Invariant operators in collinear effective theory

We consider processes which produce final state hadrons whose energy is muc h greater than their mass. In this limit interactions involving collinear f ermions and gluons are constrained by a symmetry, and we give a general set of rules for constructing leading and subleading invariant operators. Wils on coefficients C(mu, (P) over bar) are functions of a label operator (P) o ver bar, and do not commute with collinear fields. The symmetry is used to reproduce a two-loop result for factorization in B --> D pi in a simple way . (C) 2001 Published by Elsevier Science B.V.