Expansion of the boost operator in powers of the coupling constant

The representation of the boost operator as an ordered exponential is analyzed. A method of expanding this operator in powers of the coupling constant of interest in the description of the interaction of composite systems with large momenta is given. This expansion yields a mixed representation for the form factor of the composite particles; in it simultaneous use is made of the bound-state wave functions of ordinary field theory and null-plane field theory. The connection with calculations in the Pz → ∞ frame with longitudinal momentum transfer (q+ ≠ 0) is discussed for the example of the calculation of the asymptotic behavior