Leptogenesis and tensor polarisation from a gravitational Chern-Simons term

Within an effective field theory derived from string theory, the universal axion has to be coupled to the the gravitational Chern-Simons (gCS) term. During any era when the axion field is varying, the vacuum fluctuation of the gravitational wave amplitude will then be circularly polarised, generating an expectation value for the gCS term. The polarisation may be observable through the Cosmic Microwave Background, and the vacuum expectation value of the gCS term may generate the baryon asymmetry of the Universe. We argue here that such effects cannot be computed without further input from string theory, since the `vacuum' in question is unlikely to be the field-theoretic one.Comment: Latex file. 10 pages. v2: Minor changes in the abstract and the conclusion. References added. Version accepted for publication in Journal of High Energy Physics. v3: Typos corrected. Erratum sent for publicatio

Graphene physics via the Dirac oscillator in (2+1) dimensions

We show how the two-dimensional Dirac oscillator model can describe some properties of electrons in graphene. This model explains the origin of the left-handed chirality observed for charge carriers in monolayer and bilayer graphene. The relativistic dispersion relation observed for monolayer graphene is obtained directly from the energy spectrum, while the parabolic dispersion relation observed for the case of bilayer graphene is obtained in the non-relativistic limit. Additionally, if an external magnetic field is applied, the unusual Landau-level spectrum for monolayer graphene is obtained, but for bilayer graphene the model predicts the existence of a magnetic field-dependent gap. Finally, this model also leads to the existence of a chiral phase transition

Quantum phase transition in the chirality of the (2+1)-dimensional Dirac oscillator

We study the (2+1)-dimensional Dirac oscillator in the presence of an external uniform magnetic field (B). We show how the change of the strength of B leads to the existence of a quantum phase transition in the chirality of the system. A critical value of the strength of the external magnetic field (Bc) can be naturally defined in terms of physical parameters of the system. While for B = Bc the fermion can be considered as a free particle without defined chirality, for B Bc) the chirality is left (right) and there exist a net potential acting on the fermion. For the three regimes defined in the quantum phase transition of chirality, we observe that the energy spectra for each regime is drastically different. Then, we consider the z-component of the orbital angular momentum as an order parameter that characterizes the quantum phase transition

Tasas efectivas del impuesto de renta para sectores de la economía colombiana entre 2001 y 2015

El principal objetivo de este trabajo es calcular la tasa efectiva del impuesto neto de renta generado por las personas jurídicas, pertenecientes a cada uno de los sectores de la economía colombiana, usando la información disponible en los agregados de declaraciones de renta de la Dirección de Impuestos y Aduanas Nacionales (DIAN), durante el periodo 2000-2015. Las tasas efectivas obtenidas muestran que el impuesto neto de renta real pagado por las personas jurídicas no es tan alto como se cree, además de que los valores de estas tasas presentan diferencias significativas, para los diferentes sectores económicos, debido a la existencia de beneficios tributarios que impactan de forma diferente a cada sector. Estas diferencias demuestran la inequidad horizontal del sistema tributario colombiano, dado que sectores que no son tan relevantes en generación de empleo, tales como el financiero y el minero, fueron los que presentaron los mayores montos de beneficios tributarios.The main objective of this paper is to calculate the effective rate of the net income tax generated by legal entities of each of the sectors of the Colombian economy, using information available in the income tax aggregates of the Colombian National Office of Taxes and Customs (DIAN in Spanish), during the period 2000-2015. The effective rates identified show that the net real income tax paid by legal persons is not as high as believed. Additionally, the values of these rates show significant differences among the different economic sectors due to tax benefits that have different impacts on each one of them. These differences prove the horizontal inequity within the Colombian tax system, since sectors that are not so relevant for employment generation, such as the financial and the mining sectors, perceived the highest amounts of tax benefit