Precision tests of the Standard Model (SM) of particle physics are currently largely constrained by the precision of theoretical calculations. QCD, the gauge theory of quarks and gluons, is nonperturbative at low energies and precise results for QCD processes can only be obtained from numerical simulations using lattice QCD. Advances in lattice QCD have recently led to results with the precision of a few percent, enabling comparison of theoretical predictions and experimental data and providing stringent tests of the SM.
The unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix is one such test. Lattice simulations of B physics processes serve as nonperturbative inputs into CKM unitarity fits. I will introduce lattice perturbation theory and discuss its role in extracting B physics results from lattice QCD in the context of two examples, the mass of the b quark and the B meson decay constant.