The goal of the experiment is to measure the strong interaction ground state shift and width in pionic hydrogen with a high resolution crystal spectrometer. The experiment will concentrate on the improvement of the precision of the width measurement by one order of magnitude. The knowledge of the value for the shift will be improved also by almost an order of magnitude because of better statistics and by extensive studies of sources of systematic errors like the pressure dependence of the measured shift.
It will be demonstrated below that the combination of the cyclotron trap II and spherically bent crystals with an extended CCD detector provides a sufficient resolution with acceptable rates and measuring times.
The main difficulty in the experiment is to quantify the influence of Doppler broadening on the line shape caused by Coulomb deexcitation. One way to solve this problem would be to perform the measurement with transitions from high levels where no sizable Coulomb acceleration can occur. Unfortunately the yield of these transtions is too low for a high statistics study of the line broadening. Also using low pressures will not eliminate the Doppler broadening completely as recent experiments  show the Coulomb acceleration still exists in the mbar region. The high statistics experiment is thus restricted to pressures and to the transitions from for pionic hydrogen and for muonic hydrogen because of intensity reasons.
The present experiment will be performed with the and the transitions in pionic hydrogen. The corresponding transitions in muonic hydrogen will be measured simultaneously with a special arrangement of two crystals working with different reflection planes. This saves beam time and, more importantly guarantees that no systematic error occurs because of change of experimental parameters. The effect of the Doppler broadening as determined from the muonic measurement will deliver the input parameters for the fitting of the pionic lines as discussed in chapter 2.2.4. The soundness of this procedure is then checked by comparing the results for the width extracted from both the and the transitions measured at different pressures, typically between and . The strong interaction width must be independent of the transition line and the pressure if all corrections have been treated properly . This will also allow us to check that other minor effects like the X-ray broadening due to transitions during the Stark collisions do not affect the accuracy of the measurement.
The measurement of higher transitions in pionic hydrogen, like the transition and transitions at the series limit, are presently not considered as they require different Bragg crystals, but they could be the subject of a following proposal.
As we heavily rely on the two crystal method for a simultaneous measurement of pionic and muonic atoms, we propose to do extensive preparatory experiments with electronic X-rays before setting up the experiment at the pion beam. These experiments are planned to take place in the NA hall at PSI. They will use an ECR source and fluorescence X-rays as mentioned in chapter 4.1. The corresponding experimental set-up will be described in chapter 6.1.