Beyond Neutrino Mass: Observable $n$-$\overline{n}$ Oscillations in UV Complete Seesaw Models

TL;DR

The paper investigates two SU(5)-based seesaw implementations that connect neutron-antineutron oscillations to the same dynamics responsible for charged-fermion masses and neutrino masses. Model A realizes Type II seesaw with two color-sextet scalars (Topology A), while Model B employs Type I+III with a color-sextet and a color-octet fermion (Topology B); both yield observable $n-ar{n}$ transitions linked to the flavor structure. The authors show that, with at least one TeV-scale colored state and consistent flavor/phenomenology constraints, the other colored states can lie as heavy as $ obreak 10^{11} ext{ GeV}$, making $n-ar{n}$ oscillations a powerful low-energy probe of grand unification. They provide explicit fermion-mass fits, gauge-coupling unification analyses, and benchmark scenarios demonstrating that upcoming experiments such as DUNE and NNBAR can probe substantial portions of the viable parameter space, while non-observation would push these states to still-heavier scales rather than falsify the models.

Abstract

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 $ΔB = 2$ baryon number violation, with particularly significant gains expected at the latter. The discovery of such a rare $ΔB = 2$ process would indicate physics beyond the Standard Model and could point to specific unified theories that allow observable $n-\overline{n}$ transitions. We accordingly examine $n-\overline{n}$ 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 $n-\overline{n}$ 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 $n-\overline{n}$ 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 $ΔB = 2$ processes can probe for a presence of the other colored states with masses up to $10^{11}\,\mathrm{GeV}$, well beyond the reach of colliders. This positions $n-\overline{n}$ oscillations as a rare low-energy portal to grand unification and ultra-heavy new physics.

Figures (11)

• Figure 2: Schematic diagram of the neutrino mass generation in Model A.
• Figure 3: $n-\overline{n}$ oscillation diagram in Model A.
• Figure 4: Tree-level $K^0-\overline K^0$ oscillation present in Model A. Analogous diagrams yield $B_{d,s}^0-\overline B_{d,s}^0$ oscillations.
• Figure 5: One-loop $K^0-\overline K^0$ oscillation due to either $\Delta_6$ or $\Sigma_6$ exchange. Analogous diagrams generate $B_{d,s}^0-\overline B_{d,s}^0$ and $D^0-\overline D^0$ oscillations.
• Figure 6: Observable $n-\overline{n}$ range for the Model A (NO) and Model A (IO) scenarios, as indicated. The shaded blue and magenta regions correspond to the viable parts of the parameter space that can be probed in the future experiments, namely, through NNBAR and DUNE experiments, respectively, whereas the shaded gray region plotted on top of the magenta and blue regions shows the current SK exclusion. Light-green vertical shaded region shows the part excluded by $K^0-\overline K^0$ oscillation. Moreover, the pink shaded part highlights the current LHC bound due to dijet resonance. Projected sensitivity for dijet resonances from FCC-hh is presented by the dash-dotted red line, which will probe a significant part of the existing parameter space. NO/IO here refers to normal/inverted mass ordering for neutrinos. Both panels are obtained for $\tilde{\mu}=m_{\Delta_6}$.