48,276 research outputs found
Influence of the Dirac sea on proton electromagnetic knockout
We use the relativistic distorted-wave impulse approximation (RDWIA) to study
the effects of negative-energy components of Dirac wave functions on the
left-right asymmetry for (e,e'p) reactions on 16-O with 0.2 < Q^2 < 0.8 and
12-C with 0.6 < Q^2 < 1.8 (GeV/c)^2. Spinor distortion is more important for
the bound state than for the ejectile and the net effect decreases with Q^2.
Spinor distortion breaks Godon equivalence and the data favor the CC2 operator
with intermediate coupling to the sea. The left-right asymmetry for Q^2 < 1.2
(GeV/c)^2 is described well by RDWIA calcuations, but at Q^2 = 1.8 (GeV/c)^2
the observed variation with missing momentum is flatter than predicted.Comment: 12 pages, 9 figures, to be submitted to PR
Decoding mode-mixing in black-hole merger ringdown
Optimal extraction of information from gravitational-wave observations of
binary black-hole coalescences requires detailed knowledge of the waveforms.
Current approaches for representing waveform information are based on
spin-weighted spherical harmonic decomposition. Higher-order harmonic modes
carrying a few percent of the total power output near merger can supply
information critical to determining intrinsic and extrinsic parameters of the
binary. One obstacle to constructing a full multi-mode template of merger
waveforms is the apparently complicated behavior of some of these modes;
instead of settling down to a simple quasinormal frequency with decaying
amplitude, some modes show periodic bumps characteristic of
mode-mixing. We analyze the strongest of these modes -- the anomalous
harmonic mode -- measured in a set of binary black-hole merger waveform
simulations, and show that to leading order, they are due to a mismatch between
the spherical harmonic basis used for extraction in 3D numerical relativity
simulations, and the spheroidal harmonics adapted to the perturbation theory of
Kerr black holes. Other causes of mode-mixing arising from gauge ambiguities
and physical properties of the quasinormal ringdown modes are also considered
and found to be small for the waveforms studied here.Comment: 15 pages, 10 figures, 2 tables; new version has improved Figs. 1-3,
consistent labelling of simulations between Tables I & II,
additional/corrected references, and extra hyphen
The Final Merger of Black-Hole Binaries
Recent breakthroughs in the field of numerical relativity have led to
dramatic progress in understanding the predictions of General Relativity for
the dynamical interactions of two black holes in the regime of very strong
gravitational fields. Such black-hole binaries are important astrophysical
systems and are a key target of current and developing gravitational-wave
detectors. The waveform signature of strong gravitational radiation emitted as
the black holes fall together and merge provides a clear observable record of
the process. After decades of slow progress, these mergers and the
gravitational-wave signals they generate can now be routinely calculated using
the methods of numerical relativity. We review recent advances in understanding
the predicted physics of events and the consequent radiation, and discuss some
of the impacts this new knowledge is having in various areas of astrophysics.Comment: 57 pages; 9 figures. Updated references & fixed typos. Published
version is at
http://www.annualreviews.org/doi/abs/10.1146/annurev.nucl.010909.08324
Black-hole binaries, gravitational waves, and numerical relativity
Understanding the predictions of general relativity for the dynamical
interactions of two black holes has been a long-standing unsolved problem in
theoretical physics. Black-hole mergers are monumental astrophysical events,
releasing tremendous amounts of energy in the form of gravitational radiation,
and are key sources for both ground- and space-based gravitational-wave
detectors. The black-hole merger dynamics and the resulting gravitational
waveforms can only be calculated through numerical simulations of Einstein's
equations of general relativity. For many years, numerical relativists
attempting to model these mergers encountered a host of problems, causing their
codes to crash after just a fraction of a binary orbit could be simulated.
Recently, however, a series of dramatic advances in numerical relativity has
allowed stable, robust black-hole merger simulations. This remarkable progress
in the rapidly maturing field of numerical relativity, and the new
understanding of black-hole binary dynamics that is emerging is chronicled.
Important applications of these fundamental physics results to astrophysics, to
gravitational-wave astronomy, and in other areas are also discussed.Comment: 54 pages, 42 figures. Some typos corrected & references updated.
Essentially final published versio
Prediction of thickness limits of ideal polar ultrathin films
Competition between electronic and atomic reconstruction is a constantly
recurring theme in transition-metal oxides. We use density functional theory
calculations to study this competition for a model system consisting of a thin
film of the polar, infinite-layer structure ACuO2 (A=Ca, Sr, Ba) grown on a
nonpolar, perovskite SrTiO3 substrate. A transition from the bulk planar
structure to a chain-type thin film accompanied by substantial changes to the
electronic structure is predicted for a SrCuO2 film fewer than five unit cells
thick. An analytical model explains why atomic reconstruction becomes more
favorable than electronic reconstruction as the film becomes thinner, and
suggests that similar considerations should be valid for other polar films
Workshop on evaluating personal search
The first ECIR workshop on Evaluating Personal Search was
held on 18th April 2011 in Dublin, Ireland. The workshop
consisted of 6 oral paper presentations and several discussion sessions. This report presents an overview of the scope and contents of the workshop and outlines the major outcomes
Band gaps in pseudopotential self-consistent GW calculations
For materials which are incorrectly predicted by density functional theory to
be metallic, an iterative procedure must be adopted in order to perform GW
calculations. In this paper we test two iterative schemes based on the
quasi-particle and pseudopotential approximations for a number of inorganic
semiconductors whose electronic structures are well known from experiment.
Iterating just the quasi-particle energies yields a systematic, but modest
overestimate of the band gaps, confirming conclusions drawn earlier for CaB_6
and YH_3. Iterating the quasi-particle wave functions as well gives rise to an
imbalance between the Hartree and Fock potentials and results in bandgaps in
far poorer agreement with experiment.Comment: 5 pages, 2 figures, 2 table
Performance assessment of tariff-based air source heat pump load shifting in a UK detached dwelling featuring phase change-enhanced buffering
Using a detailed building simulation model, the amount of thermal buffering, with and without phase change material (PCM), needed to time-shift an air source heat pump's operation to off-peak periods, as defined by the UK 'Economy 10' tariff, was investigated for a typical UK detached dwelling. The performance of the buffered system was compared to the case with no load shifting and with no thermal buffering. Additionally, the load shifting of a population of buffered heat pumps to off-peak periods was simulated and the resulting change in the peak demand on the electricity network was assessed. The results from this study indicate that 1000 L of hot water buffering or 500 L of PCM-enhanced hot water buffering was required to move the operation of the heat pump fully to off-peak periods, without adversely affecting the provision of space heating and hot water for the end user. The work also highlights that buffering and load shifting increased the heat pump's electrical demand by over 60% leading to increased cost to the end user and increased CO2 emissions (depending on the electricity tariff applied and time varying CO2 intensity of the electricity generation mix, respectively). The study also highlights that the load-shifting of populations of buffered heat pumps wholly to off-peak periods using crude instruments such as tariffs increased the peak loading on the electrical network by over 50% rather than reducing it and that careful consideration is needed as to how the load shifting of a group of heat pumps is orchestrated
STS-8 bet results
The final Best Estimate Trajectory (BET) products, i.e., the reconstructed trajectory, the Extended BET, AEROBET and MMLE input files, generated for the eighth NASA Space Shuttle flight are documented. The reconstructed trajectory (inertial BET) for this Challenger flight, the first night landing is discussed. State (position, velocity, and attitude) plus three accelerometer scale factors were determined from fitting the Guam S-band data, seven C-band passes, and pseudo Doppler and altimeter during rollout on Runway 22. The anchor epoch utilized for the batch weighted-least-squares determination was Sept. 5, 1983 7h1m50s.0 (25310 GMT seconds). The spacecraft altitude at epoch is approx. 617 kft. IMU2 data were selected for the reconstruction
Summary of shuttle data processing and aerodynamic performance comparisons for the first 11 flights
NASA Space Shuttle aerodynamic and aerothermodynamic research is but one part of the most comprehensive end-to-end flight test program ever undertaken considering: the extensive pre-flight experimental data base development; the multitude of spacecraft and remote measurements taken during entry flight; the complexity of the Orbiter aerodynamic configuration; the variety of flight conditions available across the entire speed regime; and the efforts devoted to flight data reduction throughout the aerospace community. Shuttle entry flights provide a wealth of research quality data, in essence a veritable flying wind tunnel, for use by researchers to verify and improve the operational capability of the Orbiter and provide data for evaluations of experimental facilities as well as computational methods. This final report merely summarizes the major activities conducted by the AMA, Inc. under NASA Contract NAS1-16087 as part of that interesting research. Investigators desiring more detailed information can refer to the glossary of AMA publications attached herein as Appendix A. Section I provides background discussion of software and methodology development to enable Best Estimate Trajectory (BET) generation. Actual products generated are summarized in Section II as tables which completely describe the post-flight products available from the first three-year Shuttle flight history. Summary results are presented in Section III, with longitudinal performance comparisons included as Appendices for each of the flights
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