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HK: Physik der Hadronen und Kerne
HK 4: Elektromagnetische und hadronische Proben II
HK 4.2: Gruppenbericht
Montag, 19. März 2001, 17:00–17:30, C
Towards a new high–precision determination of the pion–nucleon scattering lengths — •M. Hennebach1, D.F. Anagnostopoulos2, S. Biri3, G. Borchert1, W. Breunlich4, J.-P. Egger5, D. Gotta1, A. Gruber4, P. Indelicato6, T. Jensen7,8, Y.-W. Liu9, B. Manil6, V. Markushin9, N. Nelms10, L.M. Simons9, and H. Zmeskal4 — 1IKP, FZ Jülich — 2Dept. of Material Science, Univ. of Ioannina — 3ATOMKI, Hung. Academie of Science, Debrecen — 4IMEP, Österr. Akademie der Wiss., Wien — 5Inst. of Phys., Univ. of Neuchâtel — 6Lab. Kastler-Brossel, Univ. P. et M. Curie, Paris — 7Aarhus Univ. — 8ETH Zürich — 9PSI, Villigen — 10Dept. of Phys. and Astr., Univ. of Leicester
A new series of precision experiments with exotic atom X–rays has been started at PSI. Pions are slowed down into a target cell by using the cyclotron trap to form pionic and muonic atoms. The resultant X–rays are reflected onto a large–area CCD by a Bragg spectrometer. The first aim is to improve the accuracy of the π− mass from 3 to 1ppm. This experiment has been finished in spring 2000. Analysis of the data is under way. The second aim is a new determination of the strong–interaction shift є1s and broadening Γ1s of the ground state in pionic hydrogen. This constitutes a direct measurement of the π N scattering lengths and is an important test of the methods of chiral perturbation theory. In a first step, we aim for a precision of 0.2 % for є1s and 3 % for Γ1s, where a new method of energy calibration is used: By measuring pionic hydrogen simultaneously with a calibration line from pionic oxygen, systematic errors are suppressed. Results from a recent engineering run will be shown; the production run will follow in spring 2001. The goal for the second step is an accuracy of 1 % for Γ1s which can be achieved by determining the spectrometer characteristics precisely with an ECR source and a study of the cascade effects in muonic hydrogen.