  Fundamental symmetry tests of baryon number violation in low-energy
experiments can probe beyond the Standard Model (BSM) explanations of the
matter-antimatter asymmetry of the universe. Neutron-antineutron oscillations
are predicted to be a signature of many baryogenesis mechanisms involving
low-scale baryon number violation. This work presents first-principles
calculations of neutron-antineutron matrix elements needed to accurately
connect measurements of the neutron-antineutron oscillation rate to constraints
on $|\Delta B|=2$ baryon number violation in BSM theories. Several important
systematic uncertainties are controlled by using a state-of-the-art lattice
gauge field ensemble with physical quark masses and approximate chiral
symmetry, performing nonperturbative renormalization with perturbative matching
to the $\overline{\text{MS}}$ scheme, and studying excited state effects in
two-state fits. Phenomenological implications are highlighted by comparing
expected bounds from proposed neutron-antineutron oscillation experiments to
predictions of a specific model of post-sphaleron baryogenesis. Quantum
chromodynamics is found to predict at least an order of magnitude more events
in neutron-antineutron oscillation experiments than previous estimates based on
the "MIT bag model" for fixed BSM parameters. Lattice artifacts and other
systematic uncertainties that are not controlled in this pioneering calculation
are not expected to significantly change this conclusion.
