23,096 research outputs found
The generalized F-statistic: multiple detectors and multiple GW pulsars
The F-statistic, derived by Jaranowski, Krolak & Schutz (1998), is the
optimal (frequentist) statistic for the detection of nearly periodic
gravitational waves from known neutron stars, in the presence of stationary,
Gaussian detector noise. The F-statistic was originally derived for the case of
a single detector, whose noise spectral density was assumed constant in time,
and for a single known neutron star. Here we show how the F-statistic can be
straightforwardly generalized to the cases of 1) a network of detectors with
time-varying noise curves, and 2) a population of known sources. Fortunately,
all the important ingredients that go into our generalized F-statistics are
already calculated in the single-source/single-detector searches that are
currently implemented, e.g., in the LIGO Software Library, so implementation of
optimal multi-detector, multi-source searches should require negligible
additional cost in computational power or software development.Comment: 6 pages, 0 figures, submitted to PRD; section IV substantially
enlarged and revised, and a few typos correcte
Applications of Partially Quenched Chiral Perturbation Theory
Partially quenched theories are theories in which the valence- and sea-quark
masses are different. In this paper we calculate the nonanalytic one-loop
corrections of some physical quantities: the chiral condensate, weak decay
constants, Goldstone boson masses, B_K and the K+ to pi+ pi0 decay amplitude,
using partially quenched chiral perturbation theory. Our results for weak decay
constants and masses agree with, and generalize, results of previous work by
Sharpe. We compare B_K and the K+ decay amplitude with their real-world values
in some examples. For the latter quantity, two other systematic effects that
plague lattice computations, namely, finite-volume effects and unphysical
values of the quark masses and pion external momenta are also considered. We
find that typical one-loop corrections can be substantial.Comment: 22 pages, TeX, refs. added, minor other changes, version to appear in
Phys. Rev.
The isentropic equation of state of 2-flavor QCD
Using Taylor expansions of the pressure obtained previously in studies of
2-flavor QCD at non-zero chemical potential we calculate expansion coefficients
for the energy and entropy densities up to in the quark
chemical potential. We use these series in to determine lines of
constant entropy per baryon number () that characterize the expansion of
dense matter created in heavy ion collisions. In the high temperature regime
these lines are found to be well approximated by lines of constant .
In the low temperature phase, however, the quark chemical potential is found to
increase with decreasing temperature. This is in accordance with resonance gas
model calculations. Along the lines of constant we calculate the energy
density and pressure. Within the accuracy of our present analysis we find that
the ratio for as well as the softest point of the equation
of state, , show no significant dependence on
.Comment: 7 pages, 10 figure
Finite-volume two-pion energies and scattering in the quenched approximation
We investigate how L\"uscher's relation between the finite-volume energy of
two pions at rest and pion scattering lengths has to be modified in quenched
QCD. We find that this relation changes drastically, and in particular, that
``enhanced finite-volume corrections" of order and occur at
one loop ( is the linear size of the box), due to the special properties of
the in the quenched approximation. We define quenched pion scattering
lengths, and show that they are linearly divergent in the chiral limit. We
estimate the size of these various effects in some numerical examples, and find
that they can be substantial.Comment: 22 pages, uuencoded, compressed postscript fil
On Lattice Computations of K+ --> pi+ pi0 Decay at m_K =2m_pi
We use one-loop chiral perturbation theory to compare potential lattice
computations of the K+ --> pi+ pi0 decay amplitude at m_K=2m_pi with the
experimental value. We find that the combined one-loop effect due to this
unphysical pion to kaon mass ratio and typical finite volume effects is still
of order minus 20-30%, and appears to dominate the effects from quenching.Comment: 4 pages, revte
A Classification of random Dirac fermions
We present a detailed classification of random Dirac hamiltonians in two
spatial dimensions based on the implementation of discrete symmetries. Our
classification is slightly finer than that of random matrices, and contains
thirteen classes. We also extend this classification to non-hermitian
hamiltonians with and without Dirac structure.Comment: 15 pages, version2: typos in the table of classes are correcte
The QCD spectrum with three quark flavors
We present results from a lattice hadron spectrum calculation using three
flavors of dynamical quarks - two light and one strange, and quenched
simulations for comparison. These simulations were done using a one-loop
Symanzik improved gauge action and an improved Kogut-Susskind quark action. The
lattice spacings, and hence also the physical volumes, were tuned to be the
same in all the runs to better expose differences due to flavor number. Lattice
spacings were tuned using the static quark potential, so as a byproduct we
obtain updated results for the effect of sea quarks on the static quark
potential. We find indications that the full QCD meson spectrum is in better
agreement with experiment than the quenched spectrum. For the 0++ (a0) meson we
see a coupling to two pseudoscalar mesons, or a meson decay on the lattice.Comment: 38 pages, 20 figures, uses epsf. 5/29/01 revision responds to
referee's Comments, changes pion fits and tables, and corrects Fig. 10 and
some minor error
Testing improved actions for dynamical Kogut-Susskind quarks
We extend tests of "Naik" and "fat link" improvements of the Kogut-Susskind
quark action to full QCD simulations, and verify that the improvements
previously demonstrated in the quenched approximation apply also to dynamical
quark simulations. We extend the study of flavor symmetry improvement to the
complete set of pions, and find that the nonlocal pions are significantly
heavier than the local non-Goldstone pion. These results can be used to
estimate the lattice spacing necessary for realistic simulations with this
action.Comment: 16 pages, LaTeX, PostScript figures include
Physical Results from Unphysical Simulations
We calculate various properties of pseudoscalar mesons in partially quenched
QCD using chiral perturbation theory through next-to-leading order. Our results
can be used to extrapolate to QCD from partially quenched simulations, as long
as the latter use three light dynamical quarks. In other words, one can use
unphysical simulations to extract physical quantities - in this case the quark
masses, meson decay constants, and the Gasser-Leutwyler parameters L_4-L_8. Our
proposal for determining L_7 makes explicit use of an unphysical (yet
measurable) effect of partially quenched theories, namely the double-pole that
appears in certain two-point correlation functions. Most of our calculations
are done for sea quarks having up to three different masses, except for our
result for L_7, which is derived for degenerate sea quarks.Comment: 26 pages, 12 figures (discussion on discretization errors at end of
sec. IV clarified; minor improvements in presentation; results unchanged
Méthodologie de validation des données hydrométriques en temps réel dans un réseau d'assainissement urbain
L'objectif du présent travail est l'élaboration d'une méthodologie de validation des données hydrométriques mesurées dans un réseau d'assainissement. L'information validée est utilisée aussi bien en temps réel, pour optimiser les consignes de gestion, qu'en temps différé, pour poser le véritable diagnostic et évaluer, sur une base quotidienne, l'efficacité des systèmes d'assainissement.Le principe de base de la méthodologie proposée repose sur la redondance analytique de l'information provenant d'une part de la mesure directe du débit sur le terrain et d'autre part du débit simulé à partir des variables météorologiques. On compare ainsi d'une part, l'écart entre la valeur prévue par un modèle autorégressif (AR) et la valeur mesurée et d'autre part, l'écart entre la valeur prévue par ce même modèle AR et la valeur simulée par un modèle hydrologique. Parmi les valeurs, mesurée et simulée, celle qui se rapproche le plus de la valeur prévue est retenue. Afin de considérer des modèles non stationnaires et d'éviter le biais d'estimation des paramètres de régression par la méthode standard des moindres carrés, le filtre de Kalman est utilisé pour identifier les paramètres du modèle AR.La méthodologie proposée a été testée avec succès sur un bassin urbain de la municipalité de Verdun. L'hydrogramme mesuré a été bruité artificiellement à la fois par un bruit blanc et par un certain nombre de perturbations de grandes amplitudes et de différentes formes. Le processus de validation a permis de retrouver pratiquement les mesures initiales, non bruitées. Les critères de performance introduits sont largement concluants.We developed an automated methodology for real-time validation of hydrometric data in a sewer network. Our methodology uses real-time validated data to optimise system management and non-real-time data to evaluate day-to-day performance.Two approaches can be used to validate and correct hydrometric data; the choice depends on the number of level gauges present in a system. In single gauge systems, univariate filtering is used to smooth data. For example, frequency filtering systematically eliminates values corresponding to frequencies higher than a predetermined threshold frequency. In systems with several gauging stations-duplex, triplex, or multiplex systems-the multivariate filtering method proposed here can be used to validate data series from each gauge. Material redundancy in duplex or higher order systems makes it possible to detect a deficient gauge, using a decision rule to set aside erroneous readings before averaging accepted values. Part of the underlying principle of this methodology is heavier reliance on gauges that give readings consistent with previous and subsequent validated values in a given series. Thus isolated positive or negative variations within a series are eliminated if corresponding variation values at other gauges are more consistent. To evaluate persistence, a reading is compared to a value predicted by an autoregressive (AR) model calibrated by the previous validated reading.This filtering technique constitutes an intelligent alternative to the frequency filtering method mentioned above. In more practical terms, it compares the deviation of an AR model prediction from a measured value with the deviation of the same AR model prediction from a value estimated by a regressive model at other stations in the network. Among the values measured and estimated by the regressive model, the one nearest the AR model prediction is retained.Our methodology also relies on analytical redundancy generated by direct measurement of flow and hydrological simulation. More precisely, the deviation of the AR model prediction from the measured value is compared with the deviation of the same AR model prediction from a value obtained from a hydrological simulation model. Among measured and simulated values, the one nearest the AR model prediction is retained. To allow consideration of nonstationary models and to avoid the well-known bias of the least squares method, the Kalman filter is used to identify the parameters of the AR model.The methodology we propose employs three models. The first generates analytical redundancy using hydrological modelling. An autoregressive model is then used to predict future runoff rate values. Finally, a voting process model is used to compare measured and simulated values.The proposed methodology was tested on the Verdun sewer system in Quebec with successful results. Two types of artificial disturbance of the measured hydrograph were created: white noise was added to measured values and disturbances of large amplitude and various forms were introduced. The methodology produced the initial values and performance criteria were conclusive. Thus on-site testing confirms that this approach allows completely automated detection and correction of most anomalies. Flood peaks were neither underestimated nor overestimated, and total runoff volumes were retained
- …