30,957 research outputs found
Phase Fluctuations in Strongly Coupled -Wave Superconductors
We present a numerically exact solution for the BCS Hamiltonian at any
temperature, including the degrees of freedom associated with classical phase,
as well as amplitude, fluctuations via a Monte Carlo (MC) integration. This
allows for an investigation over the whole range of couplings: from weak
attraction, as in the well-known BCS limit, to the mainly unexplored
strong-coupling regime of pronounced phase fluctuations. In the latter, for the
first time two characteristic temperatures and , associated with
short- and long-range ordering, respectively, can easily be identified in a
mean-field-motivated Hamiltonian. at the same time corresponds to the
opening of a gap in the excitation spectrum. Besides introducing a novel
procedure to study strongly coupled d-wave superconductors, our results
indicate that classical phase fluctuations are not sufficient to explain the
pseudo-gap features of high-temperature superconductors (HTS).Comment: 5 pages, 3 figure
A Renormalization Group Analysis of the NCG constraints m_{top} = 2\,m_W},
We study the evolution under the renormalization group of the restrictions on
the parameters of the standard model coming from Non-Commutative Geometry,
namely and . We adopt the point of
view that these relations are to be interpreted as {\it tree level} constraints
and, as such, can be implemented in a mass independent renormalization scheme
only at a given energy scale . We show that the physical predictions on
the top and Higgs masses depend weakly on .Comment: 7 pages, FTUAM-94/2, uses harvma
Observational Constraints on Transverse Gravity: a Generalization of Unimodular Gravity
We explore the hypothesis that the set of symmetries enjoyed by the theory
that describes gravity is not the full group of diffeomorphisms Diff(M), as in
General Relativity, but a maximal subgroup of it, TransverseDiff(M), with its
elements having a jacobian equal to unity; at the infinitesimal level, the
parameter describing the coordinate change, xi^mu (x), is transverse, i.e.,
partial_mu(xi^mu)=0. Incidentally, this is the smaller symmetry one needs to
propagate consistently a graviton, which is a great theoretical motivation for
considering these theories. Also, the determinant of the metric, g, behaves as
a "transverse scalar", so that these theories can be seen as a generalization
of the better-known unimodular gravity. We present our results on the
observational constraints on transverse gravity, in close relation with the
claim of equivalence with general scalar-tensor theory. We also comment on the
structure of the divergences of the quantum theory to the one-loop order.Comment: Prepared for the First Mediterranean Conference on Classical and
Quantum Gravity, MCCQG, Kolymbari (Crete, Greece), 14-18 September, 2009;
also, ERE2009: Gravitation in the Large, Bilbao (Spain), 7-11 September, 200
Analysis of process variables via CFD to evaluate the performance of a FCC riser
Feedstock conversion and yield products are studied through a 3D model simulating the main reactor of the fluid catalytic cracking (FCC) process. Computational fluid dynamic (CFD) is used with Eulerian-Eulerian approach to predict the fluid catalytic cracking behavior. The model considers 12 lumps with catalyst deactivation by coke and poisoning by alkaline nitrides and polycyclic aromatic adsorption to estimate the kinetic behavior which, starting from a given feedstock, produces several cracking products. Different feedstock compositions are considered. The model is compared with sampling data at industrial operation conditions. The simulation model is able to represent accurately the products behavior for the different operating conditions considered. All the conditions considered were solved using a solver ANSYS CFX 14.0. The different operation process variables and hydrodynamic effects of the industrial riser of a fluid catalytic cracking (FCC) are evaluated. Predictions from the model are shown and comparison with experimental conversion and yields products are presented; recommendations are drawn to establish the conditions to obtain higher product yields in the industrial process
Gauge Symmetry and Consistent Spin-Two Theories
We study Lagrangians with the minimal amount of gauge symmetry required to
propagate spin-two particles without ghosts or tachyons. In general, these
Lagrangians also have a scalar mode in their spectrum. We find that, in two
cases, the symmetry can be enhanced to a larger group: the whole group of
diffeomorphisms or a enhancement involving a Weyl symmetry. We consider the
non-linear completions of these theories. The intuitive completions yield the
usual scalar-tensor theories except for the pure spin-two cases, which
correspond to two inequivalent Lagrangians giving rise to Einstein's equations.
A more constructive self-consistent approach yields a background dependent
Lagrangian.Comment: 7 pages, proceedings of IRGAC'06; typo correcte
Scale Factor Duality: A Quantum Cosmological Approach
We consider the minisuperspace model arising from the lowest order string
effective action containing the graviton and the dilaton and study solutions of
the resulting Wheeler-Dewitt equation. The scale factor duality symmetry is
discussed in the context of our quantum cosmological model.Comment: 10 pages, plain tex, uses panda.tex (appended
Searching for Very High Energy Emission from Pulsars Using the High Altitude Water Cherenkov (HAWC) Observatory
There are currently over 160 known gamma-ray pulsars. While most of them are
detected only from space, at least two are now seen also from the ground. MAGIC
and VERITAS have measured the gamma ray pulsed emission of the Crab pulsar up
to hundreds of GeV and more recently MAGIC has reported emission at
TeV. Furthermore, in the Southern Hemisphere, H.E.S.S. has detected the Vela
pulsar above 30 GeV. In addition, non-pulsed TeV emission coincident with
pulsars has been detected by many groups, including the Milagro Collaboration.
These GeV-TeV observations open the possibility of searching for
very-high-energy (VHE, > 100GeV) pulsations from gamma-rays pulsars in the HAWC
field of view.Comment: Presented at the 34th International Cosmic Ray Conference (ICRC2015),
The Hague, The Netherlands. See arXiv:1508.03327 for all HAWC contribution
Charges and fluxes in Maxwell theory on compact manifolds with boundary
We investigate the charges and fluxes that can occur in higher-order Abelian
gauge theories defined on compact space-time manifolds with boundary. The
boundary is necessary to supply a destination to the electric lines of force
emanating from brane sources, thus allowing non-zero net electric charges, but
it also introduces new types of electric and magnetic flux. The resulting
structure of currents, charges, and fluxes is studied and expressed in the
language of relative homology and de Rham cohomology and the corresponding
abelian groups. These can be organised in terms of a pair of exact sequences
related by the Poincar\'e-Lefschetz isomorphism and by a weaker flip symmetry
exchanging the ends of the sequences. It is shown how all this structure is
brought into play by the imposition of the appropriately generalised Maxwell's
equations. The requirement that these equations be integrable restricts the
world-volume of a permitted brane (assumed closed) to be homologous to a cycle
on the boundary of space-time. All electric charges and magnetic fluxes are
quantised and satisfy the Dirac quantisation condition. But through some
boundary cycles there may be unquantised electric fluxes associated with
quantised magnetic fluxes and so dyonic in nature.Comment: 28 pages, plain Te
The LOFT (Large Observatory for X-ray Timing) background simulations
The Large Observatory For X-ray Timing (LOFT) is an innovative medium-class
mission selected for an assessment phase in the framework of the ESA M3 Cosmic
Vision call. LOFT is intended to answer fundamental questions about the
behaviour of matter in the very strong gravitational and magnetic fields around
compact objects. With an effective area of ~10 m^2 LOFT will be able to measure
very fast variability in the X-ray fluxes and spectra. A good knowledge of the
in-orbit background environment is essential to assess the scientific
performance of the mission and to optimize the instrument design. The two main
contributions to the background are cosmic diffuse X-rays and high energy
cosmic rays; also, albedo emission from the Earth is significant. These
contributions to the background for both the Large Area Detector and the Wide
Field Monitor are discussed, on the basis of extensive Geant-4 simulations of a
simplified instrumental mass model.Comment: Proceedings of SPIE, Vol. 8443, Paper No. 8443-209, 201
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