Recent results from the CBELSA/TAPS experiment at ELSA

In order to gain a better understanding of the dynamics inside the nucleon and of the non-perturbative regime of QCD, the nucleon excitation spectra and the properties of nucleon resonances are investigated. An essential experimental tool to achieve this goal is the study of different photoproduction reactions. Partial wave analyses are performed in order to obtain information about the contributing resonances. A complete experiment is needed to extract the underlying amplitudes unambiguously, which requires the measurement of carefully chosen single and double polarization observables in addition to the unpolarized cross section. The CBELSA/TAPS experiment in Bonn offers the possibility to measure several polarization observables using a linearly or circularly polarized photon beam and with a longitudinally or transversely polarized target. This contribution gives an overview of recently measured polarization observables in different final states. The impact of the new data is discussed.

π-N Drell-Yan Process in TMD Factorization

This article presents the review of the current understanding on the pion-nucleon Drell-Yan process from the point of view of the TMD factorization. Using the evolution formalism for the unpolarized and polarized TMD distributions developed recently, we provide the theoretical expression of the relevant physical observables, namely, the unpolarized cross section, the Sivers asymmetry, and the cos⁡2ϕ asymmetry contributed by the double Boer-Mulders effects. The corresponding phenomenology, particularly at the kinematical configuration of the COMPASS πN Drell-Yan facility, is displayed numerically.

Measurement of the proton scalar polarizabilities at MAMI

The electric (αE1) and magnetic (βM1) scalar polarizabilities describe the response of the nucleon to an applied electric or magnetic field. They are not only fundamental properties related to the internal structure and dynamics of the nucleon, but they are important also in other areas of physics, such as atomic structure. The values of αE1 and βM1 quoted by the Particle Data Group were determined using data on the unpolarized differential cross-section of the Compton scattering $\gamma p \to \gamma p$. The measurement of the beam asymmetry Σ3, provides an alternative approach to the extraction of the scalar polarizabilities, with different sensitivity and systematics compared to the unpolarized crosssection. This asymmetry was measured recently for the first time below the pion photoproduction threshold by the A2 Collaboration with the Crystal Ball/TAPS experiment at MAMI (Mainz, Germany). A new high precision measurement of both a unpolarized cross-section and beam asymmetry Σ3a is ongoing at MAMI and the polarizabilities αE1 and βM1 will be extracted with unprecedented precision.

STUDY OF THE η MESON PRODUCTION IN THE $\vec{d}+d\to{}^4He+\eta$ REACTION USING THE POLARIZED BEAM

The measurement of the η meson production in [Formula: see text] reaction has been performed at the Big Karl magnetic spectograph at COSY synchrotron in Jülich. The vector and tensor polarized deuetron beam at the excess energy of Q=16 MeV was used. The preliminary results for the unpolarized cross section are presented here.

TWO-PHOTON ANNIHILATION INTO PROTON-ANTIPROTON IN A QUARK-DIQUARK SCHEME

We consider [Formula: see text] in the framework of the Brodsky-Farrar-Lepage scheme allowing for (scalar) diquarks as quasi-elementary constituents. We compute the unpolarized cross section using different proton wave functions. Their normalization is fixed by computing the electromagnetic form factor of the proton in the same framework. Good agreement with the experimental data can be obtained. Two-photon annihilation into other baryons is also discussed.

SIMPLE REACTION STRUCTURES IN A "COMPLICATED" KINEMATIC RANGE

The energy range between 500 and 1000 MeV is usually considered a "complicated" one for elastic proton-proton scattering in which there is no simple description. It is shown that in that energy range, for much of the angular range, two simple features of the magnitudes and relative phases of the invariant amplitudes in the planar transverse frame give a good approximation of the observed data on unpolarized cross section as well as all the various polarization measurements. The features appear to be particularly pronounced at the larger scattering angles and higher energies, thus suggesting a connection with the small distance part of the internucleon force which thus may shed light on the validity of various quark models. Similar regularities also appear in a similar energy and angular range for pion-nucleon scattering. A simple way is presented on a quark level to summarize these regularities.