We present a novel finite-volume QED action designed to improve the infinite-volume extrapolation of hadronic observables in precision lattice QCD+QED calculations. The new action, which we call QED$_\text{r}$, is designed to remove kinematics-independent finite-volume corrections that appear at $\mathrm{O}(1/L^3)$ in the commonly used QED$_\text{L}$ formulation, where $L$ is the spatial extent of the physical volume. For a number of key observables, these effects depend on the internal structure of the hadrons and are difficult to evaluate non-perturbatively, making an analytical subtraction of the finite-volume effects impractical. We show that the QED$_\text{r}$ action proposed here corresponds to a particular subset of the infrared-improved QED actions presented by Davoudi et al. in 2019. We explicitly study the QED$_\text{r}$ electromagnetic finite-size effects on hadron masses and leptonic decay rates, relevant for Standard Model precision tests using the Cabibbo-Kobayashi-Maskawa matrix elements. In addition, we propose methods to remove the kinematics-dependent $\mathrm{O}(1/L^3)$ effects in leptonic decays. The removal of such contributions, shifting the leading contamination to $\mathrm{O}(1/L^4)$, will help to reduce the systematic uncertainties associated with finite-volume effects in future lattice QCD+QED calculations.

We determine the topological susceptibility and the excess kurtosis of $SU(3)$ pure gauge theory in four space-time dimensions. The result is based on high-statistics studies at seven lattice spacings and in seven physical volumes, allowing for a controlled continuum and infinite volume extrapolation. We use a gluonic topological charge measurement, with gradient flow smoothing in the operator. Two complementary smoothing strategies are used (one keeps the flow time fixed in lattice units, one in physical units). Our data support a recent claim that both strategies yield a universal continuum limit; we find $\chi_\mathrm{top}^{1/4}r_0=0.4769(14)(11)$ or $\chi_\mathrm{top}^{1/4}=197.8(0.7)(2.7)\,\mathrm{MeV}$.

This paper aims to serve as an introductory resource for disseminating the concept to individuals with interests in quantum chromodynamics (QCD) for hadrons. We discuss several topological aspects of the QCD vacuum and briefly review recent progress on this intuitive unifying framework for the lowlying hadron physics rooted in QCD by introducing the vacuum as a liquid of instantons and anti-instantons. We develop systematic density expansion on the dilute vacuum with diagrammatical Feynman rules to calculate the vacuum expectation values and generalize the calculations to hadronic matrix element (charges), and hadronic form factors using the instanton liquid (IL) ensemble. The IL ensemble prediction are well-consistent with those of recent lattice QCD calculations.