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Pionic Hydrogen: Status and Outlook

D. F. Anagnostopoulos$^{1}$, S. Biri$^{2}$, G. Borchert$^{3}$, W. H. Breunlich$^{4}$, M. Cargnelli$^{4}$, J.-P. Egger$^{5}$, B. Gartner$^{4}$, D. Gotta$^{3}$, P. Hauser$^{6}$, M. Hennebach$^{4}$, P. Indelicato$^{7}$, T. Jensen$^{6}$, R. King$^{4}$, F. Kottmann$^{8}$, B. Lauss$^{4}$, Y. W. Liu$^{6}$, V. E. Markushin$^{6}$, J. Marton$^{4}$, N. Nelms$^{9}$, G. C. Oades$^{10}$, G. Rasche$^{11}$, P. A. Schmelzbach$^{6}$, L. M. Simons$^{6}$, J. Zmeskal$^{4}$

$^{1}$ Department of Material Science, University of Ioannina, GR-45110 Ioannina, Greece
$^{2}$ Institute of Nuclear Research (ATOMKI) of the Hungarian Academy of Sciences, H-4001,Debrecen, Hungary
$^{3}$ Institut für Kernphysik, Forschungszentrum Jülich, D-52425 Jülich, Germany
$^{4}$ Institut für Mittelenergiephysik,A-1090 Wien, Österreichische Akademie der Wissenschaften, Austria
$^{5}$ Institut de Physique de l'Université de Neuchâtel, CH-2000 Neuchâtel, Switzerland
$^{6}$ Paul-Scherrer-Institut (PSI), CH-5232 Villigen, Switzerland
$^{7}$ Laboratoire Kastler-Brossel, Université Pierre et Marie Curie, F-75252 Paris, France
$^{8}$ Institut für Teilchenphysik, ETH Zürich, CH-8057 Zürich , Switzerland
$^{9}$ Department of Astronomy and Physics, University of Leicester, Leicester LEI7RH, England
$^{10}$ Institute of Physics, Aarhus University, DK-8000 Aarhus, Denmark
$^{11}$ Institut für Theoretische Physik, Universität Zürich, Switzerland


The measurement of the strong interaction shift and width of the ground state in the pionic hydrogen atom determines two different linear combinations of the two isospin separated s-wave scattering lengths of the pion nucleon system. If both quantities are measured with a precision of about $1\%$ a stringent test of chiral perturbation theory and a determination of the pion nucleon coupling constant can be obtained. Past measurements determined the shift with an accuracy better than $1\%$, and the width with an accuracy of $9\%$. Additional information from pionic deuterium measurements has been used in order to extract isospin separated scattering lengths with sufficient accuracy. Future measurements plan to directly measure the width of pionic hydrogen with an accuracy on the level on $1\%$.

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Pionic Hydrogen Collaboration