1,854 research outputs found
Is the energy density of the ground state of the sine-Gordon model unbounded from below for beta^2 > 8 pi ?
We discuss Coleman's theorem concerning the energy density of the ground
state of the sine-Gordon model proved in Phys. Rev. D 11, 2088 (1975).
According to this theorem the energy density of the ground state of the
sine-Gordon model should be unbounded from below for coupling constants beta^2
> 8 pi. The consequence of this theorem would be the non-existence of the
quantum ground state of the sine-Gordon model for beta^2 > 8 pi. We show that
the energy density of the ground state in the sine-Gordon model is bounded from
below even for beta^2 > 8 pi. This result is discussed in relation to Coleman's
theorem (Comm. Math. Phys. 31, 259 (1973)), particle mass spectra and
soliton-soliton scattering in the sine-Gordon model.Comment: 22 pages, Latex, no figures, revised according to the version
accepted for publication in Journal of Physics
On the estimate of the sigma^(I = 1)_(KN)(0)-term value from the energy level shift of kaonic hydrogen in the ground state
Using the experimental data on the energy level shift of kaonic hydrogen in
the ground state (the DEAR Collaboration, Phys. Rev. Lett. 94, 212302 (2005))
and the theoretical value of the energy level shift, calculated within the
phenomenological quantum field theoretic approach to the description of strong
low-energy anti-K N and anti-K NN interactions developed at Stefan Meyer
Institut fuer subatomare Physik in Vienna, we estimate the value of the
sigma^(I = 1)_(KN)(0)-term of low-energy anti-K N scattering. We get sigma^(I =
1)_(KN)(0) = (433 +/- 85) MeV. This testifies the absence of strange quarks in
the proton structure.Comment: 7 pages, no figure
On renormalizability of the massless Thirring model
We discuss the renormalizability of the massless Thirring model in terms of
the causal fermion Green functions and correlation functions of left-right
fermion densities. We obtain the most general expressions for the causal
two-point Green function and correlation function of left-right fermion
densities with dynamical dimensions of fermion fields, parameterised by two
parameters. The region of variation of these parameters is constrained by the
positive definiteness of the norms of the wave functions of the states related
to components of the fermion vector current. We show that the dynamical
dimensions of fermion fields calculated for causal Green functions and
correlation functions of left-right fermion densities can be made equal. This
implies the renormalizability of the massless Thirring model in the sense that
the ultra-violet cut-off dependence, appearing in the causal fermion Green
functions and correlation functions of left-right fermion densities, can be
removed by renormalization of the wave function of the massless Thirring
fermion fields only.Comment: 17 pages, Latex, the contribution of fermions with opposite chirality
is added,the parameterisation of fermion determinant by two parameters is
confirmed,it is shown that dynamical dimensions of fermion fields calculated
from different correlation functions can be made equal.This allows to remove
the dependence on the ultra-violet cut-off by the renormalization of the wave
function of Thirring fermion fields onl
Model-Dependence of Shapiro Time Delay and the "Speed of Gravity/Speed of Light" Controversy
Fomalont and Kopeikin have recently succeeded in measuring the
velocity-dependent component of the Shapiro time delay of light from a quasar
passing behind Jupiter. While there is general agreement that this observation
tests the gravitomagnetic properties of the gravitational field, a controversy
has emerged over the question of whether the results depend on the speed of
light, , or the speed of gravity, . By analyzing the Shapiro time delay
in a set of ``preferred frame'' models, I demonstrate that this question is
ill-posed: the distinction can only be made in the context of a class of
theories in which , and the answer then depends on the specific class
of theories one chooses. It remains true, however, that for a large class of
theories ``close enough'' to general relativity, the leading contribution to
the time delay depends on and not ; within this class, observations
are thus not yet accurate enough to measure the speed of gravity.Comment: 12 pages, LaTeX; v2: added discussion of present observational
limits, and of relative importance of various contributions to time delay;
new references; v3: minor clarifications in response to refere
Macroscopic models for superconductivity
This paper reviews the derivation of some macroscopic models for superconductivity and also some of the mathematical challenges posed by these models. The paper begins by exploring certain analogies between phase changes in superconductors and those in solidification and melting. However, it is soon found that there are severe limitations on the range of validity of these analogies and outside this range many interesting open questions can be posed about the solutions to the macroscopic models
The DLV System for Knowledge Representation and Reasoning
This paper presents the DLV system, which is widely considered the
state-of-the-art implementation of disjunctive logic programming, and addresses
several aspects. As for problem solving, we provide a formal definition of its
kernel language, function-free disjunctive logic programs (also known as
disjunctive datalog), extended by weak constraints, which are a powerful tool
to express optimization problems. We then illustrate the usage of DLV as a tool
for knowledge representation and reasoning, describing a new declarative
programming methodology which allows one to encode complex problems (up to
-complete problems) in a declarative fashion. On the foundational
side, we provide a detailed analysis of the computational complexity of the
language of DLV, and by deriving new complexity results we chart a complete
picture of the complexity of this language and important fragments thereof.
Furthermore, we illustrate the general architecture of the DLV system which
has been influenced by these results. As for applications, we overview
application front-ends which have been developed on top of DLV to solve
specific knowledge representation tasks, and we briefly describe the main
international projects investigating the potential of the system for industrial
exploitation. Finally, we report about thorough experimentation and
benchmarking, which has been carried out to assess the efficiency of the
system. The experimental results confirm the solidity of DLV and highlight its
potential for emerging application areas like knowledge management and
information integration.Comment: 56 pages, 9 figures, 6 table
Morphologies of z~0.7 AGN host galaxies in CANDELS : no trend of merger incidence with AGN luminosity
PS would like to acknowledge funding through grant ASI I/005/11/0. DKoo would like to acknowledge funding through grant NSF AST-0808133. SJ acknowledges financial support from the EC through an ERC grant StG-257720.The processes that trigger active galactic nuclei (AGN) remain poorly understood. While lower luminosity AGN may be triggered by minor disturbances to the host galaxy, stronger disturbances are likely required to trigger luminous AGN. Major wet mergers of galaxies are ideal environments for AGN triggering since they provide large gas supplies and galaxy scale torques. There is however little observational evidence for a strong connection between AGN and major mergers. We analyse the morphological properties of AGN host galaxies as a function of AGN and host galaxy luminosity and compare them to a carefully matched sample of control galaxies. AGN are X-ray selected in the redshift range 0.5 < z < 0.8 and have luminosities 41 ≲ log (LX [erg s−1]) ≲ 44.5. ‘Fake AGN’ are simulated in the control galaxies by adding point sources with the magnitude of the matched AGN. We find that AGN host and control galaxies have comparable asymmetries, Sérsic indices and ellipticities at rest frame ∼950 nm. AGN host galaxies show neither higher average asymmetries nor higher fractions of very disturbed objects. There is no increase in the prevalence of merger signatures with AGN luminosity. At 95 per cent confidence we find that major mergers are responsible for <6 per cent of all AGN in our sample as well as <40 per cent of the highest luminosity AGN (log  (LX [erg s−1]) ∼ 43.5). Major mergers therefore either play only a very minor role in the triggering of AGN in the luminosity range studied or time delays are too long for merger features to remain visible.PostprintPeer reviewe
Gravitational signals emitted by a point mass orbiting a neutron star: a perturbative approach
We compute the energy spectra of the gravitational signals emitted when a
pointlike mass moves on a closed orbit around a non rotating neutron star,
inducing a perturbation of its gravitational field and its internal structure.
The Einstein equations and the hydrodynamical equations are perturbed and
numerically integrated in the frequency domain. The results are compared with
the energy spectra computed by the quadrupole formalism which assumes that both
masses are pointlike, and accounts only for the radiation emitted because the
orbital motion produces a time dependent quadrupole moment. The results of our
perturbative approach show that, in general, the quadrupole formalism
overestimates the amount of emitted radiation, especially when the two masses
are close. However, if the pointlike mass is allowed to move on an orbit so
tight that the keplerian orbital frequency resonates with the frequency of the
fundamental quasi-normal mode of the star (2w_K=w_f), this mode can be excited
and the emitted radiation can be considerably larger than that computed by the
quadrupole approach.Comment: 36 pages, 7 figures, submimtted to Phys. Rev.
T-PHOT: A new code for PSF-matched, prior-based, multiwavelength extragalactic deconfusion photometry
We present T-PHOT, a publicly available software aimed at extracting accurate
photometry from low-resolution images of deep extragalactic fields, where the
blending of sources can be a serious problem for the accurate and unbiased
measurement of fluxes and colours. T-PHOT has been developed within the
ASTRODEEP project and it can be considered as the next generation to TFIT,
providing significant improvements above it and other similar codes. T-PHOT
gathers data from a high-resolution image of a region of the sky, and uses it
to obtain priors for the photometric analysis of a lower resolution image of
the same field. It can handle different types of datasets as input priors: i) a
list of objects that will be used to obtain cutouts from the real
high-resolution image; ii) a set of analytical models; iii) a list of
unresolved, point-like sources, useful e.g. for far-infrared wavelength
domains. We show that T-PHOT yields accurate estimations of fluxes within the
intrinsic uncertainties of the method, when systematic errors are taken into
account (which can be done thanks to a flagging code given in the output).
T-PHOT is many times faster than similar codes like TFIT and CONVPHOT (up to
hundreds, depending on the problem and the method adopted), whilst at the same
time being more robust and more versatile. This makes it an optimal choice for
the analysis of large datasets. In addition we show how the use of different
settings and methods significantly enhances the performance. Given its
versatility and robustness, T-PHOT can be considered the preferred choice for
combined photometric analysis of current and forthcoming extragalactic optical
to far-infrared imaging surveys. [abridged]Comment: 23 pages, 20 figures, 2 table
Galaxy Zoo: Are Bars Responsible for the Feeding of Active Galactic Nuclei at 0.2 < z < 1.0?
We present a new study investigating whether active galactic nuclei (AGN)
beyond the local universe are preferentially fed via large-scale bars. Our
investigation combines data from Chandra and Galaxy Zoo: Hubble (GZH) in the
AEGIS, COSMOS, and GOODS-S surveys to create samples of face-on, disc galaxies
at 0.2 < z < 1.0. We use a novel method to robustly compare a sample of 120 AGN
host galaxies, defined to have 10^42 erg/s < L_X < 10^44 erg/s, with inactive
control galaxies matched in stellar mass, rest-frame colour, size, Sersic
index, and redshift. Using the GZH bar classifications of each sample, we
demonstrate that AGN hosts show no statistically significant enhancement in bar
fraction or average bar likelihood compared to closely-matched inactive
galaxies. In detail, we find that the AGN bar fraction cannot be enhanced above
the control bar fraction by more than a factor of two, at 99.7% confidence. We
similarly find no significant difference in the AGN fraction among barred and
non-barred galaxies. Thus we find no compelling evidence that large-scale bars
directly fuel AGN at 0.2<z<1.0. This result, coupled with previous results at
z=0, implies that moderate-luminosity AGN have not been preferentially fed by
large-scale bars since z=1. Furthermore, given the low bar fractions at z>1,
our findings suggest that large-scale bars have likely never directly been a
dominant fueling mechanism for supermassive black hole growth.Comment: 13 pages, 5 figures, 2 tables, accepted by MNRA
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