Measurement of the beam-helicity asymmetry I-circle dot in the photoproduction of pi(0)pi(+/-) pairs off protons and off neutrons

Crystal Ball MAMI Collaboration, Crystal Ball TAPS Collaboration, Crystal Ball A2 Collaboration, M. Oberle*, J. Ahrens, J. R. M. Annand, H. J. Arends, K. Bantawa, P. A. Bartolome, R. Beck, V. Bekrenev, H. Berghauser, A. Braghieri, D. Branford, W. J. Briscoe, J. Brudvik, S. Cherepnya, B. Demissie, M. Dieterle, E. J. DownieP. Drexler, L. V. Fil'kov, A. Fix, D. I. Glazier, E. Heid, D. Hornidge, D. Howdle, G. M. Huber, O. Jahn, I. Jaegle, T. C. Jude, A. Kaeser, V. L. Kashevarov, I. Keshelashvili, R. Kondratiev, M. Korolija, S. P. Kruglov, B. Krusche, A. Kulbardis, V. Lisin, K. Livingston, I. J. D. MacGregor, Y. Maghrbi, J. Mancell, D. M. Manley, Z. Marinides, M. Martinez, J. C. McGeorge, E. McNicoll, D. Mekterovic, V. Metag, S. Micanovic, D. P. Watts

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions gamma p -> n pi(0)pi(+) off free protons and gamma d -> (p)p pi(0)pi(-) and gamma d -> (p)n pi(0)pi(+) off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from pi(0) mesons were detected in the 4 pi electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.

Original languageEnglish
Article number54
Number of pages18
JournalEuropean Physical Journal A: Hadrons and Nuclei
Volume50
Issue number3
DOIs
Publication statusPublished - 10 Mar 2014

Keywords

  • DOUBLE-PION-PHOTOPRODUCTION
  • TAGGED PHOTON SPECTROMETER
  • CROSS-SECTION MEASUREMENT
  • 2ND RESONANCE REGION
  • PI(0) PHOTOPRODUCTION
  • BARYON RESONANCES
  • PARTIAL-WAVE
  • MICROTRON
  • THRESHOLD
  • DEUTERON

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