Observation of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:msup><mml:mi>η</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math> Cusp in the New Precise Beam Asymmetry <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="normal">Σ</mml:mi></mml:math> Data for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi><mml:mi>p</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:mi>p</mml:mi><mml:mi>η</mml:mi></mml:math>
F. Afzal, Y. Wunderlich, A. V. Anisovich, D. Bayadilov, R. Beck, M. Becker, E. Blanke, K.-Th. Brinkmann, S. Ciupka, V. Credé, M. Dieterle, H. Dutz, D. Elsner, S. Friedrich, F. Frommberger, A. Gridnev, M. Gottschall, M. Grüner, E. Gutz, Ch. Hammann, J. Hannappel, J. Hartmann, W. Hillert, J. Hoff, P. Hoffmeister, Christiane Honisch, T. C. Jude, H. Kalinowsky, F. Kalischewski, I. Keshelashvili, P. Klassen, Ф. Клейн, E. Klempt, K. Koop, Philipp Kroenert, B. Krusche, Μ. Lang, I. Lopatin, P. Mahlberg, Ulf-G. Meißner, F. Messi, V. Metag, W. Meyer, Bence Mitlasóczki, J. Müller, J. Müllers, M. Nanova, K. Nikonov, V. A. Nikonov, V. Novinskiy, R. Novotny, D. Piontek, G. Reicherz, L. Richter, D. Rönchen, T. Rostomyan, B. Salisbury, A. Sarantsev, D. Schaab, Christian Schmidt, H. Schmieden, J. Schultes, T. Seifen, V. Sokhoyan, C. Sowa, K. Spieker, N. Stausberg, A. Thiel, U. Thoma, T. Triffterer, M. Urban, G. Urff, H. van Pee, D. Walther, Ch. Wendel, U. Wiedner, A. Wilson, A. Winnebeck, L. Witthauer
Abstract
Data on the beam asymmetry $\mathrm{\ensuremath{\Sigma}}$ in the photoproduction of $\ensuremath{\eta}$ mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from $W=1748\text{ }\text{ }\mathrm{MeV}$ to $W=2045\text{ }\text{ }\mathrm{MeV}$). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the $p{\ensuremath{\eta}}^{\ensuremath{'}}$ threshold [${E}_{\ensuremath{\gamma}}=1447\text{ }\text{ }\mathrm{MeV}$ ($W=1896\text{ }\text{ }\mathrm{MeV}$)]. The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the $N(1895){\frac{1}{2}}^{\ensuremath{-}}$ nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.