Beyond neutrino mass: observable $n$-$bar{n}$ oscillations in UV complete seesaw models

Next-generation experiments, such as the Deep Underground Neutrino Experiment and the European Spallation Source, are set to improve sensitivity to neutron-antineutron oscillation, a direct probe of ∆B = 2 baryon number violation, with particularly significant gains expected at the latter. The discovery of such a rare ∆B = 2 process would indicate physics beyond the Standard Model and could point to specific unified theories that allow observable ▫$n$▫-▫$bar{n}$▫ transitions. We accordingly examine ▫$n$▫-▫$bar{n}$▫ oscillations within a unified framework that accounts for charged fermion masses and generates viable neutrino masses via the seesaw mechanism. More specifically, we show that ▫$n$▫-▫$bar{n}$▫ oscillations can arise from two specific topologies within two distinct SU(5) scenarios. One topology requires a presence of two color-sextet scalars in the Type II seesaw framework, whereas the other involves a scalar sextet and a color-octet fermion in the Type III seesaw framework. While the former topology can be realized in the SO(10)/Pati-Salam frameworks, the latter finds a natural embedding in SU(5), which constitutes one of the key novelties of our work. Remarkably enough, the same dynamics responsible for fermion masses also induces baryon number violation, thus linking ▫$n$▫-▫$bar{n}$▫ oscillations to the flavor structure of the theory. We show that, given a TeV-scale mass for one of the colored states, upcoming searches for such ∆B = 2 processes can probe for a presence of the other colored states with masses up to 1011 GeV, well beyond the reach of colliders. This positions ▫$n$▫-▫$bar{n}$▫ oscillations as a rare low-energy portal to grand unification and ultra-heavy new physics.