Primordial Magnetic Fields via Spontaneous Breaking of Lorentz Invariance

Spontaneous breaking of Lorentz invariance compatible with observational limits may realistically take place in the context of string theories, possibly endowing the photon with a mass. In this process the conformal symmetry of the electromagnetic action is broken allowing for the possibility of generating large scale ($\sim Mpc$) magnetic fields within inflationary scenarios. We show that for reheating temperatures safe from the point of view of the gravitino and moduli problem, T_{RH} \laq 10^{9} GeV for $m_{3/2} \approx 1 TeV$, the strength of the generated seed fields is, in our mechanism, consistent with amplification by the galactic dynamo processes and can be even as large as to explain the observed galactic magnetic fields through the collapse of protogalactic clouds.Comment: Final version to appear in Physics Letters

Impact of thixotropy on flow patterns induced in a stirred tank : numerical and experimental studies

Agitation of a thixotropic shear-thinning fluid exhibiting a yield stress is investigated both experimentally and via simulations. Steady-state experiments are conducted at three impeller rotation rates (1, 2 and 8 s−1) for a tank stirred with an axial-impeller and flow-field measurements are made using particle image velocimetry (PIV) measurements. Threedimensional numerical simulations are also performed using the commercial CFD code ANSYS CFX10.0. The viscosity of the suspension is determined experimentally and is modelled using two shear-dependant laws, one of which takes into account the flow instabilities of such fluids at low shear rates. At the highest impeller speed, the flow exhibits the familiar outward pumping action associated with axial-flow impellers. However, as the impeller speed decreases, a cavern is formed around the impeller, the flow generated in the vicinity of the agitator reorganizes and its pumping capacity vanishes. An unusual flow pattern, where the radial velocity dominates, is observed experimentally at the lowest stirring speed. It is found to result from wall slip effects. Using blades with rough surfaces prevents this peculiar behaviour and mainly resolves the discrepancies between the experimental and computational results

Gamma-Ray Emission From Be/X-Ray Binaries

Be/X-ray binaries are systems formed by a massive Be star and a magnetized neutron star, usually in an eccentric orbit. The Be star has strong equatorial winds occasionally forming a circumstellar disk. When the neutron star intersects the disk the accretion rate dramatically increases and a transient accretion disk can be formed around the compact object. This disk can last longer than a single orbit in the case of major outbursts. If the disk rotates faster than the neutron star, the Cheng-Ruderman mechanism can produce a current of relativistic protons that would impact onto the disk surface, producing gamma-rays from neutral pion decays and initiating electromagnetic cascades inside the disk. In this paper we present calculations of the evolution of the disk parameters during both major and minor X-ray events, and we discuss the generation of gamma-ray emission at different energies within a variety of models that include both screened and unscreened disks.Comment: 14 pages, to appear in: "The multiwavelength approach to unidentified gamma-ray sources", Eds. K. S. Cheng & G.E. Romero, Kluwer Academic Publisher (Astrophysics and Space Sciences Journal). The present version has two additional figures respect to the version to be published in the journa

The Functional Renormalization Group and O(4) scaling

The critical behavior of the chiral quark-meson model is studied within the Functional Renormalization Group (FRG). We derive the flow equation for the scale dependent thermodynamic potential at finite temperature and density in the presence of a symmetry-breaking external field. Within this scheme, the critical scaling behavior of the order parameter, its transverse and longitudinal susceptibilities as well as the correlation lengths near the chiral phase transition are computed. We focus on the scaling properties of these observables at non-vanishing external field when approaching the critical point from the symmetric as well as from the broken phase. We confront our numerical results with the Widom-Griffiths form of the magnetic equation of state, obtained by a systematic epsilon-expansion of the scaling function. Our results for the critical exponents are consistent with those recently computed within Lattice Monte-Carlo studies of the O(4) spin system.Comment: 14 pages, 11 figure

Observations of Coronal Mass Ejections with the Coronal Multichannel Polarimeter

The Coronal Multichannel Polarimeter (CoMP) measures not only the polarization of coronal emission, but also the full radiance profiles of coronal emission lines. For the first time, CoMP observations provide high-cadence image sequences of the coronal line intensity, Doppler shift and line width simultaneously in a large field of view. By studying the Doppler shift and line width we may explore more of the physical processes of CME initiation and propagation. Here we identify a list of CMEs observed by CoMP and present the first results of these observations. Our preliminary analysis shows that CMEs are usually associated with greatly increased Doppler shift and enhanced line width. These new observations provide not only valuable information to constrain CME models and probe various processes during the initial propagation of CMEs in the low corona, but also offer a possible cost-effective and low-risk means of space weather monitoring.Comment: 6 figures. Will appear in the special issue of Coronal Magnetism, Sol. Phy

Charged lepton Flavor Violation in Supersymmetry with Bilinear R-Parity Violation

The simplest unified extension of the Minimal Supersymmetric Standard Model with bi-linear R-parity violation naturally predicts a hierarchical neutrino mass spectrum, suitable to explain atmospheric and solar neutrino fluxes. We study whether the individual violation of the lepton numbers L_{e,mu,tau} in the charged sector can lead to measurable rates for BR(mu->e gamma)and $BR(tau-> mu gamma). We find that some of the R-parity violating terms that are compatible with the observed atmospheric neutrino oscillations could lead to rates for mu->e gamma measurable in projected experiments. However, the Delta m^2_{12} obtained for those parameters is too high to be compatible with the solar neutrino data, excluding therefore the possibility of having measurable rates for mu->e gamma in the model.Comment: 29 pages, 8 figures. Constraint from solar neutrino data included, conclusions changed respect v

Quasi-particle model for lattice QCD: quark-gluon plasma in heavy ion collisions

We propose a quasi-particle model to describe the lattice QCD equation of state for pure SU(3) gauge theory in its deconfined state, for $T \ge 1.5T_c$. The method involves mapping the interaction part of the equation of state to an effective fugacity of otherwise non-interacting quasi-gluons. We find that this mapping is exact. Using the quasi-gluon distribution function, we determine the energy density and the modified dispersion relation for the single particle energy, in which the trace anomaly is manifest. As an application, we first determine the Debye mass, and then the important transport parameters, {\it viz}, the shear viscosity, $\eta$ and the shear viscosity to entropy density ratio, $\eta/{\mathcal S}$. We find that both $\eta$ and $\eta/{\mathcal S}$ are sensitive to the interactions, and that the interactions significantly lower both $\eta$ and $\eta/\mathcal S$.Comment: 10 pages, 8 figures, epj class file, version accepted for publication in Euro. Phys.J

Environment-Induced Decoherence and the Transition From Quantum to Classical

We study dynamics of quantum open systems, paying special attention to those aspects of their evolution which are relevant to the transition from quantum to classical. We begin with a discussion of the conditional dynamics of simple systems. The resulting models are straightforward but suffice to illustrate basic physical ideas behind quantum measurements and decoherence. To discuss decoherence and environment-induced superselection einselection in a more general setting, we sketch perturbative as well as exact derivations of several master equations valid for various systems. Using these equations we study einselection employing the general strategy of the predictability sieve. Assumptions that are usually made in the discussion of decoherence are critically reexamined along with the ``standard lore'' to which they lead. Restoration of quantum-classical correspondence in systems that are classically chaotic is discussed. The dynamical second law -it is shown- can be traced to the same phenomena that allow for the restoration of the correspondence principle in decohering chaotic systems (where it is otherwise lost on a very short time-scale). Quantum error correction is discussed as an example of an anti-decoherence strategy. Implications of decoherence and einselection for the interpretation of quantum theory are briefly pointed out.Comment: 80 pages, 7 figures included, Lectures given by both authors at the 72nd Les Houches Summer School on "Coherent Matter Waves", July-August 199

Quantum mechanics: Myths and facts

A common understanding of quantum mechanics (QM) among students and practical users is often plagued by a number of "myths", that is, widely accepted claims on which there is not really a general consensus among experts in foundations of QM. These myths include wave-particle duality, time-energy uncertainty relation, fundamental randomness, the absence of measurement-independent reality, locality of QM, nonlocality of QM, the existence of well-defined relativistic QM, the claims that quantum field theory (QFT) solves the problems of relativistic QM or that QFT is a theory of particles, as well as myths on black-hole entropy. The fact is that the existence of various theoretical and interpretational ambiguities underlying these myths does not yet allow us to accept them as proven facts. I review the main arguments and counterarguments lying behind these myths and conclude that QM is still a not-yet-completely-understood theory open to further fundamental research.Comment: 51 pages, pedagogic review, revised, new references, to appear in Found. Phy

Improved Holographic QCD

We provide a review to holographic models based on Einstein-dilaton gravity with a potential in 5 dimensions. Such theories, for a judicious choice of potential are very close to the physics of large-N YM theory both at zero and finite temperature. The zero temperature glueball spectra as well as their finite temperature thermodynamic functions compare well with lattice data. The model can be used to calculate transport coefficients, like bulk viscosity, the drag force and jet quenching parameters, relevant for the physics of the Quark-Gluon Plasma.Comment: LatEX, 65 pages, 28 figures, 9 Tables. Based on lectures given at several Schools. To appear in the proceedinds of the 5th Aegean School (Milos, Greece