Heavy Quark Effective Theory, Interpolating Fields and Bethe-Salpeter   Amplitudes
  We use the LSZ reduction theorem and interpolating fields, alongwith the heavy quark effective theory, to investigate the structure of the Bethe-Salpeter amplitude for heavy hadrons. We show how a simple form of this amplitude, used extensively in heavy hadron decay calculations, follows naturally upto $O(1/M)$ from these field theoretic considerations. 
F. Hussain and G. Thompson