6,985 research outputs found
Chirping compact stars: gravitational radiation and detection degeneracy with binary systems A conceptual pathfinder for space-based gravitational-wave observatories
Compressible, Riemann S-type ellipsoids can emit gravitational waves (GWs)
with a chirp-like behavior (hereafter chirping ellipsoids, CELs). We show that
the GW frequency-amplitude evolution of CELs (mass ~M, radius
~km, polytropic equation of state with index ) is
indistinguishable from that emitted by double white dwarfs (DWDs) and by
extreme mass-ratio inspirals (EMRIs) composed of an intermediate-mass
(e.g.~) black hole and a planet-like (e.g.~)
companion, in a specific frequency interval within the detector sensitivity
band in which the GWs of all these systems are quasi-monochromatic. We estimate
that for reasonable astrophysical assumptions, the rates in the local Universe
of CELs, DWDs and EMRIs in the mass range considered here, are very similar,
posing a detection-degeneracy challenge for space-based GW detectors. The
astrophysical implications of this CEL-binary detection degeneracy by
space-based GW-detection facilities, are outlined.Comment: Submitted to Phys. Rev.
Worst-case bounds on the quality of max-product fixed-points
We study worst-case bounds on the quality of any fixed point assignment of the max-product algorithm for Markov Random Fields (MRF). We start proving a bound
independent of the MRF structure and parameters. Afterwards, we show how this bound can be improved for MRFs with particular structures such as bipartite graphs or grids.
Our results provide interesting insight into the behavior of max-product. For example, we prove that max-product provides very good results (at least 90% of the optimal) on MRFs
with large variable-disjoint cycles (MRFs in which all cycles are variable-disjoint, namely that they do not share any edge and in which each cycle contains at least 20 variables)
The dynamical equation of the effective gluon mass
In this article we derive the integral equation that controls the momentum
dependence of the effective gluon mass in the Landau gauge. This is
accomplished by means of a well-defined separation of the corresponding
"one-loop dressed" Schwinger-Dyson equation into two distinct contributions,
one associated with the mass and one with the standard kinetic part of the
gluon. The entire construction relies on the existence of a longitudinally
coupled vertex of nonperturbative origin, which enforces gauge invariance in
the presence of a dynamical mass. The specific structure of the resulting mass
equation, supplemented by the additional requirement of a positive-definite
gluon mass, imposes a rather stringent constraint on the derivative of the
gluonic dressing function, which is comfortably satisfied by the large-volume
lattice data for the gluon propagator, both for SU(2) and SU(3). The numerical
treatment of the mass equation, under some simplifying assumptions, is
presented for the aforementioned gauge groups, giving rise to a gluon mass that
is a non-monotonic function of the momentum. Various theoretical improvements
and possible future directions are briefly discussed.Comment: 38 pages, 17 figure
Gluon mass through ghost synergy
In this work we compute, at the 'one-loop-dressed' level, the nonperturbative contribution of the ghost loops to the self-energy of the gluon propagator, in the Landau gauge. This is accomplished within the PT-BFM formalism, which guarantees the gauge-invariance of the emerging answer. In particular, the contribution of the ghost-loops is automatically transverse, by virtue of the QED-like Ward identities satisfied in this framework. Using as nonperturbative input the available lattice data for the ghost dressing function, we show that the ghost contributions have a rather sizable effect on the overall shape of the gluon propagator, both for d=3,4. Then, by exploiting a recently introduced dynamical equation for the effective gluon mass, whose solutions depend crucially on the characteristics of the gluon propagator at intermediate energies, we show that if the ghost loops are removed from the gluon propagator then the gluon mass vanishes. These findings strongly suggest that, at least at the level of the Schwinger-Dyson equations, the effects of gluons and ghosts are inextricably connected, and must be combined suitably in order to reproduce the results obtained in the recent lattice simulations
Kinetic study of nordihydroguaiaretic acid recovery from Larrea tridentata by microwave-assisted extraction
Nordihydroguaiaretic acid (NDGA) is a powerful antioxidant that can be found in plants like
Larrea tridentata (Zygophyllaceae), also known as creosote bush, which grows in semidesert
areas of Southwestern United States and Northern Mexico [1]. Several studies have
demonstrated that NDGA has important biological activities with great interest in the health area,
such as antiviral, cancer chemopreventive, and antitumorgenic activities [2]. Extraction of
bioactive compounds from plants is conventionally performed using a heat‐reflux extraction
method. However, different techniques have been developed in order to decrease extraction
time and solvent consumption, as well as to increase the extraction yield and enhance the
extracts quality [3]. The objective of this study was to develop a microwave‐assisted extraction
(MAE) method for NDGA recovery from Larrea tridentata leaves, and compare the obtained
results with those found by using the conventional heat‐reflux extraction (HRE)
Massless bound-state excitations and the Schwinger mechanism in QCD
The gauge invariant generation of an effective gluon mass proceeds through
the well-known Schwinger mechanism, whose key dynamical ingredient is the
nonperturbative formation of longitudinally coupled massless bound-state
excitations. These excitations introduce poles in the vertices of the theory,
in such a way as to maintain the Slavnov-Taylor identities intact in the
presence of massive gluon propagators. In the present work we first focus on
the modifications induced to the nonperturbative three-gluon vertex by the
inclusion of massless two-gluon bound-states into the kernels appearing in its
skeleton-expansion. Certain general relations between the basic building blocks
of these bound-states and the gluon mass are then obtained from the
Slavnov-Taylor identities and the Schwinger-Dyson equation governing the gluon
propagator. The homogeneous Bethe-Salpeter equation determining the
wave-function of the aforementioned bound state is then derived, under certain
simplifying assumptions. It is then shown, through a detailed analytical and
numerical study, that this equation admits non-trivial solutions, indicating
that the QCD dynamics support indeed the formation of such massless bound
states. These solutions are subsequently used, in conjunction with the
aforementioned relations, to determine the momentum-dependence of the dynamical
gluon mass. Finally, further possibilities and open questions are briefly
discussed.Comment: 37 pages, 20 figure
Nonperturbative study of the four gluon vertex
In this paper we study the nonperturbative structure of the SU(3) four-gluon vertex in the Landau gauge, concentrating on contributions quadratic in the metric. We employ an approximation scheme where 'one-loop' diagrams are computed using fully dressed gluon and ghost propagators, and tree-level vertices. When a suitable kinematical configuration depending on a single momentum scale p is chosen, only two structures emerge: the tree-level four-gluon vertex, and a tensor orthogonal to it. A detailed numerical analysis reveals that the form factor associated with this latter tensor displays a change of sign (zero-crossing) in the deep infrared, and finally diverges logarithmically. The origin of this characteristic behavior is proven to be entirely due to the masslessness of the ghost propagators forming the corresponding ghost-loop diagram, in close analogy to a similar effect established for the three-gluon vertex. However, in the case at hand, and under the approximations employed, this particular divergence does not affect the form factor proportional to the tree-level tensor, which remains finite in the entire range of momenta, and deviates moderately from its naive tree-level value. It turns out that the kinematic configuration chosen is ideal for carrying out lattice simulations, because it eliminates from the connected Green's function all one-particle reducible contributions, projecting out the genuine one-particle irreducible vertex. Motivated by this possibility, we discuss in detail how a hypothetical lattice measurement of this quantity would compare to the results presented here, and the potential interference from an additional tensorial structure, allowed by Bose symmetry, but not encountered within our scheme
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