Precision studies on strong interaction in pionic hydrogen

The goal of the research program of the Pionic Hydrogen Collaboration at the Paul Scherrer Institute is a precise determination of the shift and width of the picnic hydrogen ground state induced by strong interaction. A crystal spectrometer is used to measure the X-ray transitions to the ground state in order to extract the strong interaction shift and width with a precision of similar to 0.2% and similar to 1% respectively. The improvement in the precision of the width will amount to almost an order of magnitude compared to the most precise former experiment. The isospin-dependent pion-nucleon scattering lengths can be determined from the shift and width. The high intensity pion beam available at PSI, a cyclotron trap and a high resolution crystal spectrometer provide optimal experimental conditions. For determining the resolution function of the crystal spectrometer the Bragg crystals are characterized using the high intensity X-rays from He-like ions produced in an electron cyclotron resonance ion trap. Measurements of X-ray de-excitation in muonic hydrogen - an exotic atom without strong interaction - were performed to study the effect of Doppler broadening due to Coulomb transition within the electromagnetic cascade which is an important systematic effect in picnic hydrogen and has to be understood in detail. This will allow the determination of the strong interaction width with unsurpassed accuracy, thus resulting in a precise value of the isovector pion-nucleon scattering length. The present status of the experiment and the preliminary results on the strong interaction shift and width are presented.