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 Eγ = 1447 MeV (W = 1896 MeV). 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.
The transversely polarized target (PT) of the COMPASS (NA58) collaboration at CERN has been used for Drell–Yan measurements in 2015 and 2018. The transverse spin structure of the proton has been ...studied using a negative pion beam and a solid ammonia target. Employing the dynamic nuclear polarization (DNP) method, proton polarization values of more than 80% have been routinely achieved after one day, at a homogeneous magnetic field of 2.5 T and using a 3He/4He dilution refrigerator. During the data-taking the target operates in a transversely oriented magnetic dipole field at 0.6 T. This so-called frozen spin operation mode without the DNP pumping process leads to a slow depolarization of the target material, which is further accelerated by the heat input of the pion beam, produced secondary particles and radiation damage effects to the target material. Ammonia has the highest resistance against radiation-induced depolarization among known solid target materials. The proton polarization has been measured by the nuclear magnetic resonance (NMR). Relaxation times of about 1100 h have been observed for the proton polarization resulting in an average polarization between 68% and 76% during about two weeks long data-taking periods. To achieve a systematic uncertainty of the polarization ΔP/P as low as 3.2% and a statistical one of less than 1.8% two large target cells with appropriate positioning of the NMR-coils have been built.
The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is ...through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Δ(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields γ(E1E1)=-3.5±1.2, γ(M1M1)=3.16±0.85, γ(E1M2)=-0.7±1.2, and γ(M1E2)=1.99±0.29, in units of 10(-4) fm(4).
We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the γover →pover →→ηp reaction up to a center-of-mass energy of W=1.9 GeV. The ...data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of η meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.
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 first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300 MeV, covering nearly the ...full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed, which are traced to differences in the helicity amplitudes of well-known and established resonances. Precise values for the helicity amplitudes of several resonances are reported.
The electron spin-lattice relaxation time (T 1 e) of TEMPO- and trityl-doped butanol samples at 2.5 T and temperatures between 0.95 K and 2.17 K was studied by pulsed nuclear magnetic resonance (NMR) ...using the nuclear-electron double resonance (NEDOR) method. This method is based on the idea to measure the NMR lineshift produced by the local field of paramagnetic impurities, whose polarization can be manipulated. This is of technical advantage as measurements can be performed under conditions typically used for the dynamic nuclear polarization (DNP) process - in our case 2.5 T and temperatures around 1 K - where a direct measurement on the electronic spins would be far more complicated to perform.
.
Results from measurements of the photoproduction of
η
mesons from quasifree protons and neutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at the electron ...accelerator ELSA in Bonn using the
η
→
3
π
0
→
6
γ
decay. A liquid deuterium target was used for the measurement of total cross sections and angular distributions. The results confirm earlier measurements from Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitation function of
γ
n
→
n
η
. The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow
P
11
state. Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularly polarized photon beam were analyzed to determine the double polarization observable
E
. Both data sets together were also used to extract the helicity-dependent cross sections
σ
1
/
2
and
σ
3
/
2
. The narrow structure in the excitation function of
γ
n
→
n
η
appears associated with the helicity-1/2 component of the reaction.
New data on the polarization observables T, P, and H for the reaction γp→pπ(0) are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a ...linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.
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