Supersymmetry signals in electroweak precision tests at LEP (original) (raw)

Precision measurements in electroweak physics and supersymmetry

Nuclear Physics B, 1990

In the minimal supersymmetric standard model we analyse the radiative corrections to M~. the p-parameter and the lepton asymmetries at the Z°pole. We show how the radiative corrections of the standard model with a light Higgs give a lower bound on the corrections in the supersymmetric model. This feature, once the top quark mass is known, may serve to distinguish between the two different theories.

Supersymmetric Hints from Precision Electroweak Data?

The Standard Model does not provide a very good fit to the most recent precision electroweak data from LEP, due primarily to the observed branching ratios for Z decay to bb and cc. The possibility that an extension of the Standard Model with low-energy supersymmetry can improve the agreement between data and theory is considered.

Electroweak Precision Measurements and the Minimal Supersymmetric Standard Model

Progress of Theoretical Physics Supplement, 1996

We discuss supersymmetric contributions to the electroweak precision measurements in the Minimal Supersymmetric Standard Model for two cases: the quark-lepton universality violation δ qℓ in charged currents and the ratio R b = Γ (Z → bb)/Γ (Z → hadrons). The recent experimental data suggest deviations from the Standard Model for these observables, at the 1-σ level for the former and at more than 3-σ level for the latter. We analyze the non-oblique corrections from the SUSY particles to explain these discrepancies. The observed non-zero δ qℓ may be explained by relatively light sleptons, charginos and neutralinos. Although the observed excess of R b can be *) Talk given by Y. Yamada at Yukawa International Seminar (YKIS) '95 "From the Standard Model to Grand Unified Theories", Kyoto, Japan, August 21-25, 1995. To appear in the Proceedings.

Bounds on supersymmetry from electroweak precision analysis

Nuclear Physics B, 1998

The Standard Model global fit to precision data is excellent. The Minimal Supersymmetric Standard Model can also fit the data well, though not as well as the Standard Model. At best, supersymmetric contributions either decouple or only slightly decrease the total χ 2 , at the expense of decreasing the number of degrees of freedom. In general, regions of parameter space with large supersymmetric corrections from light superpartners are associated with poor fits to the data. We contrast results of a simple (oblique) approximation with full one-loop results, and show that for the most important observables the non-oblique corrections can be larger than the oblique corrections, and must be taken into account. We elucidate the regions of parameter space in both gravity-and gauge-mediated models which are excluded. Significant regions of parameter space are excluded, especially with positive supersymmetric mass parameter µ. We give a complete listing of the bounds on all the superpartner and Higgs boson masses. For either sign of µ, and for all supersymmetric models considered, we set a lower limit on the mass of the lightest CP-even Higgs scalar, m h ≥ 78 GeV. Also, the first and second generation squark masses are constrained to be above 280 (325) GeV in the supergravity (gauge-mediated) model.

Confronting electroweak precision measurements with New Physics models

The European Physical Journal C, 2000

Precision experiments, such as those performed at LEP and SLC, offer us an excellent opportunity to constrain extended gauge model parameters. To this end, it is often assumed, that in order to obtain more reliable estimates, one should include the sizable one-loop Standard Model (SM) corrections, which modify the Z 0 couplings as well as other observables. This conviction is based on the belief that the higher order contributions from the "extension sector" will be numerically small. However, the structure of higher order corrections can be quite different when comparing the SM with its extension, thus one should avoid assumptions which do not care about such facts. This is the case for all models with ρtree ≡ M 2 W /(M 2 Z cos 2 ΘW ) = 1. As an example, both the manifest left-right symmetric model and the SU (2)L ⊗ U (1)Y ⊗Ũ (1) model, with an additional Z ′ boson, are discussed and special attention to the top contribution to ∆ρ is given. We conclude that the only sensible way to confront a model with the experimental data is to renormalize it self-consistently, if not, parameters which depend strongly on quantum effects should be left free in fits, though essential physics is lost in this way. We should note that arguments given here allow us to state that at the level of loop corrections (indirect effects) there is nothing like a "model independent global analysis" of the data.

How conventional measures overestimate electroweak fine-tuning in supersymmetric theory

2013

The lack of evidence for superparticles at the CERN LHC, along with the rather high value of the Higgs mass, has sharpened the perception that supersymmetric model parameter space is highly electroweak fine-tuned (EWFT). We compare three measures of fine-tuning in SUSY models. 1. \Del_HS measures a terms containing large log contributions to m_Z that are inevitable in models defined at scales much higher than the electroweak scale. 2. \Del_BG measures variation in m_Z against variation of model parameters and allows for correlations among high scale parameters which are not included in \Del_HS. 3. The model-independent \Del_EW measures how naturally a model can generate the measured value of m_Z=91.2 GeV in terms of weak scale parameters alone. We hypothesize an overarching Ultimate Theory (UTH) wherein the high scale soft terms are all correlated. The UTH might be contained within the more general effective SUSY theories popular in the literature. For \Del_HS, EWFT can be overestim...

A detailed comparison of LEP data with the predictions of the minimal supersymmetric SU(5) GUT

Nuclear Physics B, 1992

We confront the precise LEP determinations of SIn 2OW and the strong coupling 0 3(mz) with the predictions of the minimal supersymmetric SU(5) GUT. We incorporate O(0cm03) effects in the extraction of sin 2O~from LEP data. We incorporate distinct thresholds for the supersymmetric partners of the different species of Standard Model particles, parameterized in terms of a scalar mass m 0 and a gaugino mass m1/2 that are assumed to be universal at the GUT scale. We also allow for uncertainties in the top, higgs and higgsino masses. We use the full two-loop renormalization group equations including top, bottom and tau Yukawa couplings. We show that GUT threshold effects are small because proton stability prevents triplet Higgs particles from weighing much less than 1016 GeV. Using 1-u errors for the experimental inputs and plausible ranges for unknown supersymmetric model parameters, in particular an upper bound of 300 GeV on the higgs mixing parameter~z, we find that either 3.0x 1012 TeV> m1/2>21 TeV or 011/2 <65 GeV, with the intermediate range allowed at the 2-u level. An upper bound of = 500 GeV excludes 011/2 from 80 GeV to 5 TeV at 1-u, and an upper bound of~z= I TeV excludes m1/2 from 110 GeV to 620 GeV at 1o. It is not possible at present to fix the supersymmetry breaking scale with any precision.

Z′-bosons in supersymmetric E6 models confront electroweak data

Nuclear Physics B, 1998

We study constraints on additional Z ′ bosons predicted in the supersymmetric (SUSY) E 6 models by using the updated results of electroweak experiments-Z-pole experiments, m W measurements and low-energy neutral current (LENC) experiments. We find that the effects of Z-Z ′ mixing are parametrized by (i) a tree-level contribution to the T-parameter, (ii) the effective Z-Z ′ mass mixing angleξ. In addition, the effect of the direct exchange of the heavier mass eigenstate Z 2 in the LENC processes is parametrized by (iii) a contact term g 2 E /c 2 χ m 2 Z 2. We give the theoretical predictions for the observables in the electroweak experiments together with the standard model radiative corrections. Constraints on T new andξ from the Z-pole and m W experiments and those on g 2 E /c 2 χ m 2 Z 2 from the LENC experiments are separately shown. Impacts of the kinetic mixing between the U(1) Y and U(1) ′ gauge bosons on the χ 2-analysis are studied. We show the 95% CL lower mass limit of Z 2 as a function of the effective Z-Z ′ mixing parameter ζ, a combination of the mass and kinetic mixings. Theoretical prediction on ζ and g E is found for the χ, ψ, η and ν models by assuming the minimal particle content of the SUSY E 6 models. In a certain region of the parameter space, the Z 2 boson mass in the detectable range of LHC is still allowed.

Implications of Precision Electroweak Measurements for Physics Beyond the SM

1999

We perform a global analysis of electroweak precision measurements to find constraints on physics beyond the Standard Model. In particular, we discuss oblique parameters, which are useful to constrain additional matter fields, as well as extra ZZZ bosons, and supersymmetry. We also summarize the present information on the Higgs boson mass.