179 research outputs found
Macroscopic limit of a solvable dynamical model
The interaction between an ultrarelativistic particle and a linear array made
up of two-level systems (^^ ^^ AgBr" molecules) is studied by making use of
a modified version of the Coleman-Hepp Hamiltonian. Energy-exchange processes
between the particle and the molecules are properly taken into account, and the
evolution of the total system is calculated exactly both when the array is
initially in the ground state and in a thermal state. In the macroscopic limit
(), the system remains solvable and leads to interesting
connections with the Jaynes-Cummings model, that describes the interaction of a
particle with a maser. The visibility of the interference pattern produced by
the two branch waves of the particle is computed, and the conditions under
which the spin array in the limit behaves as a ^^ ^^
detector" are investigated. The behavior of the visibility yields good insights
into the issue of quantum measurements: It is found that, in the
thermodynamical limit, a superselection-rule space appears in the description
of the (macroscopic) apparatus. In general, an initial thermal state of the ^^
^^ detector" provokes a more substantial loss of quantum coherence than an
initial ground state. It is argued that a system decoheres more as the
temperature of the detector increases. The problem of ^^ ^^ imperfect
measurements" is also shortly discussed.Comment: 30 pages, report BA-TH/93-13
One-loop matching coefficients for improved staggered bilinears
We calculate one-loop matching factors for bilinear operators composed of
improved staggered fermions. We compare the results for different improvement
schemes used in the recent literature, all of which involve the use of smeared
links. These schemes aim to reduce, though not completely eliminate, O(a^2)
discretization errors. We find that all these improvement schemes substantially
reduce the size of matching factors compared to unimproved staggered fermions.
The resulting corrections are comparable to, or smaller than, those found with
Wilson and domain-wall fermions. In the best case (``Fat-7'' and mean-field
improved HYP links) the corrections are 10 % or smaller at 1/a = 2 GeV.Comment: 13 pages, 1 figure (misleading sentence in sec. II removed; version
to appear in Physical Review D
Nuclear Magnetic Resonance and Magnetization Studies of the Ferromagnetic Ordering Temperature Suppression in Ru Deficient SrRuO3
The synthesis of SrRuO3 under high-pressure oxygen produces a
nonstoichiometric form with randomly distributed vacancies on the Ru-sites,
along with a significantly reduced ferromagnetic ordering temperature. In order
to gain additional insight into the suppression of the ferromagnetism, local
studies utilizing 99,101 Ru zero-field spin-echo NMR, and Ru K-edge XAFS, along
with complimentary magnetization and x-ray diffraction measurements, have been
carried out on samples of SrRuO3 annealed at both ("ambient") atmospheric
pressure and "high-pressure" oxygen (600 atm). Consistent with previous work,
the NMR spectrum for "ambient" SrRuO3 consists of two well-defined peaks at
64.4 MHz and 72.2 MHz corresponding to the 99Ru and 101Ru isotopes,
respectively, and a hyperfine field of 329 kG. Although the magnetization
measurements show a lower ferromagnetic ordering temperature for the
"high-pressure" oxygen sample (90 K compared to 160 K for the "ambient"
sample), the NMR spectrum shows no significant shift in the two peak
frequencies. However, the two peaks exhibit considerable broadening, along with
structure on both the low and high frequency sides which is believed to be
quadrupolar in origin. Analysis of the Ru K-edge XAFS reveals more disorder in
the Ru-O bond for the "high-pressure" oxygen sample compared to the "ambient"
sample. Furthermore, XANES of Ru K-edge analysis indicates no difference in the
valence of Ru between the two samples. The magnetic behavior indicates the
existence of some vacancies on the Ru sites for the "high-pressure" oxygen
sample.Comment: Proceedings of the 3rd Polish-US Workshop on Magnetism and
Superconductivity of Advanced Materials, July 14-19, 2002, Ladek Zdroj
(Poland), to appear in Physica
A purely algebraic construction of a gauge and renormalization group invariant scalar glueball operator
This paper presents a complete algebraic proof of the renormalizability of
the gauge invariant operator to all orders of
perturbation theory in pure Yang-Mills gauge theory, whereby working in the
Landau gauge. This renormalization is far from being trivial as mixing occurs
with other gauge variant operators, which we identify explicitly. We
determine the mixing matrix to all orders in perturbation theory by using
only algebraic arguments and consequently we can uncover a renormalization
group invariant by using the anomalous dimension matrix derived from
. We also present a future plan for calculating the mass of the lightest
scalar glueball with the help of the framework we have set up.Comment: 17 page
Light propagation in statistically homogeneous and isotropic universes with general matter content
We derive the relationship of the redshift and the angular diameter distance
to the average expansion rate for universes which are statistically homogeneous
and isotropic and where the distribution evolves slowly, but which have
otherwise arbitrary geometry and matter content. The relevant average expansion
rate is selected by the observable redshift and the assumed symmetry properties
of the spacetime. We show why light deflection and shear remain small. We write
down the evolution equations for the average expansion rate and discuss the
validity of the dust approximation.Comment: 42 pages, no figures. v2: Corrected one detail about the angular
diameter distance and two typos. No change in result
The NuSTAR Extragalactic Survey: A First Sensitive Look at the High-energy Cosmic X-Ray Background Population
We report on the first 10 identifications of sources serendipitously detected by the Nuclear Spectroscopic Telescope Array (NuSTAR) to provide the first sensitive census of the cosmic X-ray background source population at gsim 10 keV. We find that these NuSTAR-detected sources are â100 times fainter than those previously detected at gsim 10 keV and have a broad range in redshift and luminosity (z = 0.020-2.923 and L 10-40 keV â 4 Ă 1041-5 Ă 1045 erg sâ1); the median redshift and luminosity are z â 0.7 and L 10-40 keV â 3 Ă 1044 erg sâ1, respectively. We characterize these sources on the basis of broad-band â0.5-32 keV spectroscopy, optical spectroscopy, and broad-band ultraviolet-to-mid-infrared spectral energy distribution analyses. We find that the dominant source population is quasars with L 10-40 keV > 1044 erg sâ1, of which â50% are obscured with N H gsim 1022 cmâ2. However, none of the 10 NuSTAR sources are Compton thick (N H gsim 1024 cmâ2) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L 10-40 keV > 1044 erg sâ1) selected at gsim 10 keV of lsim 33% over the redshift range z = 0.5-1.1. We jointly fitted the rest-frame â10-40 keV data for all of the non-beamed sources with L 10-40 keV > 1043 erg sâ1 to constrain the average strength of reflection; we find R < 1.4 for Î = 1.8, broadly consistent with that found for local active galactic nuclei (AGNs) observed at gsim 10 keV. We also constrain the host-galaxy masses and find a median stellar mass of â1011 M â, a factor â5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
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