Automatic generation of quarkonium amplitudes in NRQCD

We present a simple method to automatically evaluate arbitrary tree-level amplitudes involving the production or decay of a heavy quark pair Q (Q) over bar in a generic L-2S+1(J)[1, 8], S- or P-wave state, i.e., the leading short distance coefficients appearing in the NRQCD factorization formalism. Our approach is based on extracting the relevant contributions from the open heavy quark-antiquark amplitudes through an expansion with respect to the quark-antiquark relative momentum and the application of suitable color and spin projectors. To illustrate the capabilities of the method and its implementation in MadGraph a few applications to quarkonium collider phenomenology are presented.