THE STRONG INTERACTION SHIFT AND WIDTH OF THE GROUND-STATE OF PIONIC HYDROGEN

The 3p-1s transition in pionic hydrogen was investigated with a high-r esolution crystal spectrometer system. From the precisely measured tra nsition energy, together with the (calculated) electromagnetic energy, the strong interaction shift of the 1s state was obtained as epsilon 1s = -7.127 +/- 0.028(stat.) +/- 0.036(syst.) eV (attractive). From th e natural line width, measured for the first time, we determine the de cay width of the 1s state: Gamma(1s)((decay)) = 0.97 +/- 0.10(stat.) /- 0.05(syst.) eV. With the recently calculated electromagnetic correc tions the s-wave scattering lengths of an isospin symmetric strong int eraction are deduced. The scattering length for elastic scattering of a negative pion on a proton is a(pi-p-->pi-p)(h) = 0.0885 +/- 0.0003(s tat.) +/- 0.0006(syst.)m(pi)(-1). The scattering length for single cha rge exchange is found to be a(pi-p-->pi 0n)(h) = -0.136 +/- 0.007(stat .) +/- 0.003(syst.)m(pi)(-1). The experiment was performed at the Paul Scherrer Institute (PSI) in Switzerland. A focussing crystal spectrom eter with an array of bent crystals, the cyclotron trap (a magnetic sy stem designed to increase the particle stop density) and a CCD (charge -coupled device) detector system were employed. The results from the p ionic hydrogen experiment - together with those from the pionic deuter ium experiment - were used to test the isospin symmetry of the strong interaction, The present data are still consistent with isospin symmet ry.