(A2 Collaboration at MAMI) Afzal, F; Spieker, K; Hurck, P; Abt, S; Achenbach, P; Adlarson, P; Ahmed, Z; Akondi, CS; Annand, JRM; Arends, HJ; Bashkanov, M; Beck, R; Biroth, M; Borisov, N; Braghieri, A; Briscoe, WJ; Cividini, F; Collicott, C; Costanza, S; Denig, A; Dieterle, M; Downie, EJ; Drexler, P; Fegan, S; Gardner, S; Ghosal, D; Glazier, DI; Gorodnov, I; Gradl, W; Gurevich, D; Heijkenskjöld, L; Hornidge, D; Huber, GM; Kashevarov, VL; Kay, SJD; Korolija, Marina; Krusche, B; Lazarev, A; Livingston, K; Lutterer, S; MacGregor, IJD; Macrae, RG; Manley, DM; Martel, PP; Miskimen, R; Mocanu, M; Mornacchi, E; Mullen, C; Neganov, A; Neiser, A; Oberle, M; Ostrick, M; Otte, PB; Paudyal, D; Pedroni, P; Powell, A; Reicherz, G; Rostomyan, T; Sfienti, C; Sokhoyan, V; Steffen, O; Strakovsky, II; Strub, T; Supek, Ivan; Thiel, A; Thiel, M; Thomas, A; Usov, YA; Wagner, S; Walford, NK; Watts, DP; Werthmueller, D; Wettig, J; Witthauer, L; Wolfes, M; Zachariou, N (2024) . Physical Review Letters, 132 (12). ISSN 0031-9007

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Abstract

We report the measurement of the helicity asymmetry E for the pa-degrees and na-+ 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 pa-degrees final state (230-842 MeV for the na-+ final state) and the full range of pion polar angles, 0, providing the most precise measurement of the observable E. A moment analysis gives a clear observation of the pri cusp in the pa-degrees final state.

Item Type:	Article
Uncontrolled Keywords:	pion ; helicity asymmetry ; polarized photon beam ; Crystal Ball experiment at MAMI
Subjects:	NATURAL SCIENCES > Physics
Divisions:	Division of Experimental Physics
Depositing User:	Ema Buhin Šaler
Date Deposited:	13 Apr 2026 07:04
URI:	http://fulir.irb.hr/id/eprint/11657
DOI:	10.1103/PhysRevLett.132.121902

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