Coulomb scattering in the massless Nelson model III. Ground state wave functions and non-commutative recurrence relations

Abstract

Let $H_{P,σ}$ be the single-electron fiber Hamiltonians of the massless Nelson model at total momentum $P$ and infrared cut-off $σ>0$. We establish detailed regularity properties of the corresponding $n$-particle ground state wave functions $f^n_{P,σ}$ as functions of $P$ and $σ$. In particular, we show that \[ |\partial_{P^j}f^{n}_{P,σ}(k_1,\ldots, k_n)|, \ \ |\partial_{P^j} \partial_{P^{j'}} f^{n}_{P,σ}(k_1,\ldots, k_n)| \leq \frac{1}{\sqrt{n!}} \frac{(cλ_0)^n}{σ^{δ_{λ_0}}} \prod_{i=1}^n\frac{ χ_{[σ,κ)}(k_i)}{|k_i|^{3/2}}, \] where $c$ is a numerical constant, $λ_0\mapsto δ_{λ_0}$ is a positive function of the maximal admissible coupling constant which satisfies $\lim_{λ_0\to 0}δ_{λ_0}=0$ and $χ_{[σ,κ)}$ is the (approximate) characteristic function of the energy region between the infrared cut-off $σ$ and the ultraviolet cut-off $κ$. While the analysis of the first derivative is relatively straightforward, the second derivative requires a new strategy. By solving a non-commutative recurrence relation we derive a novel formula for $f^n_{P,σ}$ with improved infrared properties. In this representation $\partial_{P^{j'}}\partial_{P^{j}}f^n_{P,σ}$ is amenable to sharp estimates obtained by iterative analytic perturbation theory in part II of this series of papers. The bounds stated above are instrumental for scattering theory of two electrons in the Nelson model, as explained in part I of this series.