Implications of an Improved Neutron-Antineutron Oscillation Search for Baryogenesis: A Minimal Effective Theory Analysis

Future neutron-antineutron ($n\text{−}\overline{n}$) oscillation experiments, such as at the European Spallation Source and the Deep Underground Neutrino Experiment, aim to find first evidence of baryon number violation. We investigate implications of an improved $n\text{−}\overline{n}$ oscillation search for baryogenesis via interactions of $n\text{−}\overline{n}$ mediators, parametrized by an effective field theory (EFT). We find that even in a minimal EFT setup there is overlap between the parameter space probed by $n\text{−}\overline{n}$ oscillation and the region that can realize the observed baryon asymmetry of the Universe. The mass scales of exotic new particles are in the tera-electron-volt–peta-electron-volt regime, inaccessible at the LHC or its envisioned upgrades. Given the innumerable high energy theories that can match, or resemble, the minimal EFT that we discuss, future $n\text{−}\overline{n}$ oscillation experiments could probe many viable theories of baryogenesis beyond the reach of other experiments.

Figure 1

Sketches of the evolution of the heavier $n\text{−}\overline{n}$ mediator abundance $Y_{X_2}$, washout rate ${\mathrm{\Gamma }}_{\mathrm{wo}}$, and baryon asymmetry $Y_B$ in the two scenarios considered in this Letter (arbitrary normalization). In the late decay scenario, the $n\text{−}\overline{n}$ mediator is long-lived and decays out of equilibrium to generate a baryon asymmetry. In the early decay scenario, departure from equilibrium (thin dotted curve) is small, but suppressed washout enables efficient baryogenesis. See the text for details.

Figure 2

Parameter space of the minimal EFT probed by $n\text{−}\overline{n}$ oscillation for the late decay scenario, assuming that $M_{X_2}=4M_{X_1}$. The thicker (solid and dashed) black lines are obtained using central values of nuclear matrix elements from [61], while the thinner dotted lines show the effect of current lattice calculation uncertainty on the Super-K limit. For ${\mathrm{\Lambda }}_{X_2}={\mathrm{\Lambda }}_c=50{\mathrm{\Lambda }}_{X_1}$, the green shaded region can accommodate $Y_B=8.6\times {10}^{-11}$. For ${\mathrm{\Lambda }}_{X_2}={\mathrm{\Lambda }}_c=25{\mathrm{\Lambda }}_{X_1}$ ($100{\mathrm{\Lambda }}_{X_1}$), the viable region is between the dashed red (dotted-dashed blue) lines. The gray shaded region marks ${\mathrm{\Lambda }}_{X_1}<M_{X_2}$, where EFT validity requires greater than $O(1)$ coupling.

Figure 3

Parameter space of the minimal EFT probed by $n-\overline{n}$ oscillation for the early decay scenario, assuming that $M_{X_2}=4M_{X_1}$. The thicker (solid and dashed) black lines are obtained using central values of nuclear matrix elements from Ref. [61], while the thinner dotted lines show the effect of current lattice calculation uncertainty on the Super-K limit. Points represent solutions with $Y_B=8.6\times {10}^{-11}$ found for several fixed ${\mathrm{\Lambda }}_{X_1}/{\mathrm{\Lambda }}_{X_2}$ ratios while scanning over ${\mathrm{\Lambda }}_c\in (M_{X_2},{\mathrm{\Lambda }}_{X_2})$. The gray shaded region marks ${\mathrm{\Lambda }}_{X_2}<M_{X_2}$, where EFT validity requires greater than $O(1)$ coupling.