Photoproduction of pi(0) mesons off protons and neutrons in the second and third nucleon resonance regions
Date Issued
2018-01-01
Author(s)
Dieterle, M.
Werthmueller, D.
Abt, S.
Afzal, F.
Bartolome, P. Aguar
Ahmed, Z.
Ahrens, J.
Annand, J. R. M.
Arends, H. J.
Bashkanov, M.
Beck, R.
Biroth, M.
Borisov, N.
Braghieri, A.
Briscoe, W. J.
Cherepnya, S.
Cividini, F.
Collicott, C.
Costanza, S.
Denig, A.
Downie, E. J.
Drexler, P.
Fil`kov, L. V.
Garni, S.
Glazier, D. I.
Gorodnov, I.
Gradl, W.
Guenther, M.
Gurevich, D.
Heijkenskjoeld, L.
Hornidge, D.
Huber, G. M.
Kaeser, A.
Kashevarov, V. L.
Kay, S.
Keshelashvili, I.
Kondratiev, R.
Korolija, M.
Lazarev, A.
Lisin, V.
Livingston, K.
Lutterer, S.
MacGregor, I. J. D.
Manley, D. M.
Martel, P. P.
McGeorge, J. C.
Metag, V.
Middleton, D. G.
Miskimen, R.
Mornacchi, E.
Mushkarenkov, A.
Neganov, A.
Neiser, A.
Oberle, M.
Ostrick, M.
Otte, P. B.
Oussena, B.
Paudyal, D.
Pedroni, P.
Polonski, A.
Prakhov, S. N.
Ron, G.
Rostomyan, T.
Sarty, A.
Sfienti, C.
Sokhoyan, V.
Spieker, K.
Steffen, O.
Strakovsky, I. I.
Strub, T.
Supek, I.
Thiel, A.
Thiel, M.
Thomas, A.
Unverzagt, M.
Usov, Yu. A.
Wagner, S.
Walford, N. K.
Watts, D. P.
Wettig, J.
Witthauer, L.
Wolfes, M.
Zana, L. A.
DOI
10.1103/physrevc.97.065205
Abstract
Background: Photoproduction of mesons off quasifree nucleons bound in the deuteron allows us to study the electromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets.Purpose: Study experimentally single pi(0) photoproduction off quasifree nucleons from the deuteron. Investigate nuclear effects by a comparison of the results for free protons and quasifree protons. Use the quasifree neutron data (corrected for nuclear effects) to test the predictions of reaction models and partial wave analysis (PWA) for gamma n -> n pi(0) derived from the analysis of the other isospin channels.Methods: High statistics angular distributions and total cross sections for the photoproduction of pi(0) mesons off the deuteron with coincident detection of recoil nucleons have been measured for the first time. The experiment was performed at the tagged photon beam of the Mainz Microtron (MAMI) accelerator for photon energies between 0.45 and 1.4 GeV, using an almost 4 pi electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion.Results: Significant effects from final-state interactions (FSI) were observed for participant protons in comparison to free proton targets (between 30% and almost 40%). The data in coincidence with recoil neutrons were corrected for such effects under the assumption that they are identical for participant protons and neutrons. Reaction model predictions and PWA for gamma n -> n pi(0), based on fits to data for the other isospin channels, disagreed between themselves and no model provided a good description of the new data.Conclusions: The results demonstrate clearly the importance of a measurement of the fully neutral final state for the isospin decomposition of the cross section. Model refits, for example from the Bonn-Gatchina analysis, show that the new and the previous data for the other three isospin channels can be simultaneously described when the contributions of several partial waves are modified. The results are also relevant for the suppression of the higher resonance bumps in total photoabsorption on nuclei, which are not well understood.