Abstract
An experiment to measure the strong interaction width (Γ 1s ,) and shift (ε 1s ) of the ground state of pionic hydrogen (π - p), in order to determine the isospin separated scattering lengths (a + and a - ) with a relative accuracy of better than 10 -2 , has been undertaken at the Paul Scherrer Institute (PSI). This measurement is based on high-precision X-ray spectroscopy of the np → 1s transitions pionic hydrogen (π - p). A preliminary analysis of the data taken in December 2000 is presented.