1,459 research outputs found
Quantum energies with worldline numerics
We present new results for Casimir forces between rigid bodies which impose
Dirichlet boundary conditions on a fluctuating scalar field. As a universal
computational tool, we employ worldline numerics which builds on a combination
of the string-inspired worldline approach with Monte-Carlo techniques.
Worldline numerics is not only particularly powerful for inhomogeneous
background configurations such as involved Casimir geometries, it also provides
for an intuitive picture of quantum-fluctuation-induced phenomena. Results for
the Casimir geometries of a sphere above a plate and a new perpendicular-plates
configuration are presented.Comment: 8 pages, 2 figures, Submitted to the Proceedings of the Seventh
Workshop QFEXT'05 (Barcelona, September 5-9, 2005), Refs updated, version to
appear in JPhys
Optical probes of the quantum vacuum: The photon polarization tensor in external fields
The photon polarization tensor is the central building block of an effective
theory description of photon propagation in the quantum vacuum. It accounts for
the vacuum fluctuations of the underlying theory, and in the presence of
external electromagnetic fields, gives rise to such striking phenomena as
vacuum birefringence and dichroism. Standard approximations of the polarization
tensor are often restricted to on-the-light-cone dynamics in homogeneous
electromagnetic fields, and are limited to certain momentum regimes only. We
devise two different strategies to go beyond these limitations: First, we aim
at obtaining novel analytical insights into the photon polarization tensor for
homogeneous fields, while retaining its full momentum dependence. Second, we
employ wordline numerical methods to surpass the constant-field limit.Comment: 13 pages, 4 figures; typo in Eq. (5) corrected (matches journal
version
Supersymmetry breaking as a quantum phase transition
We explore supersymmetry breaking in the light of a rich fixed-point
structure of two-dimensional supersymmetric Wess-Zumino models with one
supercharge using the functional renormalization group (RG). We relate the
dynamical breaking of supersymmetry to an RG relevant control parameter of the
superpotential which is a common relevant direction of all fixed points of the
system. Supersymmetry breaking can thus be understood as a quantum phase
transition analogously to similar transitions in correlated fermion systems.
Supersymmetry gives rise to a new superscaling relation between the critical
exponent associated with the control parameter and the anomalous dimension of
the field -- a scaling relation which is not known in standard spin systems.Comment: 5 pages, 2 figures, discussion of results extended, version to appear
as a Rapid Communication in Phys. Rev.
Photon propagation in a cold axion background with and without magnetic field
A cold relic axion condensate resulting from vacuum misalignment in the early
universe oscillates with a frequency m, where m is the axion mass. We determine
the properties of photons propagating in a simplified version of such a
background where the sinusoidal variation is replaced by a square wave profile.
We prove that previous results that indicated that charged particles moving
fast in such a background radiate, originally derived assuming that all momenta
involved were much larger than m, hold for long wavelengths too. We also
analyze in detail how the introduction of a magnetic field changes the
properties of photon propagation in such a medium. We briefly comment on
possible astrophysical implications of these results.Comment: 17 pages, 4 figures, revised version includes an extended discussion
on physical implication
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
Renormalization group study of the four-body problem
We perform a renormalization group analysis of the non-relativistic
four-boson problem by means of a simple model with pointlike three- and
four-body interactions. We investigate in particular the unitarity point where
the scattering length is infinite and all energies are at the atom threshold.
We find that the four-body problem behaves truly universally, independent of
any four-body parameter. Our findings confirm the recent conjectures of Platter
et al. and von Stecher et al. that the four-body problem is universal, now also
from a renormalization group perspective. We calculate the corresponding
relations between the four- and three-body bound states, as well as the full
bound state spectrum and comment on the influence of effective range
corrections.Comment: 11 pages, 6 figures; v2: revised and published versio
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