We analyze virtual Compton scattering off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, once the irregular nucleon pole terms have been subtracted in a gauge-invariant fashion, is free of poles and kinematical zeros. The covariant treatment naturally allows one to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the ep → e ′ p ′ γ reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon. 13.60.Fz,14.20.Dh Typeset using REVT E X * This work was supported by the Deutsche Forschungsgemeinschaft (SFB 201)

We analyze polarized Compton scattering which provides information on the spin-structure of the nucleon. For scattering processes with photon energies up to 100 MeV the spin-structure dependence can be encoded into four independent parameters-the so-called spin-polarizabilities γi , i = 1...4 of the nucleon, which we calculate within the framework of the "small scale expansion" in SU(2) baryon chiral perturbation theory. Specific application is made to "forward" and "backward" spinpolarizabilities.

Abstract. Eta photo-and electroproduction off the nucleon is investigated in an effective lagrangian approach that contains Born terms and both vector meson and nucleon resonance contributions. In particular, we review and develop the formalism for coincidence experiments with polarization degrees of freedom. The different response functions appearing in single and double polarization experiments have been studied. We will present calculations for structure functions and kinematical conditions that are most sensitive to details of the lagrangian, in particular with regard to contributions of nucleon resonances beyond the dominant 5'11(1535) resonance.

We discuss the low-energy behavior of the spin-averaged amplitude of virtual Compton scattering off a nucleon. Based on gauge invariance, Lorentz invariance, and the discrete symmetries, it is shown that to first order in the frequency of the final real photon only two generalized polarizabilities appear. Different lowenergy expansion schemes are discussed and put into perspective. ͓S0556-2813͑97͒02201-2͔

Using the techniques of chiral effective field theories we evaluate the so called "generalized polarizabilities", which characterize the structure dependent components in virtual Compton scattering (VCS) off the nucleon as probed in the electron scattering reaction eN → e ′ N γ. Results are given for both spin-dependent and spin-independent structure effects to O(p 3 ) in SU(2) Heavy Baryon Chiral Perturbation Theory and to O(ǫ 3 ) in the SU(2) "Small Scale Expansion".