An efficient renormalization group improved implementation of the MSSM effective potential

In the context of MSSM, a novel improving procedure based on the renormalization group equation is applied to the effective potential in the Higgs sector. We focus on the one-loop radiative corrections computed in Landau gauge by using the mass independent renormalization scheme $\bar{DR}$. Thanks to the decoupling theorem, the well-known multimass scale problem is circumvented by switching to a new effective field theory every time a new particle threshold is encountered. We find that, for any field configuration, there is a convenient renormalization scale $\tilde{Q}^*$ at which the loop expansion respects the perturbation series hierarchy and the theory retains the vital property of stability.Comment: 35 pages, 5 figures, RevTe

Unification and fermion mass relations in low string scale D-brane models

In this talk, gauge coupling evolution is analyzed in D-brane inspired models with two Higgs doublets and a U(3)xU(2)xU(1)^N gauge symmetry. In particular, we focus on D-brane configurations with two or three abelian factors. We find that the correct hypercharge assignment of the Standard Model particles is reproduced for six viable models distinguished by different brane configurations. We also investigate the bottom tau quark mass relation and find that the correct low energy m_b / m_\tau ratio is obtained for equal b-\tau Yukawa couplings at a string scale around 10^3 TeV.Comment: 10 pages, 3 figures. Talk presented at the ``Corfu Summer Institute'', Corfu-Greece, September 4-14, 2005. To appear in the proceedings of RTN workshop on the Quest for Unification Theory Confronts Experimen

Gauge coupling and fermion mass relations in low string scale brane models

We analyze the gauge coupling evolution in brane inspired models with $U(3) \times U(2) \times U(1)^N$ symmetry at the string scale. We restrict to the case of brane configurations with two and three abelian factors (N=2,3) and where only one Higgs doublet is coupled to down quarks and leptons and only one to the up quarks. We find that the correct hypercharge assignment of the Standard Model particles is reproduced for six viable models distinguished by different brane configurations. We investigate the third generation fermion mass relations and find that the correct low energy $m_b/m_{\tau}$ ratio can be obtained for $b-\tau$ Yukawa coupling equality at a string scale as low as $M_S \sim 10^3$ TeV