Photoproduction of $pieta$-pairs from nucleons has been investigated from threshold up to incident photon energies of $approx$~1.4~GeV. The quasi-free reactions $gamma prightarrow ppi^0eta$, $gamma nrightarrow npi^0eta$, $gamma prightarrow npi^+eta$,
and $gamma nrightarrow ppi^-eta$ were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the $gamma prightarrow npi^+eta$ reaction was measured for the first time). For the $pi^0eta$ final state coherent production via the $gamma drightarrow dpi^0eta$ reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost $4pi$ coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation $sigma(ppi^0eta)$ $approx$ $sigma(npi^0eta)$ $approx$ $2sigma(ppi^-eta)$ $approx$ $2sigma(npi^+eta)$ as expected for a dominant contribution from a $Delta^{star}rightarrowetaDelta(1232)rightarrowpieta N$ reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon-meson and $pi$-$eta$ pairs. The experimental results are compared to the predictions from an isobar reaction model.
A multi-cell He gas-scintillator active target, designed for the measurement of photoreaction cross sections, is described. The target has four main chambers, giving an overall thickness of 0.103 $mathrm{g/cm^{2}}$ at an operating pressure of 2 MPa.
Scintillations are read out by photomultiplier tubes and the addition of small amounts of $mathrm{N}_{2}$ to the He, to shift the scintillation emission from UV to visible, is discussed. First results of measurements at the MAX IV Laboratory tagged-photon facility show that the target has good timing resolution and can cope well with a high-flux photon beam. The determination of reaction cross sections from target yields relies on a Monte Carlo simulation, which considers scintillation light transport, photodisintegration processes in $^{4}mathrm{He}$, background photon interactions in target windows and interactions of the reaction-product particles in the gas and target container. The predictions of this simulation are compared to the measured target response.
A first comparison has been made between the pulse-shape discrimination characteristics of a novel $^{4}$He-based pressurized scintillation detector and a NE-213 liquid-scintillator reference detector using an Am/Be mixed-field neutron and gamma-ray
source and a high-resolution scintillation-pulse digitizer. In particular, the capabilities of the two fast neutron detectors to discriminate between neutrons and gamma-rays were investigated. The NE-213 liquid-scintillator reference cell produced a wide range of scintillation-light yields in response to the gamma-ray field of the source. In stark contrast, due to the size and pressure of the $^{4}$He gas volume, the $^{4}$He-based detector registered a maximum scintillation-light yield of 750~keV$_{ee}$ to the same gamma-ray field. Pulse-shape discrimination for particles with scintillation-light yields of more than 750~keV$_{ee}$ was excellent in the case of the $^{4}$He-based detector. Above 750~keV$_{ee}$ its signal was unambiguously neutron, enabling particle identification based entirely upon the amount of scintillation light produced.
The electromagnetic polarizabilities of the nucleon are fundamental properties that describe its response to external electric and magnetic fields. They can be extracted from Compton-scattering data --- and have been, with good accuracy, in the case
of the proton. In contradistinction, information for the neutron requires the use of Compton scattering from nuclear targets. Here we report a new measurement of elastic photon scattering from deuterium using quasimonoenergetic tagged photons at the MAX IV Laboratory in Lund, Sweden. These first new data in more than a decade effectively double the world dataset. Their energy range overlaps with previous experiments and extends it by 20 MeV to higher energies. An analysis using Chiral Effective Field Theory with dynamical Delta(1232) degrees of freedom shows the data are consistent with and within the world dataset. After demonstrating that the fit is consistent with the Baldin sum rule, extracting values for the isoscalar nucleon polarizabilities and combining them with a recent result for the proton, we obtain the neutron polarizabilities as alpha_n = [11.55 +/- 1.25(stat) +/- 0.2(BSR) +/- 0.8(th)] X 10^{-4} fm^3 and beta_n = [3.65 -/+ 1.25(stat) +/- 0.2(BSR) -/+ 0.8(th)] X 10^{-4} fm3, with chi^2 = 45.2 for 44 degrees of freedom.
Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions $boldsymbol{gamma}prightarrow npi^0pi^+$ off free protons and $boldsymbol{gamma}drightarrow (p)ppi^0pi^-
$ and $boldsymbol{gamma}drightarrow (n)npi^0pi^+$ off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from $pi^0$ mesons were detected in the 4$pi$ electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.
Rate-dependent effects in the electronics used to instrument the tagger focal plane at the MAX IV Laboratory were recently investigated using the novel approach of Monte Carlo simulation to allow for normalization of high-rate experimental data acqui
red with single-hit time-to-digital converters (TDCs). The instrumentation of the tagger focal plane has now been expanded to include multi-hit TDCs. The agreement between results obtained from data taken using single-hit and multi-hit TDCs demonstrate a thorough understanding of the behavior of the detector system.
The Dalitz decay eta -> e^+ e^- gamma has been measured in the gamma p -> eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together with the photon tagging facility at the Mainz Microtron MAMI. The experimental statistic used
in this work is one order of magnitude greater than in any previous measurement of eta -> e^+ e^- gamma. The value obtained for the slope parameter 1/Lambda^2 of the eta transition form factor, 1/Lambda^2 = (1.95 +/- 0.15_stat +/- 0.10_syst) [1/GeV^2], is in good agreement with recent measurements conducted in eta -> e^+ e^- gamma and eta -> mu^+ mu^- gamma decays, as well as with recent form-factor calculations. The uncertainty obtained in the value of 1/Lambda^2 is lower compared to results from previous measurements of the eta -> e^+ e^- gamma decay.
Coherent photoproduction of $eta$-mesons off $^3$He, i.e. the reaction $gamma ^3{He}rightarrow eta ^3{He}$, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accel
erator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the $etarightarrow gammagamma$ and $etarightarrow 3pi^0rightarrow 6gamma$ 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 $eta {^3{He}}$ final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of $pi^0$-proton back-to-back emission in the $gamma ^3{He}rightarrow pi^0 pX$ reaction revealed a very complicated structure of the background and could not support previous conclusions.
Cross sections for the ^{3}He(e,epn)p reaction were measured for the first time at energy transfers of 220 and 270 MeV for several momentum transfers ranging from 300 to 450 MeV/c. Cross sections are presented as a function of the momentum of the rec
oil proton and the momentum transfer. Continuum Faddeev calculations using the Argonne V18 and Bonn-B nucleon-nucleon potentials overestimate the measured cross sections by a factor 5 at low recoil proton momentum with the discrepancy becoming much smaller at higher recoil momentum.
