D. Paudyal

Measurement of

Differential cross sections for the gamma p -> pi(0)p reaction have been measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C, up to the center-of-mass energy W = 1.9 GeV. The new results, obtained with a fine energy and angular binning, increase the existing quantity of pi(0) photoproduction data by similar to 47%. Owing to the unprecedented statistical accuracy and the full angular coverage, the results are sensitive to high partial-wave amplitudes. This is demonstrated by the decomposition of the differential cross sections in terms of Legendre polynomials and by further comparison to model predictions. A new solution of the SAID partial-wave analysis obtained after adding the new data into the fit is presented.

π^0

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 N 1/2^{-} (S_{11}) and N 1/2^{+} (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.

photoproduction on the proton at MAMI C

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 → 3π0 → 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 (σ1/2). It is absent for reactions with parallel spin orientation (σ3/2) 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 Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of P11 and S11 partial waves to explain the narrow structure.

Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections

The double-polarization observable E and the helicity-dependent cross sections σ1/2 and σ3/2 have been measured for the first time for single π0 photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the π0 meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the π0N final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry E since the σ1/2 and σ3/2 components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.

Helicity-dependent cross sections and double-polarization observable E in η photoproduction from quasifree protons and neutrons

The Dalitz decay pi(0) -> e(+) e(-) gamma has been measured in the gamma p -> pi(0) p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the pi(0) electromagnetic transition form factor, a(pi) = 0.030 +/- 0.010(tot), is in agreement with existing measurements of this decay and with recent theoretical calculations. The uncertainty obtained in the value of a(pi) is lower than in previous results based on the pi(0) -> e(+) e(-) gamma decay.

First measurement of the polarization observable E and helicity-dependent cross sections in single π0 photoproduction from quasi-free nucleons

Abstract.The scalar dipole polarizabilities,

Measurement of the π0→e+e-γ Dalitz decay at the Mainz Microtron

\alpha_{E1}

Determination of the scalar polarizabilities of the proton using beam asymmetry

αE1 and

\Sigma_{3}

\beta_{M1}

in Compton scattering

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