can be much different from the SM prediction.The VQM incorporates extra quarks whose left-handed components, as well as right-handed ones, are singlets under the SU͑2͒ gauge transformation. Then, the interactions of quarks become different from those in the SM. The CabibboKobayashi-Maskawa ͑CKM͒ matrix for the interactions with the W boson is extended and not unitary. The Z boson couples directly to the quarks with different flavors. The neutral Higgs boson also mediates flavor-changing interactions at the tree level.Our study concentrates on B s B s mixing at the one-loop level through box diagrams. The contributions at the tree level have already been analyzed in the literature. The order of these tree-level diagrams is lower than the box diagrams of the SM. However, the experimental bounds on the branching ratio of the decay B→K ϩ Ϫ suggest very weak couplings for the flavor-changing interactions at the tree level. Consequently, the tree-level contributions turn out to be smaller than the one-loop contributions. On the other hand, the new box diagrams in the VQM are expected to contribute at the same order as those of the SM. Although these new contributions do not yield a drastic change from the SM prediction, precise measurements in near-future experiments, such as BTeV and the CERN Large Handron Collider ͑LHCb͒, may be able to distinguish the VQM and the SM. Indeed, the VQM could give sizable new contributions to the decay B→X s ␥ at the one-loop level ͓5͔, imposing non-trivial constraints on the model. We assume, for definiteness, that there exist two vectorlike quarks U and D with electric charge 2/3 and Ϫ1/3, respectively. The CKM matrix V is then enlarged to be a 4 ϫ4 matrix and expressed asA L u and A L d being unitary matrices which diagonalize...