Neutron-antineutron oscillations from lattice QCD

TL;DR

This paper provides first-principles lattice QCD calculations of neutron-antineutron oscillation matrix elements, improving the connection between experimental measurements and BSM theories of baryon number violation, with implications for baryogenesis models.

Contribution

It presents the first lattice QCD determination of neutron-antineutron matrix elements with controlled systematic uncertainties, enhancing the accuracy of BSM constraints from oscillation experiments.

Findings

Quantum chromodynamics predicts at least ten times more oscillation events than previous models.

Systematic uncertainties are controlled using advanced lattice techniques and nonperturbative renormalization.

Results suggest stronger experimental bounds on baryon number violation than earlier estimates.

Abstract

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.