Low-energy decomposition of Compton scattering amplitude on a hadron and single-time current commutators

The method of low-energy decomposition of the Compton scattering amplitude on a structured hadron with an arbitrary spin in the framework of the reduction formula is proposed. The key role in this decomposition is played by the single-time commutation relations of currents containing a Schwinger term and by the connection of the Schwinger term with the contact term of the Compton scattering amplitude. Expressions for generalized electric and magnetic polarizabilities versus current matrix elements are obtained