Polarization-dependent disappearance of a resonance signal: Indication for optical pumping in a storage ring? oleh W. Nörtershäuser, A. Surzhykov, R. Sánchez, B. Botermann, G. Gwinner, G. Huber, S. Karpuk, T. Kühl, C. Novotny, S. Reinhardt, G. Saathoff, T. Stöhlker, A. Wolf