Engineering topics
Spieker, K.
Publications and source records attributed to Spieker, K..
First Measurement Using Elliptically Polarized Photons of the Double-Polarization Observable E for γ p → p π 0 and γ p → n π +
We report the measurement of the helicity asymmetry E for the p π 0 and n π + final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g., G ) and circularly polarized photons (e.g., E ) and a longitudinally polarized target. The new data cover a photon energy range 270–1400 MeV for the p π 0 final state (230–842 MeV for the n π + final state) and the full range of pion polar angles, θ , providing the most precise measurement of the observable E . A moment analysis gives a clear observation of the p η cusp in the p π 0 final state. Published by the American Physical Society 2024
Neutron polarisation transfer, C x ′ , in π+ photoproduction off the proton, d ( γ ⊙ , n → π + ) n s p e c
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First measurement of polarisation transfer $C$$^{n}_{x'}$ in deuteron photodisintegration
A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron ($C$$^{n}_{x'}$) in deuterium photodisintegration has been carried out. This quantity is determined over the photon energy range 370 – 700 MeV and for neutron centre-of-mass breakup angles ~ 45 – 120°. The polarisation of the final state neutrons was determined by an ancillary large-acceptance nucleon polarimeter, surrounding a cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The polarimeter characterised (n, p) charge exchange of the ejected neutrons to determine their polarisation. The new $C$$^{n}_{x'}$ data are also compared to a theoretical model based on nucleonic and nucleon resonance degrees of freedom constrained by the current world-database of deuterium photodisintegration measurements. Structures in $C$$^{n}_{x'}$ observed in the region of the d*(2380) could not be explained by conventional models of deuteron photodisintegration.
Measurement of Compton Scattering at MAMI for the Extraction of the Electric and Magnetic Polarizabilities of the Proton
A precise measurement of the differential cross sections dσ/dΩ and the linearly polarized photon beam asymmetry Σ 3 for Compton scattering on the proton below pion threshold has been performed with a tagged photon beam and almost 4π detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic liquid hydrogen target, with the scattered photons detected in the Crystal Ball/TAPS setup. Using the highest statistics Compton scattering data ever measured on the proton along with two effective field theories (both covariant baryon and heavy-baryon) and one fixed-t dispersion relation model, constraining the fits with the Baldin sum rule, we have obtained the proton electric and magnetic polarizabilities with unprecedented precision: α E1 = 10.99 ± 0.16 ± 0.47 ± 0.17 ± 0.34, β M1 = 3.14 ± 0.21 ± 0.24 ± 0.20 ± 0.35; in units of 10 -4 fm 3 where the errors are statistical, systematic, spin polarizability dependent, and model dependent.
Single $\pi ^0$ production off neutrons bound in deuteron with linearly polarized photons
The quasifree γ → d → π 0 n(p) photon beam asymmetry, Σ, has been measured at photon energies, E γ , from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49° to 148°. In this kinematic region, polarization observables are sensitive to contributions from the Δ(1232) and N(1440) resonances. The extracted values of Σ have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID and Bonn–Gatchina analyses; while a revised SAID fit, including the new Σ measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict π 0 n photoproduction observables, based on fits to published data in the channels π 0 p, π + n and π – p.
Measurement of the helicity asymmetry E for the reaction $$ \gamma p\rightarrow \pi ^0 p$$: The CBELSA/TAPS Collaboration
Abstract A measurement of the double-polarization observable E for the reaction $$\gamma p\rightarrow \pi ^0 p$$ γ p → π 0 p is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C $$_4$$ 4 H $$_9$$ 9 OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from $$E_\gamma =600$$ E γ = 600 to 2310 MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.
Observation of the p η ' Cusp in the New Precise Beam Asymmetry Σ Data for γ p → p η
Data on the beam asymmetry Σ in the photoproduction of η mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from W = 1748 MeV to W = 2045 MeV). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the pη' threshold [E γ = 1447 MeV (W = 1896 MeV)]. Here, the latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the N(1895)1/2 – nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.
Extracting the spin polarizabilities of the proton by measurement of Compton double-polarization observables
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Helicity-Dependent Cross Sections for the Photoproduction of π 0 Pairs from Nucleons
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Signatures of the d * ( 2380 ) Hexaquark in d ( γ , p n → )
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Measurement of the beam-helicity asymmetry in photoproduction of π0η pairs on carbon, aluminum, and lead
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Cross section for γ n → π 0 n measured at the Mainz A2 experiment
The γn → π 0 n differential cross section evaluated for 27 energy bins span the photon-energy range 290–813 MeV (W = 1.195 – 1.553 GeV) and the pion c.m. polar production angles, ranging from 18° to 162°, making use of model-dependent nuclear corrections to extract π 0 production data on the neutron from measurements on the deuteron target. Additionally, the total photoabsorption cross section was measured. The tagged photon beam produced by the 883-MeV electron beam of the Mainz Microtron MAMI was used for the π 0 -meson production. Our accumulation of 3.6×10 6 γn → π 0 n events enabled a detailed study of the reaction dynamics. Our data are in reasonable agreement with previous A2 measurements and extend them to lower energies. The data are compared to predictions of previous SAID, MAID, and BnGa partial-wave analyses and to the latest SAID fit MA19 that included our data. Selected photon decay amplitudes N* → γn at the resonance poles are observed for the first time.