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Thomas, A.

Publications and source records attributed to Thomas, A..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Photoproduction of π 0 mesons off protons and neutrons in the second and third nucleon resonance regions

Background: Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the elec- tromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets. Purpose: Study experimentally single π 0 photoproduction off quasi-free nucleons from the deuteron. Investigate nuclear effects by a comparison of the results for free protons and quasi-free protons. Use the quasi-free neutron data (corrected for nuclear effects) to test the predictions of reaction models and partial wave analysis (PWA) for $\gamma$n → nπ 0 derived from the analysis of the other isospin channels. Methods: High statistics angular distributions and total cross sections for the photoproduction of π 0 mesons off the deuteron with coincident detection of recoil nucleons have been measured for the first time. The experiment was performed at the tagged photon beam of the Mainz Microtron (MAMI) accelerator for photon energies between 0.45 GeV and 1.4 GeV, using an almost 4π electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion. Results: Significant effects from final state interactions (FSI) were observed for participant protons in comparison to free proton targets (between 30% and almost 40%). The data in coincidence with recoil neutrons were corrected for such effects under the assumption that they are identical for participant protons and neutrons. Reaction model predictions and PWA for $\gamma$n → nπ 0 , based on fits to data for the other isospin channels, disagreed between themselves and no model provided a good description of the new data. Conclusions: The results demonstrate clearly the importance of a measurement of the fully neutral final state for the isospin decomposition of the cross section. Model refits, for example from the Bonn-Gatchina analysis, show that the new and the previous data for the other three isospin channels can be simultaneously described when the contributions of several partial waves are modified. Finally, the results are also relevant for the suppression of the higher resonance bumps in total photoabsorption on nuclei, which are not well understood.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Experimental study of the γ p → π 0 η p reaction with the A2 setup at the Mainz Microtron

Here, the data available from the A2 Collaboration at MAMI were analyzed to select the γp → π 0 ηp reaction on an event-by-event basis, which allows for partial-wave analyses of three-body final states to obtain more reliable results, compared to fits to measured distributions. These data provide the world’s best statistical accuracy in the energy range from threshold to E γ = 1.45 GeV, allowing a finer energy binning in the measurement of all observables needed for understanding the reaction dynamics. The results obtained for the measured observables are compared to existing models, and the impact from the new data is checked by the fit with the revised Mainz model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Measurement of the ω → π 0 e + e - and η → e + e - γ Dalitz decays with the A2 setup at the Mainz Microtron

The Dalitz decays η → e + e - γ and ω → π 0 e + e - have been measured in the γp → ηp and γp → ωp reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron. The value obtained for the slope parameter of the electromagnetic transition form factor of η, Λ$-2\atop{η}$ = (1.97 ± 0.11 tot ) GeV -2 , is in good agreement with previous measurements of the η → e + e - γ and η → μ + μ - γ decays. The uncertainty obtained in the value of Λ$-2\atop{η}$ is lower than in previous results based on the η → e + e - γ decay. The value obtained for the ω slope parameter, Λ$-2\atop{ωπ^0}$ = (1.99 ± 0.21 tot ) GeV -2 , is somewhat lower than previous measurements based on ω → π 0 μ + μ - , but the results for the ω transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Measurement of the π 0 → e + e - γ Dalitz decay at the Mainz Microtron

The Dalitz decay π 0 → e + e - γ has been measured in the γp → π 0 p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the π 0 electromagnetic transition form factor, a π = 0.030 ± 0.010 tot , is in agreement with existing measurements of this decay and with recent theoretical calculations. The uncertainty obtained in the value of a π is lower than in previous results based on the π 0 → e + e - γ decay.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Determination of the scalar polarizabilities of the proton using beam asymmetry $\Sigma_{3}$ in Compton scattering

The scalar dipole polarizabilities, α E1 and β M1 , are fundamental properties related to the internal dynamics of the nucleon. The currently accepted values of the proton polarizabilities were determined by fitting to unpolarized proton Compton scattering cross section data. The measurement of the beam asymmetry Σ 3 in a certain kinematical range provides an alternative approach to the extraction of the scalar polarizabilities. At the Mainz Microtron (MAMI) the beam asymmetry was measured for Compton scattering below pion photoproduction threshold for the first time. Finally, the results are compared with model calculations and the influence of the experimental data on the extraction of the scalar polarizabilities is determined.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Photon asymmetry measurements of $\overrightarrow{\gamma}p \rightarrow \pi^{0} p$ γ → p → π 0 p for $E_{\gamma}=$ 320-650 MeV

High-statistics measurements of the photon asymmetry Σ for the $\overrightarrow{\gamma}p \rightarrow \pi^{0} p$ reaction have been made in the center-of-mass energy range W = 1214–1450 MeV. The data were measured with the MAMI A2 real photon beam and Crystal Ball/TAPS detector systems in Mainz, Germany. The resulting measurements significantly improve the existing world data and are shown to be in good agreement with previous measurements, and with the MAID, SAID, and Bonn-Gatchina predictions. We have also combined the photon asymmetry results with recent cross-section measurements from Mainz to calculate the profile functions, $\check{Σ}$ (= σ 0 Σ), and perform a moment analysis. Comparison with calculations from the Bonn-Gatchina model shows that the precision of the data is good enough to further constrain the higher partial waves, and there is an indication of interference between the very small F-waves and the N(1520)3/2 - and N(1535)1/2 - resonances.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2016

Horizon Based Orientation Estimation for Planetary Surface Navigation

Planetary rovers navigate in extreme environments for which a Global Positioning System (GPS) is unavailable, maps are restricted to relatively low resolution provided by orbital imagery, and compass information is often lacking due to weak or not existent magnetic fields. However, an accurate rover localization is particularly important to achieve the mission success by reaching the science targets, avoiding negative obstacles visible only in orbital maps, and maintaining good communication connections with ground. This paper describes a horizon solution for precise rover orientation estimation. The detected horizon in imagery provided by the on board navigation cameras is matched with the horizon rendered over the existing terrain model. The set of rotation parameters (roll, pitch yaw) that minimize the cost function between the two horizon curves corresponds to the rover estimated pose.

localization↗
DOE OSTI2016

Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections

Here, the double polarization observable E and the helicity dependent cross sections σ 1/2 and σ 3/2 were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η→2γ and η→3π 0 . The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ 1/2 and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in η production are only N1/2 – (S11) and N1/2 + (P 11 ). Furthermore, the extracted Legendre coefficients of the angular distributions for σ1/2 are in good agreement with recent reaction model predictions assuming a narrow resonance in the P 11 wave as the origin of this structure.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Photoproduction of $\eta$ $\pi$ pairs off nucleons and deuterons

Quasi-free photoproduction of πη-pairs has been investigated from threshold up to incident photon energies of 1.4 GeV, respectively up to photon-nucleon invariant masses up to 1.9 GeV. Total cross sections, angular distributions, invariant-mass distributions of the πη and meson-nucleon pairs, and beam helicity asymmetries have been measured for the reactions γp → pπ 0 η, γn → nπ 0 η, γp → nπ + η, and γn → pπ – η from nucleons bound inside the deuteron. For the γp initial-state data for free protons have also been analyzed. Finally, the total cross sections for quasi-free production of π 0 η pairs from nucleons bound in 3He nuclei have been investigated in view of final state interaction (FSI) effects. The experiments were performed at the tagged photon beam facility of the Mainz MAMI accelerator using an almost 4π covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The shapes of all differential cross section data and the asymmetries are very similar for protons and neutrons and agree with the conjecture that the reactions are dominated by the sequential Δ *3/2 – → ηΔ(1232) → πηN decay chain, mainly with Δ(1700)3/2 – and Δ(1940)3/2 – . The ratios of the magnitude of the total cross sections also agree with this assumption. Furthermore, the absolute magnitudes of the cross sections are reduced by FSI effects with respect to free proton data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗