713 research outputs found
Geothermal Casimir Phenomena
We present first worldline analytical and numerical results for the
nontrivial interplay between geometry and temperature dependencies of the
Casimir effect. We show that the temperature dependence of the Casimir force
can be significantly larger for open geometries (e.g., perpendicular plates)
than for closed geometries (e.g., parallel plates). For surface separations in
the experimentally relevant range, the thermal correction for the
perpendicular-plates configuration exhibits a stronger parameter dependence and
exceeds that for parallel plates by an order of magnitude at room temperature.
This effect can be attributed to the fact that the fluctuation spectrum for
closed geometries is gapped, inhibiting the thermal excitation of modes at low
temperatures. By contrast, open geometries support a thermal excitation of the
low-lying modes in the gapless spectrum already at low temperatures.Comment: 8 pages, 3 figures, contribution to QFEXT07 proceedings, v2:
discussion switched from Casimir energy to Casimir force, new analytical
results included, matches JPhysA versio
On the orbital and physical parameters of the HDE 226868/Cygnus X-1 binary system
In this paper we explore the consequences of the recent determination of the
mass m=(8.7 +/- 0.8)M_Sun of Cygnus X-1, obtained from the Quasi-Periodic
Oscillation (QPO)-photon index correlation scaling, on the orbital and physical
properties of the binary system HDE 226868/Cygnus X-1. By using such a result
and the latest spectroscopic optical data of the HDE 226868 supergiant star we
get M=(24 +/- 5)M_Sun for its mass. It turns out that deviations from the third
Kepler law significant at more than 1-sigma level would occur if the
inclination i of the system's orbital plane to the plane of the sky falls
outside the range 41-56 deg: such deviations cannot be due to the first
post-Newtonian (1PN) correction to the orbital period because of its smallness;
interpreted in the framework of the Newtonian theory of gravitation as due to
the stellar quadrupole mass moment Q, they are unphysical because Q would take
unreasonably large values. By conservatively assuming that the third Kepler law
is an adequate model for the orbital period we obtain i=(48 +/- 7) deg which
yields for the relative semimajor axis a=(42 +/- 9)R_Sun. Our estimate for the
Roche's lobe of HDE 226868 is r_M = (21 +/- 6)R_Sun.Comment: Latex2e, 7 pages, 1 table, 4 figures. To appear in ApSS (Astrophysics
and Space Science
Gauge Invariant Cutoff QED
A hidden generalized gauge symmetry of a cutoff QED is used to show the
renormalizability of QED. In particular, it is shown that corresponding Ward
identities are valid all along the renormalization group flow. The exact
Renormalization Group flow equation corresponding to the effective action of a
cutoff lambda phi^4 theory is also derived. Generalization to any gauge group
is indicated.Comment: V1: 18 pages, 2 figures; V2: Discussions improved. Version accepted
for publication in Physica Script
Multi-epoch Near-Infrared Interferometry of the Spatially Resolved Disk Around the Be Star Zeta Tau
We present interferometric observations of the Be star Zeta Tau obtained
using the MIRC beam combiner at the CHARA Array. We resolved the disk during
four epochs in 2007-2009. We fit the data with a geometric model to
characterize the circumstellar disk as a skewed elliptical Gaussian and the
central Be star as a uniform disk. The visibilities reveal a nearly edge-on
disk with a FWHM major axis of ~ 1.8 mas in the H-band. The non-zero closure
phases indicate an asymmetry within the disk. Interestingly, when combining our
results with previously published interferometric observations of Zeta Tau, we
find a correlation between the position angle of the disk and the spectroscopic
V/R ratio, suggesting that the tilt of the disk is precessing. This work is
part of a multi-year monitoring campaign to investigate the development and
outward motion of asymmetric structures in the disks of Be stars.Comment: Accepted for publication in the Astronomical Journal. 27 pages, 7
Figure
Coherence properties of the two-dimensional Bose-Einstein condensate
We present a detailed finite-temperature Hartree-Fock-Bogoliubov (HFB)
treatment of the two-dimensional trapped Bose gas. We highlight the numerical
methods required to obtain solutions to the HFB equations within the Popov
approximation, the derivation of which we outline. This method has previously
been applied successfully to the three-dimensional case and we focus on the
unique features of the system which are due to its reduced dimensionality.
These can be found in the spectrum of low-lying excitations and in the
coherence properties. We calculate the Bragg response and the coherence length
within the condensate in analogy with experiments performed in the
quasi-one-dimensional regime [Richard et al., Phys. Rev. Lett. 91, 010405
(2003)] and compare to results calculated for the one-dimensional case. We then
make predictions for the experimental observation of the quasicondensate phase
via Bragg spectroscopy in the quasi-two-dimensional regime.Comment: 9 pages, 9 figure
Towards a renormalizable standard model without fundamental Higgs scalar
We investigate the possibility of constructing a renormalizable standard
model with purely fermionic matter content. The Higgs scalar is replaced by
point-like fermionic self-interactions with couplings growing large at the
Fermi scale. An analysis of the UV behavior in the point-like approximation
reveals a variety of non-Gaussian fixed points for the fermion couplings. If
real, such fixed points would imply nonperturbative renormalizability and evade
triviality of the Higgs sector. For point-like fermionic self-interactions and
weak gauge couplings, one encounters a hierarchy problem similar to the one for
a fundamental Higgs scalar.Comment: 18 pages, 4 figure
Soliton solutions in an effective action for SU(2) Yang-Mills theory: including effects of higher-derivative term
The Skyrme-Faddeev-Niemi (SFN) model which is an O(3) model in three
dimensional space upto fourth-order in the first derivative is regarded as a
low-energy effective theory of SU(2) Yang-Mills theory. One can show from the
Wilsonian renormalization group argument that the effective action of
Yang-Mills theory recovers the SFN in the infrared region. However, the thoery
contains an additional fourth-order term which destabilizes the soliton
solution. In this paper, we derive the second derivative term perturbatively
and show that the SFN model with the second derivative term possesses soliton
solutions.Comment: 7 pages, 3 figure
Effective action for the order parameter of the deconfinement transition of Yang-Mills theories
The effective action for the Polyakov loop serving as an order parameter for
deconfinement is obtained in one-loop approximation to second order in a
derivative expansion. The calculation is performed in dimensions,
mostly referring to the gauge group SU(2). The resulting effective action is
only capable of describing a deconfinement phase transition for
. Since, particularly in , the system is
strongly governed by infrared effects, it is demonstrated that an additional
infrared scale such as an effective gluon mass can change the physical
properties of the system drastically, leading to a model with a deconfinement
phase transition.Comment: 23 pages, 4 figures, minor improvements, version to appear in PR
Asymptotically free scalar curvature-ghost coupling in Quantum Einstein Gravity
We consider the asymptotic-safety scenario for quantum gravity which
constructs a non-perturbatively renormalisable quantum gravity theory with the
help of the functional renormalisation group. We verify the existence of a
non-Gaussian fixed point and include a running curvature-ghost coupling as a
first step towards the flow of the ghost sector of the theory. We find that the
scalar curvature-ghost coupling is asymptotically free and RG relevant in the
ultraviolet. Most importantly, the property of asymptotic safety discovered so
far within the Einstein-Hilbert truncation and beyond remains stable under the
inclusion of the ghost flow.Comment: 8 pages, 3 figures, RevTe
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