4,534 research outputs found
The Hubble rate in averaged cosmology
The calculation of the averaged Hubble expansion rate in an averaged
perturbed Friedmann-Lemaitre-Robertson-Walker cosmology leads to small
corrections to the background value of the expansion rate, which could be
important for measuring the Hubble constant from local observations. It also
predicts an intrinsic variance associated with the finite scale of any
measurement of H_0, the Hubble rate today. Both the mean Hubble rate and its
variance depend on both the definition of the Hubble rate and the spatial
surface on which the average is performed. We quantitatively study different
definitions of the averaged Hubble rate encountered in the literature by
consistently calculating the backreaction effect at second order in
perturbation theory, and compare the results. We employ for the first time a
recently developed gauge-invariant definition of an averaged scalar. We also
discuss the variance of the Hubble rate for the different definitions.Comment: 12 pages, 25 figures, references added, clarity improved, frame
switching subtlety fixed, results unchanged, v3 minor typos fixe
Rewriting Logic Semantics of a Plan Execution Language
The Plan Execution Interchange Language (PLEXIL) is a synchronous language
developed by NASA to support autonomous spacecraft operations. In this paper,
we propose a rewriting logic semantics of PLEXIL in Maude, a high-performance
logical engine. The rewriting logic semantics is by itself a formal interpreter
of the language and can be used as a semantic benchmark for the implementation
of PLEXIL executives. The implementation in Maude has the additional benefit of
making available to PLEXIL designers and developers all the formal analysis and
verification tools provided by Maude. The formalization of the PLEXIL semantics
in rewriting logic poses an interesting challenge due to the synchronous nature
of the language and the prioritized rules defining its semantics. To overcome
this difficulty, we propose a general procedure for simulating synchronous set
relations in rewriting logic that is sound and, for deterministic relations,
complete. We also report on two issues at the design level of the original
PLEXIL semantics that were identified with the help of the executable
specification in Maude
Naked shell singularities on the brane
By utilizing non-standard slicings of 5-dimensional Schwarzschild and
Schwarzschild-AdS manifolds based on isotropic coordinates, we generate static
and spherically symmetric braneworld spacetimes containing shell-like naked
null singularities. For planar slicings, we find that the brane-matter sourcing
the solution is a perfect fluid with an exotic equation of state and a pressure
singularity where the brane crosses the bulk horizon. From a relativistic point
of view, such a singularity is required to maintain matter infinitesimally
above the surface of a black hole. From the point of view of the AdS/CFT
conjecture, the singular horizon can be seen as one possible quantum correction
to a classical black hole geometry. Various generalizations of planar slicings
are also considered for a Ricci-flat bulk, and we find that singular horizons
and exotic matter distributions are common features.Comment: REVTeX4, 13 pages, 6 figures, references and comments adde
Energetics of the Einstein-Rosen spacetime
A study covering some aspects of the Einstein--Rosen metric is presented. The
electric and magnetic parts of the Weyl tensor are calculated. It is shown that
there are no purely magnetic E--R spacetimes, and also that a purely electric
E--R spacetime is necessarily static. The geodesics equations are found and
circular ones are analyzed in detail. The super--Poynting and the
``Lagrangian'' Poynting vectors are calculated and their expressions are found
for two specific examples. It is shown that for a pulse--type solution, both
expressions describe an inward radially directed flow of energy, far behind the
wave front. The physical significance of such an effect is discussed.Comment: 19 pages Latex.References added and updated.To appear in
Int.J.Theor.Phy
Measurement of the cross-section ratio sigma_{psi(2S)}/sigma_{J/psi(1S)} in deep inelastic exclusive ep scattering at HERA
The exclusive deep inelastic electroproduction of and
at an centre-of-mass energy of 317 GeV has been studied with the ZEUS
detector at HERA in the kinematic range GeV,
GeV and GeV, where is the photon virtuality, is the
photon-proton centre-of-mass energy and is the squared four-momentum
transfer at the proton vertex. The data for GeV were taken in
the HERA I running period and correspond to an integrated luminosity of 114
pb. The data for GeV are from both HERA I and HERA II
periods and correspond to an integrated luminosity of 468 pb. The decay
modes analysed were and for the
and for the . The cross-section ratio
has been measured as a function of
and . The results are compared to predictions of QCD-inspired
models of exclusive vector-meson production.Comment: 24 pages, 8 figure
PHIL photoinjector test line
LAL is now equiped with its own platform for photoinjectors tests and
Research and Developement, named PHIL (PHotoInjectors at LAL). This facility
has two main purposes: push the limits of the photoinjectors performances
working on both the design and the associated technology and provide a low
energy (MeV) short pulses (ps) electron beam for the interested users. Another
very important goal of this machine will be to provide an opportunity to form
accelerator physics students, working in a high technology environment. To
achieve this goal a test line was realised equipped with an RF source, magnets
and beam diagnostics. In this article we will desrcibe the PHIL beamline and
its characteristics together with the description of the first two
photoinjector realised in LAL and tested: the ALPHAX and the PHIN RF Guns
Combined QCD and electroweak analysis of HERA data
A simultaneous fit of parton distribution functions (PDFs) and electroweak
parameters to HERA data on deep inelastic scattering is presented. The input
data are the neutral current and charged current inclusive cross sections which
were previously used in the QCD analysis leading to the HERAPDF2.0 PDFs. In
addition, the polarisation of the electron beam was taken into account for the
ZEUS data recorded between 2004 and 2007. Results on the vector and
axial-vector couplings of the Z boson to u- and d-type quarks, on the value of
the electroweak mixing angle and the mass of the W boson are presented. The
values obtained for the electroweak parameters are in agreement with Standard
Model predictions.Comment: 32 pages, 10 figures, accepted by Phys. Rev. D. Small corrections
from proofing process and small change to Fig. 12 and Table
Limits on the effective quark radius from inclusive scattering at HERA
The high-precision HERA data allows searches up to TeV scales for Beyond the
Standard Model contributions to electron-quark scattering. Combined
measurements of the inclusive deep inelastic cross sections in neutral and
charged current scattering corresponding to a luminosity of around 1
fb have been used in this analysis. A new approach to the beyond the
Standard Model analysis of the inclusive data is presented; simultaneous
fits of parton distribution functions together with contributions of "new
physics" processes were performed. Results are presented considering a finite
radius of quarks within the quark form-factor model. The resulting 95% C.L.
upper limit on the effective quark radius is cm.Comment: 10 pages, 4 figures, accepted by Phys. Lett.
Search for a narrow baryonic state decaying to and in deep inelastic scattering at HERA
A search for a narrow baryonic state in the and
system has been performed in collisions at HERA with the ZEUS detector
using an integrated luminosity of 358 pb taken in 2003-2007. The search
was performed with deep inelastic scattering events at an centre-of-mass
energy of 318 GeV for exchanged photon virtuality, , between 20 and 100
. Contrary to evidence presented for such a state around 1.52
GeV in a previous ZEUS analysis using a sample of 121 pb taken in
1996-2000, no resonance peak was found in the invariant-mass
distribution in the range 1.45-1.7 GeV. Upper limits on the production cross
section are set.Comment: 16 pages, 4 figures, accepted by Phys. Lett. B. Minor changes from
journal reviewing process, including a small correction to figure
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