The potential of the variable DA white dwarf G117-B15A as a tool for Fundamental Physics

White dwarfs are well studied objects. The relative simplicity of their physics allows to obtain very detailed models which can be ultimately compared with their observed properties. Among white dwarfs there is a specific class of stars, known as ZZ-Ceti objects, which have a hydrogen-rich envelope and show periodic variations in their light curves. G117-B15A belongs to this particular set of stars. The luminosity variations have been successfully explained as due to g-mode pulsations. G117-B15A has been recently claimed to be the most stable optical clock ever found, being the rate of change of its 215.2 s period very small: \dot{P}= (2.3 +- 1.4)x10^{-15} s s^-1, with a stability comparable to that of the most stable millisecond pulsars. The rate of change of the period is closely related to its cooling timescale, which can be accurately computed. In this paper we study the pulsational properties of G117-B15A and we use the observed rate of change of the period to impose constraints on the axion emissivity and, thus, to obtain a preliminary upper bound to the mass of the axion. This upper bound turns out to be 4cos^{2}{\beta} meV at the 95% confidence level. Although there are still several observational and theoretical uncertainties, we conclude that G117-B15A is a very promising stellar object to set up constraints on particle physics.Comment: 32 pages, 14 figures, accepted for publication in New Astronom

Banking Sector Performance in Latin America: Market Power versus Efficiency

TSince the mid-1990s the banking sector in the Latin American emerging markets has experienced profound changes due to financial liberalisation, a significant increase in foreign investments and greater mergers activities often occurring following financial crises. The wave of consolidation and the rapid increase in market concentration that took place in most countries has generated concerns about the rise in banks’ market power and its potential effects on consumers. This paper advances the existing literature by testing the market power (Structure-Conduct-Performance and Relative Market Power) and efficient structure (X- and scale efficiency) hypotheses for a sample of over 2,500 bank observations in nine Latin American countries over 1997-2005. We use the Data Envelopment Analysis technique to obtain reliable efficiency measures. We produce evidence supporting the efficient structure hypotheses. The findings are particularly robust for the largest banking markets in the region, namely Brazil, Argentina and Chile. Finally, capital ratios and bank size seem to be among the most important factors in explaining higher than normal profits for Latin American banks.Structure-Conduct-Performance; Efficient Structure; Latin American banking; Data Envelopment Analysis (DEA).

Deep Strong Coupling Regime of the Jaynes-Cummings model

We study the quantum dynamics of a two-level system interacting with a quantized harmonic oscillator in the deep strong coupling regime (DSC) of the Jaynes-Cummings model, that is, when the coupling strength g is comparable or larger than the oscillator frequency w (g/w > 1). In this case, the rotating-wave approximation cannot be applied or treated perturbatively in general. We propose an intuitive and predictive physical frame to describe the DSC regime where photon number wavepackets bounce back and forth along parity chains of the Hilbert space, while producing collapse and revivals of the initial population. We exemplify our physical frame with numerical and analytical considerations in the qubit population, photon statistics, and Wigner phase space.Comment: Published version, note change of title: DSC regime of the JC mode