189 research outputs found
The VLQ Calorimeter of H1 at HERA: A Highly Compact Device for Measurements of Electrons and Photons under Very Small Scattering Angles
In 1998, the detector H1 at HERA has been equipped with a small backward
spectrometer, the Very Low Q^2 (VLQ) spectrometer comprising a silicon tracker,
a tungsten - scintillator sandwich calorimeter, and a Time-of-Flight system.
The spectrometer was designed to measure electrons scattered under very low
angles, equivalent to very low squared four - momentum transfers Q^2, and high
energy photons with good energy and spatial resolution. The VLQ was in
operation during the 1999 and 2000 run periods. This paper describes the design
and construction of the VLQ calorimeter, a compact device with a fourfold
projective energy read-out, and its performance during test runs and in the
experiment.Comment: 32 pages, 25 figures, 2 tables (To be submitted to Nucl. Instrum.
Meth. A
Spherically symmetric ADM gravity with variable G and Lambda(c)
This paper investigates the Arnowitt--Deser--Misner (hereafter ADM) form of
spherically symmetric gravity with variable Newton parameter G and cosmological
term Lambda(c). The Newton parameter is here treated as a dynamical variable,
rather than being merely an external parameter as in previous work on closely
related topics. The resulting Hamilton equations are obtained; interestingly, a
static solution exists, that reduces to Schwarzschild geometry in the limit of
constant G, describing a Newton parameter ruled by a nonlinear differential
equation in the radial variable r. A remarkable limiting case is the one for
which the Newton parameter obeys an almost linear growth law at large r. An
exact solution for G as a function of r is also obtained in the case of
vanishing cosmological constant. Some observational implications of these
solutions are obtained and briefly discussed.Comment: 16 pages, 2 figures. The presentation has been improved in all
section
Ghost wave-function renormalization in Asymptotically Safe Quantum Gravity
Motivated by Weinberg's asymptotic safety scenario, we investigate the
gravitational renormalization group flow in the Einstein-Hilbert truncation
supplemented by the wave-function renormalization of the ghost fields. The
latter induces non-trivial corrections to the beta-functions for Newton's
constant and the cosmological constant. The resulting ghost-improved phase
diagram is investigated in detail. In particular, we find a non-trivial
ultraviolet fixed point in agreement with the asymptotic safety conjecture,
which also survives in the presence of extra dimensions. In four dimensions the
ghost anomalous dimension at the fixed point is , supporting
space-time being effectively two-dimensional at short distances.Comment: 23 pages, 4 figure
Background Independence and Asymptotic Safety in Conformally Reduced Gravity
We analyze the conceptual role of background independence in the application
of the effective average action to quantum gravity. Insisting on a background
independent renormalization group (RG) flow the coarse graining operation must
be defined in terms of an unspecified variable metric since no rigid metric of
a fixed background spacetime is available. This leads to an extra field
dependence in the functional RG equation and a significantly different RG flow
in comparison to the standard flow equation with a rigid metric in the mode
cutoff. The background independent RG flow can possess a non-Gaussian fixed
point, for instance, even though the corresponding standard one does not. We
demonstrate the importance of this universal, essentially kinematical effect by
computing the RG flow of Quantum Einstein Gravity in the ``conformally
reduced'' Einstein--Hilbert approximation which discards all degrees of freedom
contained in the metric except the conformal one. Without the extra field
dependence the resulting RG flow is that of a simple -theory. Including
it one obtains a flow with exactly the same qualitative properties as in the
full Einstein--Hilbert truncation. In particular it possesses the non-Gaussian
fixed point which is necessary for asymptotic safety.Comment: 4 figures
Scale-dependent metric and causal structures in Quantum Einstein Gravity
Within the asymptotic safety scenario for gravity various conceptual issues
related to the scale dependence of the metric are analyzed. The running
effective field equations implied by the effective average action of Quantum
Einstein Gravity (QEG) and the resulting families of resolution dependent
metrics are discussed. The status of scale dependent vs. scale independent
diffeomorphisms is clarified, and the difference between isometries implemented
by scale dependent and independent Killing vectors is explained. A concept of
scale dependent causality is proposed and illustrated by various simple
examples. The possibility of assigning an "intrinsic length" to objects in a
QEG spacetime is also discussed.Comment: 52 page
On the renormalization group flow of f(R)-gravity
We use the functional renormalization group equation for quantum gravity to
construct a non-perturbative flow equation for modified gravity theories of the
form . Based on this equation we show that certain
gravitational interactions monomials can be consistently decoupled from the
renormalization group (RG) flow and reproduce recent results on the asymptotic
safety conjecture. The non-perturbative RG flow of non-local extensions of the
Einstein-Hilbert truncation including and interactions is investigated in detail. The inclusion of
such interactions resolves the infrared singularities plaguing the RG
trajectories with positive cosmological constant in previous truncations. In
particular, in some -truncations all physical trajectories emanate from
a Non-Gaussian (UV) fixed point and are well-defined on all RG scales. The RG
flow of the -truncation contains an infrared attractor which drives a
positive cosmological constant to zero dynamically.Comment: 55 pages, 7 figures, typos corrected, references added, version to
appear in Phys. Rev.
One Loop Beta Functions in Topologically Massive Gravity
We calculate the running of the three coupling constants in cosmological,
topologically massive 3d gravity. We find that \nu, the dimensionless
coefficient of the Chern-Simons term, has vanishing beta function. The flow of
the cosmological constant and Newton's constant depends on \nu, and for any
positive \nu there exist both a trivial and a nontrivial fixed point.Comment: 44 pages, 16 figure
Fractal space-times under the microscope: A Renormalization Group view on Monte Carlo data
The emergence of fractal features in the microscopic structure of space-time
is a common theme in many approaches to quantum gravity. In this work we carry
out a detailed renormalization group study of the spectral dimension and
walk dimension associated with the effective space-times of
asymptotically safe Quantum Einstein Gravity (QEG). We discover three scaling
regimes where these generalized dimensions are approximately constant for an
extended range of length scales: a classical regime where , a
semi-classical regime where , and the UV-fixed point
regime where . On the length scales covered by
three-dimensional Monte Carlo simulations, the resulting spectral dimension is
shown to be in very good agreement with the data. This comparison also provides
a natural explanation for the apparent puzzle between the short distance
behavior of the spectral dimension reported from Causal Dynamical
Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic
Safety.Comment: 26 pages, 6 figure
Distribution of satellite galaxies in high redshift groups
We use galaxy groups at redshifts between 0.4 and 1.0 selected from the Great
Observatories Origins Deep Survey (GOODS) to study the color-morphological
properties of satellite galaxies, and investigate possible alignment between
the distribution of the satellites and the orientation of their central galaxy.
We confirm the bimodal color and morphological type distribution for satellite
galaxies at this redshift range: the red and blue classes corresponds to the
early and late morphological types respectively, and the early-type satellites
are on average brighter than the late-type ones. Furthermore, there is a {\it
morphological conformity} between the central and satellite galaxies: the
fraction of early-type satellites in groups with an early-type central is
higher than those with a late-type central galaxy. This effect is stronger at
smaller separations from the central galaxy. We find a marginally significant
signal of alignment between the major axis of the early-type central galaxy and
its satellite system, while for the late-type centrals no significant alignment
signal is found. We discuss the alignment signal in the context of shape
evolution of groups.Comment: 7 pages, 7 figures, accepted by Ap
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