The reactions γp → ηp and γp → η 0 p are measured from their thresholds up to the center-of-mass energy W ¼ 1.96 GeV with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections are obtained with unprecedented statistical accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section for η photoproduction at the energies in the vicinity of the η 0 threshold, W ¼ 1896 MeV (E γ ¼ 1447 MeV). Within the framework of a revised ηMAID isobar model, the cusp, in connection with a steep rise of the η 0 total cross section from its threshold, can only be explained by a strong coupling of the poorly known Nð1895Þ1=2 − state to both ηp and η 0 p. Including the new high-accuracy results in the ηMAID fit to available η and η 0 photoproduction data allows the determination of the Nð1895Þ1=2 − properties.

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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 Δð1232Þ 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 ¼ −3.5 AE 1.2, γ M1M1 ¼ 3.16 AE 0.85, γ E1M2 ¼ −0.7 AE 1.2, and γ M1E2 ¼ 1.99 AE 0.29, in units of 10 −4 fm 4 . DOI: 10.1103/PhysRevLett.114.112501 PACS numbers: 25.20.Lj, 13.40.-f, 13.60.Fz, 13.88.+e Electromagnetic polarizabilities are fundamental properties of composite systems such as molecules, atoms, nuclei, and hadrons [1]. Whereas magnetic moments provide information about the ground-state properties of a system, polarizabilities provide information about the excited states of the system. For atomic systems, polarizabilities are of the order of the atomic volume. For hadrons, polarizabilities are much smaller than the volume, typically of order PRL 114,

The scalar dipole polarizabilities, αE1 and βM1, 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 Σ3 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. The results are compared with model calculations and the influence of the experimental data on the extraction of the scalar polarizabilities is determined.

Precise helicity-dependent cross sections and the double-polarization observable E were measured for η photoproduction from quasifree protons and neutrons bound in the deuteron. The η → 2γ and η → 3π 0 → 6γ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of η photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin (σ 1/2 ). It is absent for reactions with parallel spin orientation (σ 3/2 ) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with * Also at Dipartimento di Fisica, Università di Pavia, I-27100 Pavia, Italy. Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of P 11 and S 11 partial waves to explain the narrow structure.