Transverse momentum dependent operator expansion at next-to-leading power

Abstract: We develop a method of transverse momentum dependent (TMD) operator expansion that yields the TMD factorization theorem on the operator level. The TMD operators are systematically ordered with respect to TMD-twist, which allows a certain separation of kinematic and genuine power corrections. The process dependence enters via the boundary conditions for the background fields. As a proof of principle, we derive the TMD factorization up to the next-to-leading power $(\sim q_T/Q)$ at the next-to-leading order for any process with two detected states.