Radiative Symmetry Breaking on D-branes at Non-supersymmetric Singularities

The possibility of radiative gauge symmetry breaking on D3-branes at non-supersymmetric orbifold singularities is examined. As an example, a simple model of D3-branes at non-supersymmetric C^3/Z_6 singularity with some D7-branes for the cancellations of R-R tadpoles in twisted sectors is analyzed in detail. We find that there are no tachyon modes in twisted sectors, and NS-NS tadpoles in twisted sectors are canceled out, though uncanceled tadpoles and tachyon modes exist in untwisted sectors. This means that this singularity background is a stable solution of string theory at tree level, though some specific compactification of six-dimensional space should be considered for a consistent untwisted sector. On D3-brane three massless "Higgs doublet fields" and three family "up-type quarks" are realized at tree level. Other fermion fields, "down-type quarks" and "leptons", can be realized as massless modes of the open strings stretching between D3-branes and D7-branes. The Higgs doublet fields have Yukawa couplings with up-type quarks, and they also have self-couplings which give a scalar potential without flat directions. Since there is no supersymmetry, the radiative corrections may naturally develop negative Higgs mass squared and "electroweak symmetry breaking". We explicitly calculate the open string one-loop correction to the Higgs mass squared from twisted sectors, and find that the negative value is indeed realized in this specific model.Comment: 20 page

SUSY-breaking Soft Terms in a MSSM Magnetized D7-brane Model

We compute the SUSY-breaking soft terms in a magnetized D7-brane model with MSSM-like spectrum, under the general assumption of non-vanishing auxiliary fields of the dilaton and Kahler moduli. As a particular scenario we discuss SUSY breaking triggered by ISD or IASD 3-form fluxes.Comment: Latex, 27 pages, v2: added reference

Infrared Gluon and Ghost Propagators from Lattice QCD. Results from large asymmetric lattices

We report on the infrared limit of the quenched lattice Landau gauge gluon and ghost propagators as well as the strong coupling constant computed from large asymmetric lattices. The infrared lattice propagators are compared with the pure power law solutions from Dyson-Schwinger equations (DSE). For the gluon propagator, the lattice data is compatible with the DSE solution. The preferred measured gluon exponent being $\sim 0.52$, favouring a null zero momentum propagator. The lattice ghost propagator shows finite volume effects and, for the volumes considered, the propagator does not follow a pure power law. Furthermore, the strong coupling constant is computed and its infrared behaviour investigated.Comment: Talk given at QNP06; final version with improved english, accepted for publication at EPJ

The Maastricht Convergence Criteria and Optimal Monetary Policy for the EMU Accession Countries.

The EMU accession countries are obliged to fulfill the Maastricht convergence criteria prior to entering the EMU. This paper uses a DSGE model of a two-sector small open economy, to address the following question: How do the Maastricht convergence criteria modify optimal monetary policy in an economy facing domestic and external shocks? First, we derive the micro founded loss function that represents the objective function of the optimal monetary policy not constrained to satisfy the criteria. We find that the optimal monetary policy should not only target inflation rates in the domestic sectors and aggregate output fluctuations but also domestic and international terms of trade. Second, we show how the loss function changes when the monetary policy is constrained to satisfy the Maastricht criteria. The loss function of such a constrained policy is characterized by additional elements penalizing fluctuations of the CPI inflation rate, the nominal interest rate and the nominal exchange rate around the new targets which are potentially different from the steady state of the unconstrained optimal monetary policy. Under the chosen parameterization, the unconstrained optimal monetary policy violates two criteria: concerning the CPI inflation rate and the nominal interest rate. The constrained optimal policy results in targeting the CPI inflation rate and the nominal interest rate that are 0.7% lower (in annual terms) than the CPI inflation rate and the nominal interest rate in the countries taken as a reference. The welfare costs associated with these constraints need to be offset against credibility gains and other benefits related to the compliance with the Maastricht criteria that are not modelled. JEL Classification: F41, E52, E58, E61EMU accession countries, Maastricht convergence criteria, optimal monetary policy