  In terms of flavor invariants, we establish the intimate connection between
leptonic CP violation in the canonical seesaw model for neutrino masses and
that in the seesaw effective field theory (SEFT). For the first time, we
calculate the Hilbert series and explicitly construct the primary flavor
invariants in the SEFT by considering both the dimension-five Weinberg operator
${\cal O}^{\alpha \beta}_5 = \overline{\ell^{}_{\alpha \rm L}} \widetilde{H}
\widetilde{H}^{\rm T} \ell^{\rm C}_{\beta \rm L}$ and the dimension-six
operator ${\cal O}^{\alpha \beta}_6 = \overline{\ell^{}_{\alpha \rm L}}
\widetilde{H} {\rm i} \not\!\partial \widetilde{H}^\dagger \ell^{}_{\beta \rm
L}$ at the tree-level matching. The inclusion of only the Wilson coefficients
$C^{\alpha \beta}_5$ and $C^{\alpha \beta}_6$ already enables the SEFT to
incorporate all physical information about the full seesaw model. Moreover, the
minimal sufficient and necessary conditions for CP conservation both in the
SEFT and in the full theory are clarified, and the matching between the flavor
invariants in both theories is accomplished. Through the matching of flavor
invariants, the CP asymmetries necessary for successful leptogenesis are
directly linked to those in neutrino-neutrino and neutrino-antineutrino
oscillations at low energies. Surprisingly, it is revealed that the precise
measurements of $C^{\alpha \beta}_5$ and $C^{\alpha \beta}_6$ in low-energy
experiments are powerful enough to probe the full seesaw model, including CP
violation for cosmological matter-antimatter asymmetry.
