Amplitude bases in generic EFTs

Abstract: We present for the first time an efficient algorithm to find a basis of kinematically independent structures built of (massless and massive) spinor helicity variables in four dimensions. This method provides a classification of independent contact terms for the scattering amplitudes with generic masses, spins and multiplicity, in any effective field theory (EFT). These contact terms are in one-to-one correspondence with a complete set of irrelevant operators in the EFT. As basic applications of our method, we classify the $D^{2n} F^4$ contact terms in SU$(N)$ Yang-Mills theory for $n\leq 8$, dimension-six operators involving five $W^\pm$, $Z$ and $\gamma$ vector bosons, and spin-tidal effective interactions for spin-1 massive particles in gravitational theories.