First measurement using elliptically polarized photons of the double-polarization observable $E$ for $\gamma p \to p \pi^0$ and $\gamma p \to n \pi^+$

Author:

A2 Collaboration, F. Afzal, K. Spieker, P. Hurck, S. Abt, P. Achenbach, P. Adlarson, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, E. J. Downie, P. Drexler, S. Fegan, S. Gardner, D. Ghosal, D. I. Glazier, I. Gorodnov, W. Gradl, D. Gurevich, L. Heijkenskjöld, D. Hornidge, G. M. Huber, V. L. Kashevarov, S. J. D. Kay, M. Korolija, B. Krusche, A. Lazarev, K. Livingston, S. Lutterer, I. J. D. MacGregor, R. G. Macrae, D. M. Manley, P. P. Martel, R. Miskimen, M. Mocanu, E. Mornacchi, C. Mullen, A. Neganov, A. Neiser, M. Oberle, M. Ostrick, P. B. Otte, D. Paudyal, P. Pedroni, A. Powell, G. Reicherz, T. Rostomyan, C. Sfienti, V. Sokhoyan, O. Steffen, I. I. Strakovsky, T. Strub, I. Supek, A. Thiel, M. Thiel, A. Thomas, Yu. A. Usov, S. Wagner, N. K. Walford, D. P. Watts, D. Werthmüller, J. Wettig, L. Witthauer, M. Wolfes, N. Zachariou

Keyword:

Nuclear Experiment, Nuclear Experiment (nucl-ex)

journal:

date:

2024-02-08 00:00:00

Abstract

We report the measurement of the helicity asymmetry $E$ for the $p\pi^0$ and $n\pi^+$ final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g.~$G$) and circularly polarized photons (e.g.~$E$) and a longitudinally polarized target. The new data cover a photon energy range 270 - 1400 MeV for the $p\pi^0$ final state (230 - 842 MeV for the $n\pi^+$ final state) and the full range of pion polar angles, $\theta$, providing the most precise measurement of the observable $E$. A moment analysis gives a clear observation of the $p\eta$ cusp in the $p\pi^0$ final state.

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