학술논문
Evaluation of the E2/M1 ratio in the $N\to \Delta(1232)$ transition from the $ \vec{\gamma} \vec{p} \to p \pi^0 $ reaction
Document Type
Working Paper
Author
Mornacchi, E.; Pedroni, P.; Afzal, F.; Wunderlich, Y.; Abt, S.; Achenbach, P.; Annand, J. R. M.; Arends, H. J.; Bashkanov, M.; Biroth, M.; Beck, R.; Borisov, N.; Braghieri, A.; Briscoe, W. J.; Cividini, F.; Collicott, C.; Denig, A.; Dolzhikov, A. S.; Downie, E.; Fegan, S.; Fix, A.; Ghosal, D.; Gorodnov, I.; Gradl, W.; Gurevich, D.; Heijkenskjoeld, L.; Hornidge, D.; Huber, G. M.; Kashevarov, V. L.; Kay, S. J. D.; Korolija, M.; Lazarev, A.; Livingston, K.; Lutterer, S.; MacGregor, I. J. D.; Manley, D. M.; Martel, P. P.; Miskimen, R.; Mocanu, M.; Mullen, C.; Neganov, A.; Neiser, A.; Oberle, M.; Ostrick, M.; Otte, P. B.; Paudyal, D.; Powell, A.; Rostomyan, T.; Sokhoyan, V.; Spieker, K.; Steffen, O.; Strakovsky, I. I.; Thiel, M.; Thomas, A.; Usov, Yu. A.; Wagner, S.; Watts, D. P.; Werthmueller, D.; Wettig, J.; Witthauer, L.; Wolfes, M.; Zachariou, N.
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Subject
Language
Abstract
A new data set for the helicity-dependent differential cross section of the single-meson photoproduction reaction $\gamma p \to p \pi^{0}$ was obtained for the photon energy interval 150-400 MeV. The experiment was performed at the A2 tagged photon facility of the Mainz Microtron MAMI using a circularly polarized photon beam and a longitudinally polarized proton target. The reaction products were detected with the large acceptance Crystal Ball/TAPS calorimeter covering 97\% of the full solid angle. These new results, obtained with a fine energy and polar angle binning, greatly increase both the existing quantity and quality of the data available for this observable. A moment analysis, based on a finite expansion in Legendre polynomials, was applied to these data by using a bootstrap-based fitting method to correctly account for their systematic uncertainties. From the resulting decomposition of the differential cross sections, the $E2/M1$ ratio for the $N\to \Delta(1232)$ transition was determined to be $[-2.38 \pm 0.16{\hbox{ (stat.+sys.)}} \pm 0.10 {\hbox{ (model)}}] \%$. Combining this value with previous results also allowed us to evaluate the most precise available estimate of the $E2/M1$ ratio to be used for all further reference and model comparisons.
Comment: 16 pages, 14 figures
Comment: 16 pages, 14 figures