We present a simple model for the γN → ππN reaction which reproduces the cross sections of the π + π − p, π + π − n, π + π 0 n and π − π 0 p channels over the range of the energies 0.41−0.85 GeV. We use the dynamical model for the resonances, ∆(1232), N * (1520) and ρ-meson. The total photoabsorption off a nucleon is also discussed.
The Coulomb interaction among massless Dirac fermions in graphene is unscreened around the isotropic Dirac points, causing a logarithmic velocity renormalization and a cone reshaping. In less symmetric Dirac materials possessing anisotropic cones with tilted axes, the Coulomb interaction can provide still more exotic phenomena, which have not been… (More)