Berry Phase in Non-Perturbative QED

We study QED${}_4$ in the adiabatic approximation, incorporating global topological effects associated with the $U(1)$ Berry connection. The Berry phase accumulated by the fermionic vacuum is given by $\mathrm{\Delta }\alpha ={\oint }_{\mathcal{C}}{\gamma }_5{\mathcal{A}}^{(n)}$, where ${\mathcal{A}}^{(n)}$ is a closed but non-exact one-form defined over the space of gauge configurations. This chiral holonomy induces an emergent vacuum angle that contributes non-perturbatively to the effective action. The partition function decomposes into topological sectors weighted by this geometric phase, analogous to quantum systems on multiply connected spaces. Our results reveal that even in Abelian gauge theory, the infrared regime can exhibit global effects beyond the reach of local or perturbative descriptions.