The pionic hydrogen experiment at PSI*) D. GOTTA, M. HENNEBACH t) Institut f~r Kernphysik, Forschungszentrum dfilich, D-52425 Jfilich Y.-W. LIu, V. MhRKUSHIN,L.M. SIMONS Paul-Scherrer-Institut (PSI), CH-5232 Villigen, Switzerland H. FUHRMANN, A. GRUBER, J. ZMESKAL IMEP, Osterreichische Akademie der Wissenschaften, A -1090 Vienna, Austria e. INDELICATO, B. MANIL Laboratoire Kastler-Brossel, Universitd Pierre et Marie Curie, F-75252 Paris, France : D. ANAGNOSTOPOULOS Department of Material Science, University of loannina, GR-45110 Ioannina, Greece N. NELMS Dept. of Physics and Astronomy, University of Leicester, Leicester LEITRH, England A new high-precision determination of the strong-interaction shift (s and broad- ening (Fls) of the ground state in pionic hydrogen (TrH) has been started at PSI. This constitutes a direct measurement of the zrN scattering lengths and is an important test of the methods of chiral perturbation theory. The experiment will be conducted in two steps. In the first step we aim for a precision of 0.2 % for el~ and 3 % for FI~. A new method of energy calibration is used: by measuring ~H simultaneously with pionic oxygen (TrO), systematic errors are suppressed. The goal for the second step is an accuracy of 1% for FI~. PACS: 36.10, 13.75.G Key words: exotic atoms, pion-nucleon interaction, x-ray spectroscopy 1 Introduction Pionic hydrogen offers an opportunity to investigate the pion-nucleon (TrN) in- teraction directly at relative energy zero. Disregarding isospin-breaking effects, the s-wave 7rN interaction at threshold can be completely described by the isoscalar and isovector scattering lengths a + and a-. These, in turn, are related to the ex- perimentally accessible quantities el~ and rls by Deser-type formulae [1, 2]. If one *) Talk given at the XIVth Summer School "Understanding the Structure of Hadrons', Prague (Czech Republic), July 9 - 13, 2001. t) corresponding author Czechoslovak Journal of Physics, Vol. 52 (2002), Suppl. B B169