Photoproduction of ηπ-pairs off nucleons

Abstract

In this work, photo-production of ηπ meson pairs off protons and neutrons, bound in the deuteron, and off free protons was investigated. The data were taken at the Mainzer Microtron MAMI during 4 beam-times in 2007 and 2009, using liquid hydrogen and deuterium targets. Additionally longitudinally polarized dButanol data and carbon data were analyzed using data from 5 beam-times carried out from 2013-2016. A longitudinally polarized electron beam of up to 1.5 GeV, was converted to circularly polarized photons via a bremsstrahlung process. The photons were energy-tagged with the Glasgow tagging spectrometer and reached energies up to 1.4 GeV. Final state particles were detected in a nearly 4π covering electromagnetic calorimeter, consisting of the Crystal Ball and TAPS detectors. Charged particles were identified with a dedicated Particle Identification Detector in the CB and veto detectors in TAPS. The center of mass energy W was determined from the final state particles, using a kinematical reconstruction to remove effects of Fermi-motion inside the deuteron, and covers energies from ≈ 1600 MeV - 1900 MeV. For the reactions γp → ηπ0p, γp(n) → ηπ0p(n), γn(p) → ηπ0n(p), γp → ηπ+n, γp(n) → ηπ+n(n), and γn(p) → ηπ-p(p) total cross sections, angular distributions for different coordinate systems, mass distributions of Nη, Nπ and ηπ have been extracted as well as beam helicitiy asymmetries, and are compared to theoretical predictions. Also total cross sections were extracted for the inclusive reactions γN(N) → ηπ0(N) and γN(N) → ηπ±(N) and for the coherent reaction γd → ηπ0d. Additionally the double polarization observable E and the helicity dependent cross sections σ1/2 and σ3/2 were extracted for the ηπ0 channels. The shapes of the extracted observables are almost identical for reactions on quasi-free protons and neutrons. Compared to the results of the reactions on free protons, the results of quasi-free protons are reduced by FSI effects. The shapes of the quasi-free proton results agree nicely with the free proton results, if the total cross section is divided out. Ratios of total cross sections on quasi-free protons and neutrons are in agreement with the assumption, that ηπ photo-production is dominated by a sequential decay cascade of the form Δ* → Δ(1232)3/2+ η → πηN. Also the invariant mass distributions support this assumption, and additionally the angular distributions are in good agreement with model predictions that only take the Δ(1700)3/2- resonance into account.