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

A bstract: 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 $$ n-\overline{n} $$ n − n ¯ transitions. We accordingly examine $$ n-\overline{n} $$ n − 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 $$ n-\overline{n} $$ n − 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 $$ n-\overline{n} $$ n − 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 10 ¹¹ GeV, well beyond the reach of colliders. This positions $$ n-\overline{n} $$ n − n ¯ oscillations as a rare low-energy portal to grand unification and ultra-heavy new physics.