arXiv subjects

A. B. Lahanas

Publications and source records attributed to A. B. Lahanas.

46 records · Page 3Linked to original sources

arXiv1998

Z-Observables and the Effective Weak Mixing Angle in the MSSM

We make a comparison of the predicted effective weak mixing angle, the Z-on resonance asymmetries and the W-boson mass to the LEP and SLD data at their present status. We find that the predicted MSSM values for the effective weak mixing angle are in agreement with the LEP+SLD average value for a ``heavy'' SUSY breaking scale while we observe an agreement with SLD data in the case of a ``light'' SUSY breaking scale. The resulting values for the W-boson mass and for the electron left-right asymmetries are compatible with CDF,UA2,DO and LEP data respectively. Unexpectedly we find that the supersymmetric QCD contributions to the Z-observables tend to vanish everywhere in the M1/2-M0 plane. Furthermore, values of M1/2 which are greater than 500 GeV are favoured by the MSSM if one considers the current experimental value for the strong coupling.

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arXiv1996

Supersymmetric QCD corrections to the W-boson width

We calculate the one-loop supersymmetric QCD corrections to the width of the $W$-boson. We find that these are of order $\sim {{ α_s} \over π} {1 \over 20} { {M^2_W} \over {M_S^2}} Γ_{u {\bar d}}$, where $M_S$ is the supersymmetry breaking scale and $ Γ_{u {\bar d}}$ the tree level hadronic width for $W^+ \rightarrow u {\bar d} $. Due to the appearance of the suppression factor $\sim {1 \over 20}$ these are at least two orders of magnitude smaller than the standard QCD corrections $\sim {{ α_s} \over π} Γ_{u {\bar d}}$ and hence of the order of the two-loop electroweak effects. Therefore supersymmetric QCD corrections will only be of relevance once experiments reach that level of accuracy.

hep-ph↗

arXiv1996

Threshold effects and radiative electroweak symmetry breaking in $SU(5)$ extensions of the MSSM

We make a complete analysis of radiative symmetry breaking in the MSSM and its $SU(5)$ extensions including low- and high-energy threshold effects in the framework of the two-loop renormalization group. In particular, we consider minimal $SU(5)$, the ``missing-doublet'' $SU(5)$, a Peccei-Quinn invariant version of $SU(5)$ as well as a version with light adjoint remnants. We derive permitted ranges for the parameters of these models in relation to predicted $α_{s}$ and $M_G$ values within the present experimental accuracy. The parameter regions allowed under the constraints of radiative symmetry breaking, perturbativity and proton stability, include the experimentally designated domain for $α_s$. In the case of the minimal $SU(5)$, the values of $α_s$ obtained are somewhat large in comparison with the experimental average. The ``missing-doublet'' $SU(5)$, generally, predicts smaller values of $α_s$. In both versions of the ``missing-doublet'', the high energy threshold effects on $α_s$ operate in the opposite direction than in the case of the minimal model, leading to small values. In the case of the Peccei-Quinn version however the presence of an extra intermediate scale allows to achieve an excellent agreement with the experimental $α_s$ values. Finally, the last considered version, with light remnants, exhibits unification of couplings at string scale at the expense however of rather large $α_s$ values.

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arXiv1996

Trilinear Gauge Boson Couplings in the MSSM

We study the C and P even WWγand WWZ trilinear gauge boson vertices (TGV's), in the context of the MSSM assuming that the external W's are on their mass shell. We find that for energies less than 200 GeV squark and slepton contributions to the aforementioned couplings are two orders of magnitude smaller than those of the Standard Model (SM). In the same energy range the bulk of the supersymmetric Higgs corrections to the TGV's is due to the lightest neutral Higgs, h_0, whose contribution is like that of a Standard Model Higgs of the same mass. The contributions of the Neutralinos and Charginos are sensitive to the input value for the soft gaugino mass M_{1/2}, being more pronounced for values M_{1/2} < 100 GeV. In this case and in the unphysical region, 0 < \sqrt{s} < 2 M_W , their contributions are substantially enhanced resulting in large corrections to the static quantities of the W boson. However, such an enhancement is not observed in the physical region. In general for 2 M_W < \sqrt{s} < 200 GeV the MSSM predictions differ from those of the SM but they are of the same order of magnitude. To be detectable deviations from the SM require sensitivities reaching the per mille level and hence unlikely to be observed at LEP200. For higher energies SM and MSSM predictions exhibit a fast fall off behaviour, in accord with unitarity requirements, getting smaller,in most cases, by almost an order of magnitude already at energies \sqrt{s} 0.5 TeV.

hep-ph↗

arXiv1995

Radiative Electroweak symmetry breaking in the MSSM and Low Energy Thresholds

We study Radiative Electroweak Symmetry Breaking in the Minimal Supersymmetric Standard Model (MSSM). We employ the 2-loop Renormalization Group equations for running masses and couplings taking into account sparticle threshold effects. The decoupling of each particle below its threshold is realized by a step function in all one-loop Renormalization Group equations (RGE). This program requires the calculation of all wavefunction, vertex and mass renormalizations for all particles involved. Adapting our numerical routines to take care of the succesive decoupling of each particle below its threshold, we compute the mass spectrum of sparticles and Higgses consistent with the existing experimental constraints. The effect of the threshold corrections is in general of the same order of magnitude as the two-loop contributions with the exception of the heavy Higgses and those neutralino and chargino states that are nearly

hep-ph↗

arXiv1995

Trilinear Gauge Boson Vertices in the MSSM

We study the $C$ and $P$ even $WWγ$ and $WWZ$ trilinear gauge boson vertices (TGV's), in the context of the MSSM as functions of the soft SUSY breaking parameters $A_0, m_0, M_{1/2}$ and the momentum $q$ carried by $γ, Z$, assuming the external $W$'s are on their mass shell. We follow a complete renormalization group analysis taking into account all constraints imposed by the radiative breaking of the electroweak symmetry. It is found that for energies ${\sqrt{s}} \equiv q^2 \le 200 \; GeV$ squark and slepton contributions to the aforementioned couplings are two orders of magnitude smaller than those of the Standard Model (SM). In the same energy range the bulk of the supersymmetric Higgs corrections to the TGV's is due to the lightest neutral Higgs, $h_0$, whose contribution is like that of a Standard Model Higgs of the same mass. The rest have negligible effect due to their heaviness. The contributions of the Neutralinos and Charginos are sensitive to the input value for the soft gaugino mass $M_{1/2}$ being more pronounced for values $M_{1/2} < 100 \; GeV$. In this case and in the unphysical region, $0 < \sqrt{s} < 2 M_W $ their contributions are substantially enhanced resulting to large corrections to the static quantities of the $W$ boson. However such an enhancement is not observed in the physical region and their corrections to the TGV's are rather small. In general for $2 M_W < \sqrt{s} < 200 \; GeV $ the MSSM predictions differ from those of the SM but they are of the same order of magnitude. Deviations from the SM predictions to be detectable require sensitivities reaching the per mille level and hence unlikely to be observed at LEP200. For higher energies SM and MSSM predictions exhibit a fast fall off behaviour, in accord with unitarity requirements, getting smaller by almost an

hep-ph↗

arXiv1995

Static Quantities of the W bosons in the MSSM

I discuss the static quantities of the W boson, magnetic dipole and electric quadrupole moments, in the context of the minimal supersymmetric standard model, in which supersymmetry is broken by soft terms $A_o,\, m_o,\, M_{1/2}$. Following a renormalization group analysis it is found that the supersymmetric values of $Δk_γ$ and $ΔQ_γ$ can be largely different, in some cases, from the standard model predictions but of the same order of magnitude for values of \quad $A_0,m_0,M_{1/2} \leq {\cal O}(1 TeV) $. Therefore possible supersymmetric structure can be probed provided the accuracy of measurements for $Δk_γ$, $ΔQ_γ$ reaches $10^{-2}-10^{-3}$ and hence hard to be detected at LEP2. In cases where $M_{1/2} \ll A_0,m_0 $, the charginos and neutralinos may give substantial contributions saturating the LEP2 sensitivity limits. This occurs when their masses $m_{\tilde C},m_{\tilde Z}$ turn out to be both light satisfying $m_{\tilde C}+m_{\tilde Z} \simeq M_W $. However these extreme cases are perturbatively untrustworthy and besides unnatural for they occupy a small region in the parameter space.

hep-ph↗

arXiv1994

Low Energy Thresholds and the Renormalization Group in the MSSM

We derive the 1-loop Renormalization Group Equations for the parameters of the Minimal Supersymmetric Standard Model (MSSM) taking into account the successive decoupling of each sparticle below its threshold. This is realized by a step function at the level of each graph contributing to the Renormalization Group Equations.

hep-ph↗

arXiv1994

Static Quantities of the W-Boson in the MSSM

We systematically analyze the anomalous dipole $Δk_γ$ and quadrupole $ΔQ_γ$ moments of the W gauge bosons in the context of the minimal supersymmetric standard model as functions of the soft SUSY breaking parameters $A_0,m_0,M_{1/2}$ and the top quark mass. The severe constraints imposed by the radiative breaking mechanism of the electroweak symmetry\,$ SU(2)\times U(1)$\, are duly taken into account. The supesymmetric values of $Δk_γ$ and $ΔQ_γ$ can be largely different, in some cases, from the standard model predictions but of the same order of magnitude for values of \quad $A_0,m_0,M_{1/2} \leq {\cal O}(1 TeV) $. Therefore possible supersymmetric structure can be probed provided the accuracy of measurements for $Δk_γ$, $ΔQ_γ$ reaches $10^{-2}-10^{-3}$ and hence hard to be detected at LEP2. If deviations from the standard model predictions are observed at LEP2, most likely these are not due to an underlying supersymmetric structure. In cases where $M_{1/2} \ll A_0,m_0 $, the charginos and neutralinos may give substantial contributions saturating the LEP2 sensitivity limits. This occurs when their masses $m_{\tilde C},m_{\tilde Z}$ turn out to be both light satisfying $m_{\tilde C}+m_{\tilde Z} \simeq M_W $. However these extreme cases are perturbatively untrustworthy and besides unnatural for they occupy a small region in the parameter space. }

hep-ph↗

arXiv1993

One loop corrections to the Neutralino sector and Radiative Electroweak Breaking in the MSSM

We compute one loop radiative corrections to the physical neutralino masses in the MSSM considering the dominant top-stop contributions. We present a numerical renormalization group analysis of the feasibility of the radiative gauge symmetry breaking parametrized by the standard soft supersymmetry breaking terms. Although the above computed effects can be in principle large for extreme values of the Yukawa couplings, they do not in general exceed a few per cent for most of the parameter space. Therefore tree level constraints imposed on the gluino mass $m_{\tilde{g}}$ and on the superpotential parameter $μ$ by LEP1 and CDF experiments, are not upset by the heavy top-stop sector.

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