Chiral Dynamics in Pion-Photon Reactions: Habilitation

As the lightest particle of the strong force, the pion plays a central role in the field of strong interactions, and understanding its properties is of prime relevance for understanding the strong interaction in general. The low-energy behaviour of pions is of particular interest. Although the quark-gluon substructure and their quantum chromodynamics is not apparent then, this specific inner structure causes the presence of approximate symmetries in pion-pion interactions and in pion decays, which gives rise to the systematic description of processes involving pions in terms of few low-energy constants. Specifically, the chiral symmetry and its spontaneous and explicit breaking, treated in chiral perturbation theory (ChPT), leads to firm predictions for low-energy properties of the pion. To those belong the electromagnetic polarisabilities of the pion, describing the leading-order structure effect in pion Compton scattering. The research presented in this work is concerned with the interaction of pions and photons, including pion Compton scattering, and also the production of neutral and charged pions in pion-photon collisions. Common to all investigated processes is, that the collision energy is in the range below a few pion masses. This region is reigned by the chiral dynamics as said before. For all processes, predictions have been available, or were obtained along this work. From the year 2000 on, the COMPASS experiment at CERN has been prepared in several steps for precision measurements of such processes. The pion-photon interaction is studied through the Primakoff effect by scattering high-energetic pions off the quasi-real photons of a nuclear Coulomb field. The principal characteristic of Primakoff events is extremely small momentum transfer to the target nucleus, below $10^{-3}$\,GeV$^2/c^2$. Their reconstruction requires detectors for the charged and neutral particles at the limit of the physical feasibility. For high-precision charged-particle tracking, silicon microstrip detectors were built, and their operation continuously enhanced. Furthermore, a thorough understanding of QED effects is mandatory in order to really get a handle on the strong-interaction aspect. A good part of the present work was dedicated to solve the open QED issues for the measurements. From data taken with COMPASS in the year 2004, the Primakoff reaction of the process $\pi^-\gamma\rightarrow\pi^-\pi^-\pi^+$ could be isolated and analyzed in terms of the absolute cross-section. The published result confirms the leading-order ChPT prediction. More results could be obtained from the same data concerning the radiative widths of meson resonances, where that of the $\pi_2(1670)$ is a novel observation. The interplay of electromagnetic and strong effects could be observed with unprecedented detail and is subject to further investigations. In the year 2009 data have been taken that were shown in the present work to be of sufficiently high quality, along with a very detailed unterstanding of the apparative effects, such that the aimed-at determination of the pion polarisability is now in reach.