The four scalar degrees of freedom of the Standard Model, the Higgs and the longitudinal components of the Ws and Z, are amenable to different effective field theory descriptions. "SMEFT" wraps them up in a single Higgs doublet, whereas "HEFT" treats the Higgs and the Goldstones separately. We seek to understand the suitability of "SMEFT" and "HEFT" for describing the effects of possible heavy new physics in, e.g., LHC measurements.
We identify physical features that can only be described by HEFT, and thereby identify two classes of beyond the Standard Model physics for which a HEFT description of their low energy physics is required: i) those which contain extra sources of electroweak symmetry breaking, ii) those which contain particles getting most of their mass from the Higgs mechanism. We show how some "HEFTy" theories are still viable given current experimental constraints.