We briefly review a variant of functional matching to derive
an Effective Field Theory (EFT) for heavy particles at the one-loop level
in the top-down approach.
The method integrates out heavy fields that correspond to mass eigenstates,
i.e. after removing mixing effects by diagonalizing mass matrices.
Tree- and loop-level effects are separated by employing the background-field method,
hard and soft modes are separated with the use of the expansion by regions.
The method is exemplified for the Higgs Singlet Extension of the Standard Model where the
mass $M_H$ of the additional Higgs boson is considered large, and the Higgs
mixing angle $\alpha$ is assumed to scale like $1/M_H$, in order to
guarantee decoupling in the large-$M_H$ limit.
Our calculation is agnostic
w.r.t. the type (SMEFT vs. HEFT) of the emerging EFT.
Eventually the emerging EFT Lagrangian can be transformed into SMEFT form,
but only at the cost of introducing fermionic EFT operators, although no such
operators are directly generated upon solving the functional integral
over the heavy Higgs field.