Myriam Mondragon

Reduction of couplings and heavy top quark in the minimal SUSY GUT

Abstract Out of 256 independent reduction solutions that can be found within the minimal supersymmetric SU(5) GUT, there are exactly two asymptotically free solutions which can restrict the top quark mass m t and do not contradict the observed mass spectrum of the first two fermion generations. A numerical analysis shows that these two solutions lie on the same renormalization group invariant surface on which m t and the bottom quark mass m b assume relatively stable values for a given supersymmetry breaking scale m SUSY = 200 GeV with α S ( M Z ) = 0.12, α em ( M Z ) = (127.9) −1 and m τ = 1.78 GeV fixed, we find that on this surface m t and m t vary 2% and 3% around their central values 182 GeV and 5.3 GeV, respectively.

CONSTRAINTS ON FINITE SOFT SUPERSYMMETRY-BREAKING TERMS

A new solution to the requirement of two-loop finiteness of the soft supersymmetry breaking terms (SSB) parameters is found in Finite-Gauge-Yukawa unified theories. The new solution has the form of a sum rule for the relevant scalar masses, relaxing the universality required by the previously known solution, which leads to models with unpleasant phenomenological consequences. Using the sum rule we investigate two Finite-Gauge-Yukawa unified models and we determine their spectrum in terms of few parameters. Some characteristic features of the models are that a) the old agreement of the top quark mass prediction with the measured value remains unchanged, b) the lightest Higgs mass is predicted around 120 GeV, c) the s-spectrum starts above 200 GeV.

Testing gauge-Yukawa-unified models by Mt

Gauge-Yukawa Unification (GYU) relates the gauge and Yukawa couplings, thereby going beyond the usual GUTs, and it is assumed that the GYU in the third fermion generation implies that its Yukawa couplings are of the same order as the unified gauge coupling at the GUT scale. We re-examine carefully the recent observation that the top-bottom mass hierarchy can be explained to a certain extent in supersymmetric GYU models. It is found that there are equiv-top-mass lines in the boundary conditions of the Yukawa couplings so that two different GYU models on the same line can not be distinguished by the top mass Mt alone. If they are on different lines, they could be distinguished by Mt in principle, provided that the predicted Mts are well below the infrared value Mt(IR). We find that the ratio Mt(IR)/sin β depends on tan β for large tan β and the lowest value of Mt(IR) is ∼ 188 GeV. We focus our attention on the existing SU(5) GYU models, which are obtained by requiring finiteness and reduction of couplings. They, respectively, predict Mt = (183 + σMSSMMt ± 5) GeV and (181 + σMSSMMt ± 3) GeV, where σMSSMMt stands for the MSSM threshold correction and is ∼ −2 GeV for the case that all the MSSM superpartners have the same mass MSUSY with μH/MSUSY ⪡ 1.

Perturbative unification of soft supersymmetry-breaking terms

Abstract Perturbative unification of soft supersymmetry-breaking (SSB) parameters is proposed in gauge-Yukawa unified models. The method, which can be applied in any finite order in perturbation theory, consists in searching for renormalization group invariant relations among the SSB parameters, which are consistent with perturbative renormalizability. For the minimal gauge-Yukawa unified model based on SU (5) we find that the low energy SSB sector contains a single arbitrary parameter, the unified gaugino mass. Within a certain approximation we find that the model predicts a superpartner spectrum which is consistent with the experimental data.

Finite unified theories and the top quark mass

We present results of a study of two phenomenologically interesting SU (5) supersymmetric Grand Unified models, which are finite to all-loops before spontaneous symmetry breaking. The finiteness conditions provide us with relationships among the Yukawa and gauge couplings at the unification point, which in turn predict a heavy top quark mass (∼ 175 – 190 GeV).

Gauge-Yukawa unification in asymptotically non-free theories

Abstract We study asymptotically non-free gauge theories and search for renormalization group invariant (i.e. technically natural) relations among the couplings which lead to successful gauge-Yukawa unification. To be definite, we consider a supersymmetric model based on SU(4) × SU(2)R × SU(2)L. It is found that among the couplings of the model, which can be expressed in this way by a single one in the lowest order approximation, are the tree gauge couplings and the Yukawa coupling of the third generation. The corrections to the lowest order results are computed, and we find that the predictions on the low energy parameters resulting from those relations are in agreement with the measurements at LEP and Tevatron for a certain range of supersymmetry breaking scale.

Confronting finite unified theories with low-energy phenomenology

Finite Unified Theories (FUTs) are N = 1 supersymmetric Grand Unified Theories that can be made all-loop finite. The requirement of all-loop finiteness leads to a severe reduction of the free parameters of the theory and, in turn, to a large number of predictions. FUTs are investigated in the context of low-energy phenomenology observables. We present a detailed scanning of the all-loop finite SU(5) FUTs, where we include the theoretical uncertainties at the unification scale and we apply several phenomenological constraints. Taking into account the restrictions from the top and bottom quark masses, we can discriminate between different models. Including further low-energy constraints such as B physics observables, the bound on the lightest Higgs boson mass and the cold dark matter density, we determine the predictions of the allowed parameter space for the Higgs boson sector and the supersymmetric particle spectrum of the selected model.

RUNNING OF SOFT PARAMETERS IN EXTRA SPACE-TIME DIMENSIONS

The evolution of the parameters including those in the soft supersymmetry-breaking (SSB) sector is studied in the minimal supersymmetric standard model (MSSM) with a certain set of Kaluza-Klein towers which has been recently considered by Dienes et al. We use the continuous Wilson renormalization group technique to derive the one-loop matching condition between the effective, renormalizable and original, unrenormalizable theories. We investigate whether the assumption of a large compactification radius in the model is consistent with the gauge coupling unification, the b−τ unification and the radiative breaking of the electroweak gauge symmetry with the universal SSB terms. We calculate the superpartner spectrum under the assumption of the universal SSB parameters to find differences between the model and the MSSM.

Concepts of Electroweak Symmetry Breaking and Higgs Physics

We present an introduction to the basic concepts of electroweak symmetry breaking and Higgs physics within the Standard Model and its supersymmetric extensions. A brief overview will also be given on alternative mechanisms of electroweak symmetry breaking. In addition to the theoretical basis, the present experimental status of Higgs physics and prospects at the Tevatron, the LHC and e + e − linear colliders are discussed.

Axial couplings on the world-line

Abstract We construct a world-line representation for the fermionic one-loop effective action with axial and also vector, scalar, and pseudo-scalar couplings. We use this expression to compute a few selected scattering amplitudes. These allow us to verify that our method yields the same results as standard field theory. In particular, we are able to reproduce the chiral anomaly. Our starting point is the second order formulation for the Dirac fermion. We translate the second order expressions into a world-line action.