New Avenues for $|\Delta B|$ = 2 Processes Beyond Neutron-Antineutron Oscillations

Abstract

We explore baryon-number-violating ($|\Delta B|=2$) processes beyond the well-known neutron-antineutron ($n-\bar{n}$) oscillations, focusing on the $\Lambda -\bar{\Lambda }$ system. The presence of a strange quark in the $\Lambda$ 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 $|\Delta B|=2$ transitions and study their UV completions mediated by exotic scalar fields with trilinear interactions. We demonstrate that in these models, $\Lambda -\bar{\Lambda }$ 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 $\delta m_{\Lambda }$, deriving bounds from current experimental limits on $n-\bar{n}$ oscillations and dinucleon decays such as $pp\to K^{+}{K}^{+}$. These bounds indicate that $\Lambda -\bar{\Lambda }$ 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 $\delta m_{\Lambda }$ at BESIII.

Comments: 32 pages, 11 figures, 6 tables