Gerasimov-Drell-Hearn sum rule and nucleon structure
  A modification of the Gerasimov--Drell--Hearn sum rule suggested by the current experimental data is presented. Within the conventional theoretical framework, we find it necessary to consider the possibility of the presence of a localized region inside a nucleon, in which the electromagnetic (EM) gauge symmetry is spontaneous broken down, if the constraints of the gauge invariance, Lorentz invariance and the assumption of the commutativity of the EM charge density operator at equal-time are considered. We also discuss the propagation of a virtual photon inside a nucleon under such a scenario. A comment on some of the recent model independent works on the same subject is provided. 
S. Ying