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N. D. Tracas

Publications and source records attributed to N. D. Tracas.

26 records · Page 2Linked to original sources
arXiv1995

Gauge Coupling Unification and the Top Mass in String Models with $SU(4)\times O(4)$ Symmetry

We discuss the low energy implications of gauge coupling unification at the string scale, taking into account string threshold corrections in the $SU(4) {\times} O(4)$ model. We express $sin^2θ_W$ and $a_3$ as functions of the calculable string threshold differences and discuss simple examples of spectra which retain the successful predictions of the supersymmetric unification. Using further the low energy data and reasonable values of the common gauge coupling at the string scale, we obtain the range of the threshold corrections. Finally, we study the top Yukawa coupling ($h_t$) evolution whose initial value is determined in terms of the common gauge coupling at the string scale. We find that $h_t$ reaches its (quasi) infra-red fixed point at the weak scale and discuss the implications on the top mass.

hep-ph↗
arXiv1995

Soft SUSY Masses and the Dynamical Determination of the Gravitino Mass

We discuss in detail the possibility of determining dynamicaly the gravitino mass $m_{3/2}$, which is related to the supersymmetry breaking scale, within the minimal supersymmetric standard model (MSSM). Using the complete MSSM spectrum, we minimize the vacuum energy including one-loop corrections and a cosmological term of ${\cal O}(m_{3/2}^4)$ induced by the underlying fundamental theory. We find that both terms are necessary to determine dynamically the gravitino mass. Other useful constraints for the low energy phenomenology are also obtained.

hep-ph↗
arXiv1994

Low Energy Thresholds and the Scalar Mass Spectrum in Minimal Supersymmetry

We discuss low energy threshold effects and calculate the sparticle masses in the context of the Minimal Supersymmetric Standard Model. We pay particular attention to the top squark and the Higgs mass parameters, and calculate the top Yukawa corrections, taking into account the successive decoupling of each particle at its threshold. We discuss the phenomenological implications in the context of the radiative symmetry breaking scenario.

hep-ph↗
arXiv1994

Implications of a Heavy Top in Supersymmetric Theories

In the context of the radiative electroweak symmetry breaking scenario, we investigate the implications of a heavy top quark mass, close to its infrared fixed point, on the low energy parameters of the minimal supersymmetric standard model. We use analytic expressions to calculate the Higgs masses as well as the supersymmetric masses of the third generation. We further assume bottom-tau unification at the GUT scale and examine the constraints put by this condition on the parameter space ($\tanβ$,$α_3$), using the renormalization group procedure at the two-loop level. We find only a small fraction of the parameter space where the above conditions can be satisfied, namely $1\le \tanβ\le 2$, while $0.111\leα_3(M_Z) \le 0.118$. We further analyse the case where all three Yukawa couplings reach the perturbative limit just after the unification scale. In this latter case, the situation turns out to be very strict demanding $\tanβ\sim 63$.

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.

hep-ph↗
arXiv1992

Predictive Ansatz for Fermion Masses in SUSY GUTS

We reexamine a succesful fermion mass Ansatz proposed by Giudice for a wide range of the ratio $tanβ=\frac {<\bar h>}{ }$ (where ${\bar h},h$ are the two standard higgs fields), in the context of supersymmetric grand unified theories. We find that the 7 predictions of the ansatz, $V_{us}, V_{cb}, V_{ub}, m_u, m_d, m_s$ and $m_b$ are in good agreement with the experiment when either {\it i) } $tanβ\simeq 1$ or {\it ii)} $tanβ\geq 30$. A correct prediction for the bottom mass gives a lower limit on $m_t\geq 125$ for case {\it (i)}, in agreement with the previous analysis, while in case {\it (ii)} $m_t\geq 145$.

hep-ph↗
arXiv1992

Ansatz for Quark, Charged Lepton, and Neutrino Masses in SUSY GUTS

We extend a fermion mass matrix Ansatz by Giuduce to include neutrino masses. The previous predictions are maintained. With two additional parameters, a large Majorana neutrino mass and a hierarchy factor, we have seven {\it further} low energy predictions: the masses of the neutrinos, the mixing angles and the phase in the leptonic sector. We choose a reasonable hierarchy of Majorana masses and fit the overall mass scale according to a solution of the solar neutrino problem via the MSW mechanism, which is in agreement with the $^{37}Cl$, Kamiokande, and GALLEX data. We then also obtain a cosmologically interesting tau-neutrino mass.

hep-ph↗