Photonuclear Reactions at Intermediate Energies
The characteristic feature of the pion-nucleon interaction at intermediate energies is the excitation of the ∆-resonance, the lowest excited state of the nucleon. This can clearly be seen by looking, for example, at the total π-N cross section as a function of the pion energy (Fig. 1a). This resonance occurs when the pion and nucleon are in a relative p-wave, coupled to a total spin J = 3/2 and isospin T = 3/2. Also the interaction of a photon with a nucleon at these energies is dominated by ∆-excitation. This is illustrated by Fig. 1b, which shows the total photoabsorption cross section for a proton. The ∆-resonance is characterized by a resonance energy ER = 1232 MeV (also referred to as the ∆-mass) and a large decay width of Γ = 110 MeV, which indicates that the ∆ quickly decays again into a pion and a nucleon. In the quark picture, the excitation of a nucleon to a ∆ is described by a spin- and isospin-flip of one of the quarks. For the discussion below this more fundamental explanation of the ∆-resonance is not essential and a more phenomenological approach is chosen.
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