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Search Results: 1 - 10 of 462057 matches for " A. Sarty "
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A Measurement of the Exclusive 3He(e,e'p) Reaction Below the Quasi-Elastic Peak
A. Kozlov,A. J. Sarty
Physics , 2003, DOI: 10.1103/PhysRevLett.93.132301
Abstract: New, high-precision measurements of the 3He(e,e'p) reaction using the A1 collaboration spectrometers at the Mainz microtron MAMI are presented. These were performed in antiparallel kinematics at energy transfers below the quasi-elastic peak, and at a central momentum transfer of 685 MeV/c. Cross sections and distorted momentum distributions were extracted and compared to theoretical predictions and existing data. The longitudinal and transverse behavior of the cross section was also studied. Sizable differences in the cross-section behavior from theoretical predictions based on Plane Wave Impulse Approximation were observed in both the two- and three-body breakup channels. Full Faddeev-type calculations account for some of the observed excess cross section, but significant differences remain.
Coherent eta-photoproduction from nuclei in a relativistic impulse approximation approach
J. Piekarewicz,A. J. Sarty,M. Benmerrouche
Physics , 1997, DOI: 10.1103/PhysRevC.55.2571
Abstract: We study coherent eta-photoproduction from nuclei in a relativistic impulse approximation approach. For the elementary production amplitude we use a standard relativistic parameterization based on a set of four Lorentz- and gauge-invariant amplitudes. The photonuclear amplitude is evaluated without recourse to a nonrelativistic reduction; the full relativistic structure of the amplitude is maintained. On general arguments we show that the coherent process is sensitive to only one of the elementary amplitudes. Moreover, we show that the nuclear structure information is fully contained in the ground-state tensor density. The tensor density is evaluated in a mean-field approximation to the Walecka model and it is shown to be sensitive to relativistic effects. Distortion effects are incorporated through an eta-nucleus optical potential that is computed in a simple t-rho approximation.
Evidence for High-Momentum Enhancement in the Exclusive 3He(e,e'p) Reaction Below the Quasi-Elastic Peak
A. Kozlov,A. J. Sarty,for the A1 Mainz Collaboration
Physics , 2002, DOI: 10.1103/PhysRevLett.93.132301
Abstract: New, high-precision measurements of the 3He(e,e'p) reaction using the A1 collaboration spectrometers at the Mainz microtron MAMI are presented. These were performed in parallel kinematics at energy transfers below the quasi-elastic peak, and at a central momentum transfer of 685 MeV/c. Cross sections and distorted momentum distributions were extracted and compared to theoretical predictions and existing data. The longitudinal and transverse behavior of the cross section was also studied. Sizable enhancements of the cross sections for missing momenta larger than 100 MeV/c as compared to Plane Wave Impulse Approximation were observed in both the two- and three-body breakup channels.
Lessons to be learned from the coherent photoproduction of pseudoscalar mesons
L. J. Abu-Raddad,J. Piekarewicz,A. J. Sarty,R. A. Rego
Physics , 1998, DOI: 10.1103/PhysRevC.60.054606
Abstract: We study the coherent photoproduction of pseudoscalar mesons---particularly of neutral pions---placing special emphasis on the various sources that put into question earlier nonrelativistic-impulse-approximation calculations. These include: final-state interactions, relativistic effects, off-shell ambiguities, and violations to the impulse approximation. We establish that, while distortions play an essential role in the modification of the coherent cross section, the uncertainty in our results due to the various choices of optical-potential models is relatively small (of at most 30%). By far the largest uncertainty emerges from the ambiguity in extending the many on-shell-equivalent representations of the elementary amplitude off the mass shell. Indeed, relativistic impulse-approximation calculations that include the same pionic distortions, the same nuclear-structure model, and two sets of elementary amplitudes that are identical on-shell, lead to variations in the magnitude of the coherent cross section by up to factors of five. Finally, we address qualitatively the assumption of locality implicit in most impulse-approximation treatments, and suggest that the coherent reaction probes---in addition to the nuclear density---the polarization structure of the nucleus.
The Radiation Tail in (e,e'p) Reactions and Corrections to Experimental Data
J. A. Templon,C. E. Vellidis,R. E. J. Florizone,A. J. Sarty
Physics , 1999, DOI: 10.1103/PhysRevC.61.014607
Abstract: We present a direct calculation of the cross section for the reaction 3He(e,e'p) including the radiation tail originating from bremsstrahlung processes. This calculation is compared to measured cross sections. The calculation is carried out from within a Monte Carlo simulation program so that acceptance-averaging effects, along with a subset of possible energy losses, are taken into account. Excellent agreement is obtained between our calculation and measured data, after a correction factor for higher-order bremsstrahlung is devised and applied to the tail. Industry-standard radiative corrections fail miserably for these data, and we use the results of our calculation to dissect the failure. Implications for design and analysis of experiments in the Jefferson-Lab energy domain are discussed.
Nuclear dependence of the coherent eta photoproduction reaction in a relativistic approach
L. J. Abu-Raddad,J. Piekarewicz,A. J. Sarty,M. Benmerrouche
Physics , 1997, DOI: 10.1103/PhysRevC.57.2053
Abstract: We study the nuclear (or A) dependence of the coherent eta photoproduction reaction in a relativistic impulse approximation approach. We use a standard relativistic parameterization of the elementary amplitude, based on a set of four Lorentz- and gauge-invariant amplitudes, to calculate the coherent production cross section from He-4, C-12, and Ca-40. In contrast to nonrelativistic treatments, our approach maintains the full relativistic structure of the process. The nuclear structure affects the process through the ground-state tensor density. This density is sensitive to relativistic effects and depends on A in a different manner than the vector density used in nonrelativistic approaches. This peculiar dependence results in He-4 having a cross section significantly smaller than that of C-12---in contrast to existent nonrelativistic calculations. Distortion effects are incorporated through an eta-nucleus optical potential that is computed in a simple ``t rho'' approximation.
Fast Spinning Pulsars as Probes of Massive Black Holes' Gravity
Dinesh Singh,Kinwah Wu,Gordon E. Sarty
Physics , 2014, DOI: 10.1093/mnras/stu614
Abstract: Dwarf galaxies and globular clusters may contain intermediate mass black holes ($10^{3}$ to $10^{5}$ solar masses) in their cores. Estimates of ~$10^{3}$ neutron stars in the central parsec of the Galaxy and similar numbers in small elliptical galaxies and globular clusters along with an estimated high probability of ms-pulsar formation in those environments has led many workers to propose the use of ms-pulsar timing to measure the mass and spin of intermediate mass black holes. Models of pulsar motion around a rotating black hole generally assume geodesic motion of a "test" particle in the Kerr metric. These approaches account for well-known effects like de Sitter precession and the Lense-Thirring effect but they do not account for the non-linear effect of the pulsar's stress-energy tensor on the space-time metric. Here we model the motion of a pulsar near a black hole with the Mathisson-Papapetrou-Dixon (MPD) equations. Numerical integration of the MPD equations for black holes of mass 2 X $10^{6}$, $10^{5}$ and $10^{3}$ solar masses shows that the pulsar will not remain in an orbital plane with motion vertical to the plane being largest relative to the orbit's radial dimensions for the lower mass black holes. The pulsar's out of plane motion will lead to timing variations that are up to ~10 microseconds different from those predicted by planar orbit models. Such variations might be detectable in long term observations of millisecond pulsars. If pulsar signals are used to measure the mass and spin of intermediate mass black holes on the basis of dynamical models of the received pulsar signal then the out of plane motion of the pulsar should be part of that model.
Competitive assembly of South Pacific invasive ant communities
Philip J Lester, Kirsti L Abbott, Megan Sarty, KC Burns
BMC Ecology , 2009, DOI: 10.1186/1472-6785-9-3
Abstract: First, traditional null model tests showed support for negative species co-occurrence patterns among plots within islands, but not among islands. A plausible explanation for this result is that analyses at larger spatial scales merge heterogeneous habitats that have considerable effects on species occurrences. Second, analyses of ant abundances showed that samples with high ant abundances had fewer species than expected by chance, both within and among islands. One ant species, the invasive yellow crazy ant Anoplolepis gracilipes, appeared to have a particularly strong effect on community structure correlated with its abundance. Third, abundances of most ant species were inversely correlated with the abundances of all other ants at both spatial scales. This result is consistent with competition theory, which predicts species distributions are affected by diffuse competition with suites of co-occurring species.Our results support a pluralistic explanation for ant species abundances and assembly. Both stochastic and deterministic processes interact to determine ant community assembly, though abundance patterns clearly drive the deterministic patterns in this community. These deterministic patterns were observed at two spatial scales. Results indicate that abundance-based null models may be more sensitive in detecting non-random patterns in community assembly than species co-occurrences analyses.Opinions on the processes governing community assembly are polarized [1-3]. One school of thought maintains that ecological communities are assembled deterministically according to 'assembly rules', generated by biotic interactions [4-6]. An opposing view asserts that communities are assembled stochastically by chance dispersal of species with life history characteristics suited to local environmental conditions [7-9]. The relative importance of chance and determinism in structuring ecological communities has been fiercely debated for nearly a century and we are far from a reso
Looking into the heart of a beast: the black hole binary LS 5039
T. Szalai,L. L. Kiss,G. E. Sarty
Physics , 2010, DOI: 10.1088/1742-6596/218/1/012028
Abstract: LS 5039 is a relatively close microquasar consisting of a late O-type star and a compact object (very possibly a black hole) on a highly eccentric orbit with a period of 3.9 days. The high X-ray, gamma-ray and radio luminosity indicate light-matter interaction, which arise from the stellar wind of the primary star accreting toward the black hole. Former examinations suggest that LS 5039 could be a prototype of wind-fed high mass X-ray binaries (WXBs) with diskless main sequence O primaries. Now there is a great chance to better understand the configuration and the physical processes in the exotic system. In July 2009 LS 5039 was followed by the Canadian MOST space telescope to get ultraprecise photometric data in a month-long semi-continuous time series. Parallel to this, we have taken simultaneous high-resolution optical spectra using the 2.3m ANU telescope of the Siding Spring Observatory, supplemented with further data obtained in early August 2009 with the same instrument. Here we present the first results from the new echelle spectra, which represent the best optical spectroscopy ever obtained for this intriguing system. We determined fundamental orbital and physical parameters of LS 5039 and examined the configuration and the circumstellar environment of the system via radial velocity measurements and detailed line-profile analysis of H-Balmer, He I and He II lines.
Proton Polarimeter Calibration between 82 and 217 MeV
J. Glister,G. Ron,B. Lee,A. Beck,E. Brash,A. Camsonne,S. Choi,J. Dumas,R. Feuerbach,R. Gilman,D. W. Higinbotham,X. Jiang,M. K. Jones,S. May-Tal Beck,E. McCullough,M. Paolone,E. Piasetzky,J. Roche,Y. Rousseau,A. J. Sarty,B. Sawatzky,S. Strauch
Physics , 2009, DOI: 10.1016/j.nima.2009.04.051
Abstract: The proton analyzing power in carbon has been measured for energies of 82 to 217 MeV and proton scattering angles of 5 to 41 degrees. The measurements were carried out using polarized protons from the elastic scattering H(pol. e, pol. p) reaction and the Focal Plane Polarimeter (FPP) in Hall A of Jefferson Lab. A new parameterization of the FPP p-C analyzing power was fit to the data, which is in good agreement with previous parameterizations and provides an extension to lower energies and larger angles. The main conclusions are that all polarimeters to date give consistent measurements of the carbon analyzing power, independently of the details of their construction and that measuring on a larger angular range significantly improves the polarimeter figure of merit at low energies.
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