ABSTRACT
I shall try to make the rule for the propagator less artificial by connecting it with something you already know. Consider the π – K scattering example (Fig. 17-1). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429492501/e6a0c676-1f8d-4684-97a0-6bf3353f7ce0/content/fig17_1.tif"/>
