On Instability of Certain Bi-Metric and Massive-Gravity Theories

Stability about cosmological background solutions to the bi-metric Hassan-Rosen theory is studied. The results of this analysis are presented, and it is shown that a large class of cosmological backgrounds is classically unstable. This sets serious doubts on the physical viability of the Hassan-Rosen theory - and in turn also of the de Rham-Gadabaze-Tolley model, to which the mentioned theory is parent. A way to overcome this instability by means of curvature-type deformations is discussed.Comment: 4 pages, 2 figures; v2: minor changes to match PRD versio

Frontiers of QCD with Precision nPDFs

Searches for new physics will increasingly depend on identifying deviations from precision Standard Model (SM) predictions. Quantum Chromodynamics (QCD) will necessarily play a central role in this endeavor as it provides the framework for the parton model. However, as we move to higher orders and into extreme kinematic regions, we begin to see the full complexities of the QCD theory. Recent theoretical developments improve our ability to analyze both proton and nuclear PDFs across the full kinematic range. These developments are incorporated into the new nCTEQ15 PDFs, and we review these developments with respect to future measurements, and identify areas where additional effort is required.Comment: Published in the "6th International conference on Physics Opportunities at Electron-Ion colliders (POETIC6)

Linear scaling electronic structure calculations and accurate sampling with noisy forces

Numerical simulations based on electronic structure calculations are finding ever growing applications in many areas of physics. A major limiting factor is however the cubic scaling of the algorithms used. Building on previous work [F. R. Krajewski and M. Parrinello, Phys.Rev. B71, 233105 (2005)] we introduce a novel statistical method for evaluating the inter-atomic forces which scales linearly with system size and is applicable also to metals. The method is based on exact decomposition of the fermionic determinant and on a mapping onto a field theoretical expression. We solve exactly the problem of sampling the Boltzmann distribution with noisy forces. This novel approach can be used in such diverse fields as quantum chromodynamics, quantum Monte Carlo or colloidal physics.Comment: 5 pages, 2 figure

Ultrafast spin dynamics in II-VI diluted magnetic semiconductors with spin-orbit interaction

We study theoretically the ultrafast spin dynamics of II-VI diluted magnetic semiconductors in the presence of spin-orbit interaction. Our goal is to explore the interplay or competition between the exchange $sd$-coupling and the spin-orbit interaction in both bulk and quantum well systems. For bulk materials we concentrate on Zn$_{1-x}$Mn$_x$Se and take into account the Dresselhaus interaction, while for quantum wells we examine Hg$_{1-x-y}$Mn$_x$Cd$_y$Te systems with a strong Rashba coupling. Our calculations were performed with a recently developed formalism which incorporates electronic correlations beyond mean-field theory originated from the exchange $sd$-coupling. For both bulk and quasi-two-dimensional systems we find that, by varying the system parameters within realistic ranges, both interactions can be chosen to play a dominant role or to compete on an equal footing with each other. The most notable effect of the spin-orbit interaction in both types of systems is the appearance of strong oscillations where the exchange $sd$-coupling by itself only causes an exponential decay of the mean electronic spin components. The mean-field approximation is also studied and it is interpreted analytically why it shows a strong suppression of the spin-orbit-induced dephasing of the spin component parallel to the Mn magnetic field.Comment: 9 pages, 5 figure

Higher gauge theory -- differential versus integral formulation

The term higher gauge theory refers to the generalization of gauge theory to a theory of connections at two levels, essentially given by 1- and 2-forms. So far, there have been two approaches to this subject. The differential picture uses non-Abelian 1- and 2-forms in order to generalize the connection 1-form of a conventional gauge theory to the next level. The integral picture makes use of curves and surfaces labeled with elements of non-Abelian groups and generalizes the formulation of gauge theory in terms of parallel transports. We recall how to circumvent the classic no-go theorems in order to define non-Abelian surface ordered products in the integral picture. We then derive the differential picture from the integral formulation under the assumption that the curve and surface labels depend smoothly on the position of the curves and surfaces. We show that some aspects of the no-go theorems are still present in the differential (but not in the integral) picture. This implies a substantial structural difference between non-perturbative and perturbative approaches to higher gauge theory. We finally demonstrate that higher gauge theory provides a geometrical explanation for the extended topological symmetry of BF-theory in both pictures.Comment: 26 pages, LaTeX with XYPic diagrams; v2: typos corrected and presentation improve

The Clustering Of Galaxies Around Radio-Loud AGNs

We examine the hypothesis that mergers and close encounters between galaxies can fuel AGNs by increasing the rate at which gas accretes towards the central black hole. We compare the clustering of galaxies around radio-loud AGNs with the clustering around a population of radio-quiet galaxies with similar masses, colors and luminosities. Our catalog contains 2178 elliptical radio galaxies with flux densities greater than 2.8 mJy at 1.4 GHz from the 6dFGS survey. We find that radio AGNs with more than 200 times the median radio power have, on average, more close (r<160 kpc) companions than their radio-quiet counterparts, suggestive that mergers play a role in forming the most powerful radio galaxies. For ellipticals of fixed stellar mass, the radio power is not a function of large-scale environment nor halo mass, consistent with the radio powers of ellipticals varying by orders of magnitude over billions of years.Comment: 12 pages, 6 figure