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 Letter presents first-principles calculations of neutron-antineutron matrix elements needed to accurately connect measurements of the neutron-antineutron oscillation rate to constraints on |ΔB|=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 modified minimal subtraction 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 postsphaleron 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.
Highlights
Introduction.—Beyond the standard model (BSM) violation of baryon number conservation is necessary to explain the observed matter-antimatter asymmetry of the Universe
Results are presented as bounds on the neutron-antineutron oscillation time τn-ngoverning the time-dependent probability Pn-n 1⁄4 sin2ðt=τn-n Þ for a free neutron in vacuum to turn into an antineutron [3]
Because beyond the standard model (BSM) and quantum chromodynamics (QCD) effects are important at different scales, any BSM mechanism mediating n-noscillations can be summarized into a particular linear combination of effective six-quark operators violating baryon number by two units
Summary
Taking into account nuclear structure effects [10]. In the future, we expect underground neutrino facilities like DUNE to be able to provide competitive bounds thanks to improved background-rejection techniques [11]. Because BSM and QCD effects are important at different scales, any BSM mechanism mediating n-noscillations can be summarized into a particular linear combination of effective six-quark operators violating baryon number by two units. Their matrix elements between neutron and antineutron states are determined by nonperturbative QCD and have to be computed before any BSM predictions for τn-ncan be made. The purpose of this Letter is to present a lattice QCD (LQCD) calculation of these six-quark matrix elements and to discuss its impact on possible discovery of new physics and theories of baryogenesis. At high scales ΛBSM ≫ ΛQCD Fierz identity violations can be neglected even if BSM matching is performed at tree level.) that provide the dominant contributions to n-ntransitions in SM EFT, Q1 1⁄4 −4ðuTCPRdÞðuTCPRdÞðdTCPRdÞTAAS; Q2 1⁄4 −4ðuTCPLdÞðuTCPRdÞðdTCPRdÞTAAS; Q3 1⁄4 −4ðuTCPLdÞðuTCPLdÞðdTCPRdÞTAAS: ð2Þ
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