Publications (20)14.86 Total impact
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Article: The Determination of Pseudoscalar Meson Photoproduction Amplitudes from Complete Experiments
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ABSTRACT: A new generation of complete experiments is currently underway with the goal of performing a high precision extraction of pseudoscalar meson photo‐production amplitudes. Here we review the most general analytic form of the cross section, dependent upon the three polarization vectors of the beam, target and recoil baryon, including all single, double and triple‐polarization terms involving 16 spin‐dependent observables. Analytic expressions that determine the recoil baryon polarization are also presented. Different conventions are in use in the literature and we have used a numerical calculation of cross sections from Chew‐Goldberger‐Low‐Nambu amplitudes with arbitrary spin projections to clarify apparent sign differences. As an illustration of the use of this machinery, we carry out a multipole analysis of the γp → K+Λ reaction and examine the impact of recently published polarization measurements. In fitting multipoles, we use a combined Monte Carlo sampling of the amplitude space, with gradient minimization, and find a shallow χ2 valley pitted with a very large number of local minima, despite the inclusion of recent data on 8 different observables. We conclude that, while a mathematical solution to the problem of determining an amplitude free of ambiguities may require 8 observables, as has been pointed out in the literature, experiments with realistically achievable uncertainties will require a significantly larger number.AIP Conference Proceedings. 10/2011; 1388(1):99-105. -
Article: Determining pseudoscalar meson photoproduction amplitudes from complete experiments
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ABSTRACT: A new generation of complete experiments is focused on a high precision extraction of pseudoscalar meson photoproduction amplitudes. Here, we review the development of the most general analytic form of the cross section, dependent upon the three polarization vectors of the beam, target and recoil baryon, including all single-, double- and triple-polarization terms involving 16 spin-dependent observables. We examine the different conventions that have been used by different authors, and we present expressions that allow the direct numerical calculation of any pseudoscalar meson photoproduction observables with arbitrary spin projections from the Chew–Goldberger–Low–Nambu amplitudes. We use this numerical tool to clarify apparent sign differences that exist in the literature, in particular with the definitions of six double-polarization observables. We also present analytic expressions that determine the recoil baryon polarization, together with examples of their potential use with quasi-4π detectors to deduce observables. As an illustration of the use of the consistent machinery presented in this review, we carry out a multipole analysis of the γp → K+Λ reaction and examine the impact of recently published polarization measurements. When combining data from different experiments, we utilize the Fierz identities to fit a consistent set of scales. In fitting multipoles, we use a combined Monte Carlo sampling of the amplitude space, with gradient minimization, and find a shallow χ2 valley pitted with a very large number of local minima. This results in broad bands of multipole solutions that are experimentally indistinguishable. While these bands have been noticeably narrowed by the inclusion of new polarization measurements, many of the multipoles remain very poorly determined, even in sign, despite the inclusion of data on eight different observables. We have compared multipoles from recent PWA codes with our model-independent solution bands and found that such comparisons provide useful consistency tests which clarify model interpretations. The potential accuracy of amplitudes that could be extracted from measurements of all 16 polarization observables has been studied with mock data using the statistical variations that are expected from ongoing experiments. We conclude that, while a mathematical solution to the problem of determining an amplitude free of ambiguities may require eight observables, as has been pointed out in the literature, experiments with realistically achievable uncertainties will require a significantly larger number.Journal of Physics G Nuclear and Particle Physics 04/2011; 38(5):053001. · 4.18 Impact Factor -
Article: Determining pseudoscalar meson photo-production amplitudes from complete experiments
[show abstract] [hide abstract]
ABSTRACT: We review the development of the most general analytic form of the cross section, dependent upon the three polarization vectors of the beam, target and recoil baryon, including all single, double and triple-polarization terms involving 16 spin-dependent observables. We examine the different conventions that have been used by different authors, and we present expressions that allow the direct numerical calculation of any pseudoscalar meson photo-production observables with arbitrary spin projections from the Chew-Goldberger-Low-Nambu amplitudes. We use this numerical tool to clarify apparent sign differences that exist in the literature. We also present analytic expressions that determine the recoil baryon polarization, together with examples of their potential use with quasi-4pi detectors to deduce observables. As an illustration of the use of the consistent machinery presented in this review, we carry out a multipole analysis of the gamma p -> K^+ Lambda reaction and examine the impact of recently published polarization measurements. In fitting multipoles, we use a combined Monte Carlo sampling of the amplitude space, with gradient minimization, and find a shallow chi^2 valley pitted with a very large number of local minima. This results in broad bands of multipole solutions that are experimentally indistinguishable. While these bands have been noticeably narrowed by the inclusion of new polarization measurements, many of the multipoles remain very poorly determined, even in sign, despite the inclusion of data on 8 different observables. We have compared multipoles from recent PWA codes with our model-independent solution bands. The potential accuracy of amplitudes that could be extracted from measurements of all 16 polarization observables has been studied with mock data using the statistical variations that are expected from ongoing experiments.10/2010; -
Article: Calculations of Polarization Observables in Pseudoscalar Meson Photo-production Reactions
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ABSTRACT: In preparation for the extraction of pseudoscalar meson photo-production amplitudes from a new generation of complete experiments, we assemble the relations between experimental observables and the Chew-Goldberger-Low-Nambu amplitudes. We present expressions that allow the direct calculation of matrix elements with arbitrary spin projections and uses these to clarify sign differences that exist in the literature. Comparing to the MAID and SAID analysis codes, we have found that the implied definitions of six double-polarization observables are the negative of what has been used in comparing to recent experimental data. Comment: 21 pages, 3 figures. Eq. (6) is corrected12/2009; -
Article: Measurements of H-->D-->(gamma-->,pi) and implications for the convergence of the Gerasimov-Drell-Hern integral.
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ABSTRACT: We report new measurements of inclusive pi production from frozen-spin HD for polarized photon beams covering the Delta(1232) resonance. These provide data simultaneously on both H and D with nearly complete angular distributions of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH) sum rule. Recent results from Mainz and Bonn exceed the GDH prediction for the proton by 22 microb, suggesting as yet unmeasured high-energy components. Our pi0 data reveal a different angular dependence than assumed in Mainz analyses and integrate to a value that is 18 microb lower, suggesting a more rapid convergence. Our results for deuterium are somewhat lower than published data, considerably more precise, and generally lower than available calculations.Physical Review Letters 06/2009; 102(17):172002. · 7.37 Impact Factor -
Article: Measurements of polarized photo-pion production on longitudinally polarized HD and Implications for Convergence of the GDH Integral
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ABSTRACT: We report new measurements of inclusive pion production from frozen-spin HD for polarized photon beams covering the Delta(1232) resonance. These provide data simultaneously on both H and D with nearly complete angular distributions of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH) sum rule. Recent results from Mainz and Bonn exceed the GDH prediction for the proton by 22 microbarns, suggesting as yet unmeasured high-energy components. Our pi0 data reveal a different angular dependence than assumed in Mainz analyses and integrate to a value that is 18 microbarns lower, suggesting a more rapid convergence. Our results for deuterium are somewhat lower than published data, considerably more precise and generally lower than available calculations. Comment: 4 pages, 4 figures. Submitted for publication in Physical Review Letters08/2008; -
Conference Proceeding: A GEM based TPC for the LEGS experiment
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ABSTRACT: A compact time projection chamber (TPC) has been constructed for the LEGS (Laser Electron Gamma Source) experiment at BNL. The TPC uses double GEMs as the amplification stage. Position encoding is achieved through charge division using zigzag shaped anode pads. The TPC has a 35 cm diameter active area and a 50 cm long drift depth. It has more than 7000 channels of readout electronics in the form of custom designed ASICs. A novel peak sensing circuit is used to measure simultaneously the amplitude and timing of the signal peak from an anode pad. Test results from a shorter prototype version of the TPC as well as the construction of the final detector are discussed.Nuclear Science Symposium Conference Record, 2005 IEEE; 11/2005 -
Article: Polarized Compton scattering from^{4} He in the Δ region
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ABSTRACT: Differential cross sections and beam asymmetries of Compton scattering from 4He have been measured with linearly polarized photons in the energy range from 206 to 310 MeV. The quality of the results has the potential to provide strong constraints on the understanding of the reaction mechanism in the Δ resonance region. A phenomenological analysis of the experimental results has been performed fitting the data to a multipole expansion including dipole and quadrupole scatterings in the impulse approximation. Results indicate that quadrupole contributions should not be neglected to reproduce the general trend of the experimental results. Comparison with predictions from recent theoretical models shows that important discrepancies exist particularly at backward angles. The additional information carried by the incident photon spin increases the difficulty in achieving a comprehensive description of experimental data.Phys. Rev. C. 11/2003; 68(5). -
Article: N-->Δ transition and proton polarizabilities from measurements of p(γ-->,γ), p(γ-->,π0), and p(γ-->,π+)
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ABSTRACT: We report new high-precision measurements of p(γ-->,γ), p(γ-->,π0) and p(γ-->,π+) cross section and beam asymmetry angular distributions for photon beam energies in the range from 213 MeV to 333 MeV. The cross sections for all three channels are locked together with a small common systematic scale uncertainty of 2%. A large overdetermination of kinematic parameters was used to achieve the first complete separation of the Compton scattering and π0-production channels. This has also allowed all detector efficiencies for the p(γ-->,γ) and p(γ-->,π0) channels to be measured directly from the data itself without resorting to simulations. The new Compton results are approximately 30% higher than previous Bonn data near the peak of the Δ resonance, resolving a long-standing unitarity puzzle. However, our p(γ-->,π0) and p(γ-->,π+) cross sections are also about 10% higher than both earlier Bonn data and recent Mainz measurements, while our p(γ-->,π+) cross sections are in good agreement with results from Tokyo. Our polarization asymmetry data are of the highest precision yet available and have considerable impact upon multipole analyses. These new data have been combined with other polarization ratios in a simultaneous analysis of both Compton scattering and π production, with Compton scattering providing two new constraints on the photopion amplitude. This analysis has improved the accuracy in the E2/M1 mixing ratio for the N-->Δ transition, EMR=-[3.07+/-0.26(stat+syst)+/-0.24(model)](%), and the corresponding N-->Δ transverse helicity amplitudes, A1/2=-[135.7+/-1.3(stat+syst)+/-3.7(model)](10-3 GeV-1/2) and A3/2=-[266.9+/-1.6(stat+syst)+/-7.8(model)](10-3 GeV-1/2). From these we deduce an oblate spectroscopic deformation for the Δ+. The same simultaneous analysis has been used to extract the proton dipole polarizabilities, α¯-β¯=+[10.39+/-1.77(stat+syst)+1.02-1.87(model)](10-4 fm3) in agreement with previous low energy measurements, and α¯+β¯=+[13.25+/-0.86(stat+syst)+0.23-0.58(model)](10-4 fm3) in agreement with recent evaluations of the Baldin sum rule. Our simultaneous analysis has also provided the first determination of the proton spin polarizabilities, γπ=-[27.23+/-2.27(stat+syst)+2.24-2.10(model)](10-4 fm4), γ0=-[1.55+/-0.15(stat+syst)+0.03-0.03(model)](10-4 fm4), γ13=+[3.94+/-0.53(stat+syst)+0.20-0.18(model)](10-4 fm4), and γ14=-[2.20+/-0.27(stat+syst)+0.05-0.09(model)](10-4 fm4). The extracted value of the backward spin polarizability, γπ, is considerably different from other analyses and this has been instrumental in bringing the value of α¯-β¯ extracted from high energy data into agreement with low energy experiments.Physical Review C 07/2001; 64(2):25203. · 3.31 Impact Factor -
Article: The^{16} O (γ [over→], π^{-} p) reaction at E_ {γ}≃ 300 MeV
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ABSTRACT: Cross sections are presented for the 16O(γ⃗,π-p) reaction at incident photon energies between 290 and 325 MeV. The data are presented for specific proton and pion angles as a function of proton energy, which are compared with calculations in a local distorted wave impulse approximation model. The results are in agreement at most kinematics, although at some kinematics the data and calculations disagree by a factor of 2 or more. These data do not support the conclusion of a large modification to the mass of the Δ resonance in the nucleus.Phys. Rev. C. 04/2000; 61(5). -
Article: Photo-Production on
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ABSTRACT: 110 ffi and 130 ffi for incoming photon energies of 206, 219, 232, 245, 259, 274, 289, 306 and 322 MeV. The unpolarized cross sections are compared with the Delta Gamma hole theoretical model results available in literature. The measured photon beam asymmetry is Gamma1 as expected from conservation of angular momentum and parity. PACS: 25.20 D, 24.70, 29.27 H 1 Introduction Elastic Compton scattering and coherent ffi photoproduction provide a complementary tool to pion scattering for the study of the interaction of pions with nucleons and nuclei. In contrast to pion scattering, the mean free Preprint submitted to Elsevier Preprint 5 October 1998 path for photon interactions in nuclear matter is much larger than the nuclear size, which allows the photon to deeply penetrate the nuclear interior. Furthermore, the distortion of the photon field b11/1998; -
Article: Multipole Analyses for p(gamma,pi) and p(gamma,gamma) in the region of the P33 delta Resonance
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ABSTRACT: Multipole analyses of the p(g,pi0), p(g,pi+) and p(g,g) reactions are carried out using different data sets. With sufficient constraints from polarization observables, the ratio of E2/M1 transition amplitudes for N->delta (EMR) appears to be largely insensitive to differences between recent p(g,pi0) cross section measurements. We deduce a current best estimate of EMR = -(2.85 +/-0.34 +/-0.21)%. Back angle Compton cross sections require a value for the backward spin polarizability that is significantly lower than previous expectations, with a magnitude that is coupled to the (g,pi) cross sections.10/1998; -
Article: First Extraction of a Spin-Polarizability of the Proton
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ABSTRACT: A proton spin-polarizability characterizing backward Compton scattering has been extracted from a dispersion analysis of data between 33 and 309 MeV. This backward spin-polarizability, "delta" = 27.1 +/-2.2(stat+sys) +2.8/-2.4(model) in units of 10^-4 fm^4, differs significantly from theoretical estimates which are dominated by pi0 t-channel exchange, and indicates a new contribution from the non-perturbative spin-structure of the proton. This new value for "delta" removes an apparent inconsistency in the difference of charge polarizabilities extracted from data above pi-threshold. Our global result, alpha-beta = 10.11 +/-1.74(stat+sys) +1.22/-0.86 (model) in units of 10^-4 fm^3, is consistent with the previous world average of data below 155 MeV. Our value for alpha+beta = 13.23 +/-0.86(stat+sys) +0.20/-0.49(model) is consistent with a recent re-evaluation of the Baldin sum rule. Comment: Small corrections to LEGS data incorporated; fits recalculated; Tables and Figures updated01/1998; -
Article: Constraining the (gamma,pi) amplitude for E2 N->delta
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ABSTRACT: A recent analysis of pion photo-production multipoles using p(g,pi0) data from Mainz is repeated, successively adding constraints from other observables and varying the number of fitted partial waves. The original analysis is shown to have been underconstrained and suffered from ambiguities. The inclusion of additional observables in our analysis results in a very different, but stable, ratio of E2/M1 multipoles at the delta that is consistent with the Mainz data set.08/1997; -
Article: The E2 N{yields}{Delta} transition from polarized Compton scattering
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ABSTRACT: Short communication.04/1993; -
Article: Laser Electron Gamma Source Facility biennial progress report, June 1992
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ABSTRACT: This report briefly discusses the Laser Electron Gamma Source facility and the following experiments conducted here: polarization in D(gamma, p)n and N-N tensor forces for energies less than 225 Mev; constraints on the nuclear tensor force from D(gamma, p)n for energies less than 315 Mev; the p(gamma, neutral pion) reaction and the E2 excitation of delta resonance; quasi-two- and three-body absorption in helium 3(gamma, N-N); and the delta-nucleon interaction in D(gamma,p-negative pion)p and D(gamma, p-n)neutral pions. (LSP).07/1992; -
Article: Measurement of^{2} H (γ [over→], p) n with linearly polarized photons in the Δ resonance region
01/2002; -
Article: N -> Delta transition and proton polarizabilities from measurements of p(gamma, gamma), p(gamma, pi0) and p(gamma, pi+)
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ABSTRACT: This version is available at the following Publisher URL: http://prc.aps.org -
Article: PHYSICS WITH POLARIZED PHOTONS
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ABSTRACT: New preliminary results on the deuteron photodisintegration at LEGS are presented. The experimental activity at LADON is rapidly reviewed and some indication is given for the possible activity at GRAAL. -
Article: PHYSICS WITH POLARIZED PHOTONS
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ABSTRACT: De nouveaux résultats préliminaires sur la photodesintégration du deuton réalisée avec LEGS sont présentés. Les activités expérimentales avec LADON sont rapidement passées en revue. Quelques indications sont données sur des activités possibles avec GRAAL. New preliminary results on the deuteron photodisintegration at LEGS are presented. The experimental activity at LADON is rapidly reviewed and some indication is given for the possible activity at GRAAL.http://dx.doi.org/10.1051/jphyscol:1990657.
Top co-authors
Top Journals
Institutions
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2003–2011
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University of Virginia
- Department of Physics
Charlottesville, VA, USA
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2–2001
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Brookhaven National Laboratory
- Physics Department
New York City, NY, USA
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