Helicity dependence of gamma p → N pi below 450 MeV and contribution to the Gerasimov-Drell-Hearn sum rule
The helicity dependence of the single pion photoproduction on the proton has been measured in the energy range from 200 to 450 MeV for the first time. The experiment, performed at the Mainz microtron MAMI, used a 4 pi-detector system, a circularly polarized, tagged photon beam, and a frozen-spin target. The data obtained provide new information for multipole analyses of pion photoproduction and determine the main contributions to the Gerasimov-Drell-Hearn sum rule and the forward spin polarizability gamma(0).
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ABSTRACT: The upgraded MAMI C accelerator is delivering electrons with an energy of 1558 MeV routinely. The A2 collaboration is doing experiments with energy marked polarised real photons produced via "Bremsstrahlung". Recent results from the GDH-experiment using a longitudinal polarised frozen spin target in combination with the DAPHNE detector and from the Crystal Ball experiment at MAMI are presented. © 2009 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.
Article: Baryon spectroscopy[Show abstract] [Hide abstract]
ABSTRACT: About 120 baryons and baryon resonances are known, from the abundant nucleon with $u$ and $d$ light-quark constituents up to the recently discovered $\Omega_b^-=bss$, and the $\Xi_b^-=bsd$ which contains one quark of each generation. In spite of this impressively large number of states, the underlying mechanisms leading to the excitation spectrum are not yet understood. Heavy-quark baryons suffer from a lack of known spin-parities. In the light-quark sector, quark-model calculations have met with considerable success in explaining the low-mass excitations spectrum but some important aspects like the mass degeneracy of positive-parity and negative-parity baryon excitations are not yet satisfactorily understood. At high masses, above 1.8 GeV, quark models predict a very high density of resonances per mass interval which is not observed. In this review, issues are identified discriminating between different views of the resonance spectrum; prospects are discussed how open questions in baryon spectroscopy may find answers from photo- and electro-production experiments which are presently carried out in various laboratories. Comment: Review article, 53 pages, 40 figures, 23 Tables. Review of Modern Physics (accepted for publication)
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ABSTRACT: A measurement of the helicity dependence of the total inclusive photoabsorption cross section on the deuteron was carried out at MAMI (Mainz) in the energy range 200 < E gamma < 800 MeV. The experiment used a 4 pi detection system, a circularly polarized tagged photon beam and a frozen spin target which provided longitudinally polarized deuterons. These new results are a significant improvement on the existing data and allow a detailed comparison with state-of-the-art calculations. (c) 2009 Elsevier B.V. All rights reserved.
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