First measurement of the circular beam asymmetry in the gamma p --> pi0 eta p reaction

The circular photon asymmetry for pi0 eta photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Delta(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.Comment: 11 pages, 3 figures, submitted to Phys.Lett.

Double pion photoproduction off nuclei - Are there effects beyond final-state interaction?

Photoproduction of pi(0)pi(0) and pi(0)pi(+/-) pairs from nuclei has been measured over a wide mass range (H-2, Li-7, C-12, Ca-40, and Pb-nat) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4 pi electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant-mass spectra for heavy nuclei to small invariant masses has been observed for pi(0) pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the sigma-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for sigma-in-medium modification. (C) 2013 Elsevier B.V. All rights reserved

Beam-helicity asymmetries in double-pion photoproduction off the proton

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels γ⃗ p→π<sup>+</sup>π<sup>0</sup>n, γ⃗ p→π<sup>0</sup>π<sup>0</sup>p, and γ⃗ p→π<sup>+</sup>π<sup>−</sup>p. The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of π0 mesons were detected in a 4π electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction dynamics

Coherent photoproduction of η-mesons off ³He – search for η-mesic nuclei

Coherent photoproduction of η-mesons off 3He, i.e. the reaction γ3He→η3He, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball – TAPS detector. Angular distributions and the total cross section were measured using the η→γγ and η→3π0→6γ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong η3He final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of π0-proton back-to-back emission in the γ3He→π0pX reaction revealed a very complicated structure of the background and could not support previous conclusions