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.
We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured ...photon energy range of 300-700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around E_{γ}∼570 MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the d^{*}(2380) hexaquark.
The target asymmetry
T
, recoil asymmetry
P
, and beam-target double polarization observable
H
were determined in exclusive
π
0
and
η
photoproduction off quasi-free protons and, for the first time, ...off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the
γ
p
→
π
0
p
and
γ
p
→
η
p
reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the
η
n
system at
W
=
1.68
GeV
. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the
S
11
(
1535
)
and
S
11
(
1650
)
resonances within the
S
11
-partial wave.
The quasifree
photon beam asymmetry,
, has been measured at photon energies,
, 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
and
(1440) resonances. The extracted values of
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
measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict
photoproduction observables, based on fits to published data in the channels
,
and
.
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
.
The γ n → 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 π0 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 π0-meson production. Our accumulation of 3.6 × 106 γ n → π 0 n events allowed 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 with 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 * → γ n at the resonance poles are determined for the first time.
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