Explore the vast range of titles available.

This paper reports on measurements of the 3 He( e , e ′ pp ) reaction measured at AmPS and the 3 He( e , e ′ pn ) reaction measured at MAMI. The measurements were performed in similar kinematics to allow a comparison to be made between the cross sections of the two reactions. Results are shown for both reactions together with a comparison for the 3 He( e , e ′ pp ) data with continuum Faddeev calculations.

A first measurement of the longitudinal beam spin asymmetry ALU in the semi-inclusive electroproduction of pairs of charged pions is reported. ALU is a higher-twist observable and offers the cleanest access to the nucleon twist-3 parton distribution function e(x). Data have been collected in the Hall-B at Jefferson Lab by impinging a 5.498 GeV electron beam on a liquid-hydrogen target, and reconstructing the scattered electron and the pion pair with the CLAS detector. One-dimensional projections of the sin(phiR) moments of ALU are extracted for the kinematic variables of interest in the valence quark region. The understanding of di-hadron production is essential for the interpretation of observables in single hadron production in semi-inclusive DIS, and pioneering measurements of single spin asymmetries in di-hadron production open a new avenue in studies of QCD dynamics.

The helicity-dependent cross sections for the photoproduction of \\(\\pi^0\\eta\\) pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the \\(\\sigma_{1/2}\\) (photon and target spin antiparallel) and \\(\\sigma_{3/2}\\) (parallel spins) components to be measured for quasi-free production of \\(\\pi^0\\eta\\) pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, \\(A_{1/2}^{n,p}\\) and \\(A_{3/2}^{n,p}\\). Identical couplings for protons and neutrons are typical for \\(\\Delta\\) resonances and identical \\(A_{1/2}\\) and \\(A_{3/2}\\) components are only possible for \\(J\\geq 3/2\\) states, which constrains possible contributions of nucleon resonances.

Cross sections for the ^{3}He(e,e'pn)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 recoil 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.

Precise helicity-dependent cross sections and the double-polarization observable \\(E\\) were measured for \\(\\eta\\) photoproduction from quasi-free protons and neutrons bound in the deuteron. The \\(\\eta\\rightarrow 2\\gamma\\) and \\(\\eta\\rightarrow 3\\pi^0\\rightarrow 6\\gamma\\) 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 \\(\\eta\\) photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin (\\(\\sigma_{1/2}\\)). It is absent for reactions with parallel spin orientation (\\(\\sigma_{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 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.

Quasi-free photoproduction of \\(\\pi\\eta\\)-pairs has been investigated from threshold up to incident photon energies of 1.4 GeV, respectively up to photon-nucleon invariant masses up to 1.9 GeV. Total cross sections, angular distributions, invariant-mass distributions of the \\(\\pi\\eta\\) and meson-nucleon pairs, and beam-helicity asymmetries have been measured for the reactions \\(\\gamma p\\rightarrow p\\pi^0\\eta\\), \\(\\gamma n\\rightarrow n\\pi^0\\eta\\), \\(\\gamma p\\rightarrow n\\pi^+\\eta\\), and \\(\\gamma n\\rightarrow p\\pi^-\\eta\\) from nucleons bound inside the deuteron. For the \\(\\gamma p\\) initial state data for free protons have also been analyzed. Finally, the total cross sections for quasi-free production of \\(\\pi^0\\eta\\) pairs from nucleons bound in \\(^3\\)He nuclei have been investigated in view of final state interaction (FSI) effects. The experiments were performed at the tagged photon beam facility of the Mainz MAMI accelerator using an almost \\(4\\pi\\) covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The shapes of all differential cross section data and the asymmetries are very similar for protons and neutrons and agree with the conjecture that the reactions are dominated by the sequential \\(\\Delta^{\\star}3/2^-\\rightarrow\\eta\\Delta(1232)\\rightarrow\\pi\\eta N\\) decay chain, mainly with \\(\\Delta(1700)3/2^-\\) and \\(\\Delta(1940)3/2^-\\). The ratios of the magnitude of the total cross sections also agree with this assumption. However, the absolute magnitudes of the cross sections are reduced by FSI effects with respect to free proton data.

We report on a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector obtained from a study of single p0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron. The results were obtained up to a neutron energy of 400 MeV. They are compared to previous measurements made more than 15 years ago at the pion beam at the BNL AGS.

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 measure the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the \\(\\Delta(1232)\\) 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 \\(\\gamma_{E1E1} = -3.5 \\pm 1.2\\), \\(\\gamma_{M1M1}= 3.16 \\pm 0.85\\), \\(\\gamma_{E1M2} = -0.7 \\pm 1.2\\), and \\(\\gamma_{M1E2} = 1.99 \\pm 0.29\\), in units of \\(10^{-4}\\) fm\\(^4\\).

There is a significant discrepancy between the values of the proton electric form factor, \\(G_E^p\\), extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of \\(G_E^p\\) from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (\\(\\varepsilon\\)) and momentum transfer (\\(Q^2\\)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing \\(\\varepsilon\\) at \\(Q^2 = 1.45 \\text{ GeV}^2\\). This measurement is consistent with the size of the form factor discrepancy at \\(Q^2\\approx 1.75\\) GeV\\(^2\\) and with hadronic calculations including nucleon and \\(\\Delta\\) intermediate states, which have been shown to resolve the discrepancy up to \\(2-3\\) GeV\\(^2\\).

Precise angular distributions have been measured for the first time for the photoproduction of \\(\\pi^{0}\\)-mesons off neutrons bound in the deuteron. The effects from nuclear Fermi motion have been eliminated by a complete kinematic reconstruction of the final state. The influence of final-state-interaction effects has been estimated by a comparison of the reaction cross section for quasi-free protons bound in the deuteron to the results for free protons and then applied as a correction to the quasi-free neutron data. The experiment was performed at the tagged photon facility of the Mainz Microtron MAMI with the Crystal Ball and TAPS detector setup for incident photon energies between \\(0.45\\)~GeV and \\(1.4\\)~GeV. The results are compared to the predictions from reaction models and partial-wave analyses based on data from other isospin channels. The model predictions show large discrepancies among each other and the present data will provide much tighter constraints. This is demonstrated by the results of a new analysis in the framework of the Bonn-Gatchina coupled-channel analysis which included the present data.