The Soft Régime in NRQCD

A Lagrangean and a set of Feynman rules are presented for Non-Relativistic QCD with manifest power counting in the heavy quark velocity v. A régime is identified in which energies and momenta are of order Mv. It is neither identical to the ultrasoft régime corresponding to radiative processes with energies and momenta of order Mv2, nor to the potential régime with on shell quarks and gluons providing the Coulomb binding. In this soft régime, gluons are on shell, and the quark propagator becomes static. Examples show that it contributes to oneand two-loop corrections of scattering and production amplitudes near threshold. The results are readily generalised to any effective field theory with more than one low energy scale. Talk presented at the workshop “Nuclear Physics with Effective Field Theories” at Caltech, 26th – 27th February 1998. Email: [email protected]