Q2-Dependence of the Drell-Hearn-Gerasimov integral
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The energy and four-momentum (Q2) dependence of the photo-absorption cross section on the proton is calculated for helicity −1/2 and −3/2 states. An effective Lagrangian model is used, formulated in terms of meson and baryon degrees of freedom, which obeys crossing symmetry, unitarity, Lorentz and gauge invariance. The difference in the cross sections for the two helicity states, the Drell-Hearn-Gerasimov integral IDHG(Q2), is evaluated at different Q2. We find that at small momentum transfer the absolute value of IDHG(Q2) first increases to reach a maximum at Q2≈ 0.05 GeV2 before decreasing at higher Q2.
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