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Measurement of the ω → π 0 e + e − and η → e + e − γ Dalitz decays with the A2 setup at MAMI 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, MAMI. The value obtained for the slope parameter of the electromagnetic transition form factor of η, Λ −2 η = (1.97 ± 0.11tot ) 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 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 ωπ 0 = (1.99 ± 0.21tot) 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.

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 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 − (S 11 ) and N 1=2 þ (P 11 ). Furthermore, the extracted Legendre coefficients of the angular distributions for σ 1=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.

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.