The strong interaction shift and width of the ground state of pionic hydrogen

• Published in: Nuclear physics. A Vol. 609; no. 3; pp. 269 - 309
• Main Authors: Sigg, D., Badertscher, A., Bogdan, M., Goudsmit, P.F.A., Leisi, H.J., Schröder, H.-Ch, Zhao, Z.G., Chatellard, D., Egger, J.-P., Jeannet, E., Aschenauer, E.C., Gabathuler, K., Simons, L.M., Rusi El Hassani, A.J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 18.11.1996
• ISSN: 0375-9474
• DOI: 10.1016/S0375-9474(96)00280-1

The 3p-1s transition in pionic hydrogen was investigated with a high-resolution crystal spectrometer system. From the precisely measured transition energy, together with the (calculated) electromagnetic energy, the strong interaction shift of the 1s state was obtained as ϵ 1s = −7.127 ± 0.028(stat.)± 0.036(syst.) eV (attractive). From the natural line width, measured for the first time, we determine the decaywidth of the 1s state: Γ 1s (decay) = 0.97 ± 0.10(stat.)± 0.05(syst.) eV. With the recently calculated electromagnetic corrections the s-wave scattering lengths of an isospin symmetric strong interaction are deduced. The scattering length for elastic scattering of a negative pion on a proton is a π − p→ π − p h = 0.0885±0.00003(stat.)±0.0006(syst.) m π −1. The scattering lengthe for single charge exchange is found to be a π − p→ π 0 n h = −0.136 ± 0.007(stat.) ± 0.003(syst.) m π −1. The experiment was performed at the Paul Scherrer Institute (PSI) in Switzerland. A focussing crystal spectrometer with an array of bent crystals, the cyclotron trap (a magnetic system designed to increase the particle stop density) and a CCD (charge-coupled device) detector system were employed. The results from the pionic hydrogen experiment — together with those from the pionic deuterium experiment — were used to test the isospin symmetry of the strong interaction. The present data are still consistent with isospin sysmmetry.