Gauge-Higgs unification on the brane

From the quantum field theory point of view, matter and gauge fields are generally expected to be localised around branes or topological defects occurring in extra dimensions. Here I discuss a simple scenario where, by starting with a five dimensional SU(3) gauge theory, we end up with several 4-D parallel branes with localised "chiral" fermions and gauge fields to them. I will show that it is possible to reproduce the electroweak model confined to a single brane, allowing a simple and geometrical approach to the fermion hierarchy problem. Some nice results of this construction are: Gauge and Higgs fields are unified at the 5-D level; and new particles are predicted: a left-handed neutrino of zero hypercharge, and a massive vector field coupling together the new neutrino to other left-handed leptons.Comment: Contribution to the proceedings of the RTN workshop "The Quest for Unification: Theory Confronts Experiment", Corfu, Greece, Sept 11-18, 200

Partial preservation of chiral symmetry and colossal magnetoresistance in adatom doped graphene

We analyze the electronic properties of adatom doped graphene in the low impurity concentration regime. We focus on the Anderson localized regime and calculate the localization length ($\xi$) as a function of the electron doping and an external magnetic field. The impurity states hybridize with carbon's $p_z$ states and form a partially filled band close to the Dirac point. Near the impurity band center, the chiral symmetry of the system's effective Hamiltonian is partially preserved which leads to a large enhancement of $\xi$. The sensitivity of transport properties, namely Mott's variable range hopping scale $T_0$, to an external magnetic field perpendicular to the graphene sheet leads to a colossal magnetoresistance effect, as observed in recent experiments.Comment: 5 pages, 4 figs. Few comments and references added. To appear in PR

Localized Spins on Graphene

The problem of a magnetic impurity, atomic or molecular, absorbed on top of a carbon atom in otherwise clean graphene is studied using the numerical renormalization group. The spectral, thermodynamic, and scattering properties of the impurity are described in detail. In the presence of a small magnetic field, the low energy electronic features of graphene make possible to inject spin polarized currents through the impurity using a scanning tunneling microscope (STM). Furthermore, the impurity scattering becomes strongly spin dependent and for a finite impurity concentration it leads to spin polarized bulk currents and a large magnetoresistance. In gated graphene the impurity spin is Kondo screened at low temperatures. However, at temperatures larger than the Kondo temperature, the anomalous magnetotransport properties are recovered.Comment: 4+ pages, 4 figures. Added reference

Threshold effects In multivariate error correction models

In this paper we propose a testing procedure for assessing the presence of threshold effects in nonstationary Vector autoregressive models with or without cointegration. Our approach involves first testing whether the long run impact matrix characterising the VECM type representation of the VAR switches according to the magnitude of some threshold variable and is valid regardless of whether the system is purely I(1), I(1) with cointegration or stationary. Once the potential presence of threshold effects is established we subsequently evaluate the cointegrating properties of the system in each regime through a model selection based approach whose asymptotic and finite sample properties are also established. This subsequently allows us to introduce a novel non-linear permanent and transitory decomposition of the vector process of interest.

Semiclassical geons as solitonic black hole remnants

We find that the end state of black hole evaporation could be represented by non-singular and without event horizon stable solitonic remnants with masses of the order the Planck scale and up to 16 units of charge. Though these objects are locally indistinguishable from spherically symmetric, massive electric (or magnetic) charges, they turn out to be sourceless geons containing a wormhole generated by the electromagnetic field. Our results are obtained by interpreting semiclassical corrections to Einstein's theory in the first-order (Palatini) formalism, which yields second-order equations and avoids the instabilities of the usual (metric) formulation of quadratic gravity. We also discuss the potential relevance of these solutions for primordial black holes and the dark matter problem.Comment: 9 pages, 1 figur