New Avenues for $|ΔB|$ = 2 Processes Beyond Neutron-Antineutron Oscillations

Abstract

We explore baryon-number-violating ($|ΔB| = 2$) processes beyond the well-known neutron-antineutron ($n - \bar{n}$) oscillations, focusing on the $Λ- \bar Λ$ system. The presence of a strange quark in the $Λ$ baryon introduces a new set of six-quark operators roughly of the form $(uds)^2$, which are different from the $(udd)^2$ operators responsible for $n - \bar{n}$ oscillations. Using the Standard Model Effective Field Theory (SMEFT), we classify all dimension-9 operators that cause $|ΔB|=2$ transitions and study their UV completions mediated by exotic scalar fields with trilinear interactions. We demonstrate that in these models, $Λ- \bar Λ$ oscillations can occur at tree level, with $n - \bar{n}$ mixing potentially appearing at higher loop levels. We employ a chiral effective theory to constrain the effective mass mixing $δm_Λ$, deriving bounds from current experimental limits on $n - \bar{n}$ oscillations and dinucleon decays such as $p \,p \to K^+ K^+$. These bounds indicate that $Λ- \barΛ$ oscillations probe a complementary parameter space, sensitive to baryon-number violation at scales up to $10^2-10^3$ TeV. We show that the existing indirect bounds make it challenging to provide a competitive bound on $δm_Λ$ at BESIII.

Figures (11)

• Figure 1: Triple scalar interaction leading to $|\Delta B|=2$ processes at tree-level. The label $q$ denotes generic quark-type fermions. See main text for further details.
• Figure 2: Extraction of $\delta m_\Lambda$ from $n- \bar{n}$ oscillations at tree-level. The central dot indicates the $|\Delta B| = 2$ mass insertion from Eq. \ref{['eq:deltamLambda']}. The intermediate $\Lambda$ baryons are off-shell.
• Figure 3: The extraction of $\delta m_\Lambda$ from the $n-\bar{n}$ oscillations through a $\pi^0$ running within the loop. The central dot indicates the $|\Delta B| = 2$ mass insertion from Eq. \ref{['eq:deltamLambda']}.
• Figure 4: The extraction of $\delta m_\Lambda$ from the $n-\bar{n}$ oscillation through $K-\bar{K}$ mixing. The central dot indicates the $|\Delta B| =2$ mass insertion from Eq. \ref{['eq:deltamLambda']}.
• Figure 5: Representative tree-level Feynman diagram contributing to the extraction of $\delta m_\Lambda$ from the process $p\,p \to K^+ K^+$ in the $t$-channel. The corresponding $u$-channel diagram is obtained by exchanging the final kaons.