Characterization of the (fractional) Malliavin-Watanabe-Sobolev spaces ${\mathcal{D}}^{\alpha ,2}$ via the Bargmann-Segal norm

Motivated by an open question going back to P.Malliavin and P.-A.Meyer (and closely related to the foundational work of S.Watanabe) on whether Malliavin-Watanabe-Sobolev regularity admits a characterization in terms of a holomorphic Laplace image similar as for Hida distributions, we establish a characterization of the spaces ${\mathcal{D}}^{\alpha ,2}$ for all $\alpha \in \mathbb{R}$ via the Bargmann-Segal norm of the $S$-transform. More precisely, we express ${\mathcal{D}}^{\alpha ,2}$-regularity, $\alpha >0$, of $F\in L^2(\mu )$, as well as dual regularity of distributions, in terms of integrability, differentiability and growth properties of the function $$(0,1)\ni \lambda ⟼{\int }_{{\mathcal{S}}_{\mathbb{C}}^{\mathrm{\prime }}}\mathrm{\mid }SF(\lambda u){\mathrm{\mid }}^{2}d\nu (u)$$ involving integer-order derivatives in $\lambda$ for $\alpha \in \mathbb{N}$ and Riemann-Liouville fractional derivatives/integrals for non-integer $\alpha$. Here $\nu$ is the Gaussian Bargmann-Segal measure. This yields practical criteria for both positive and negative (including fractional) orders of Malliavin regularity and thereby bridges Malliavin calculus and Bargmann-Segal techniques from white noise analysis. Applications are worked out for Donsker’s delta, self-intersection local times of Gaussian processes, and Gauss kernels.