Search for Baryon/Lepton number violation processes at BESIII

TL;DR

This work from BESIII conducts comprehensive searches for baryon-number and lepton-number violation in the tau-charm energy regime. It first probes BNV via $\Lambda-\bar{\Lambda}$ oscillations, deriving time-dependent and time-integrated constraints on the oscillation parameter $\delta m_{\Lambda\bar{\Lambda}}$ from $J/\psi$ decays, with improved limits over prior results. It then explores LNV in several channels, including the first four-body $D_s^+$ decays to $h^-h^0e^+e^+$ and a Majorana-neutrino–mediated mode, along with LNV in light-flavor meson decays $\omega/\phi\to\pi^+\pi^+e^-e^-$, all yielding no signal but providing stringent upper bounds on the corresponding branching fractions. Finally, the study examines simultaneous BNV and LNV in $\Xi^0$ decays and sets competitive limits on $\Xi^0\to K^-e^+$ and $\Xi^0\to K^+e^-$ BF, contributing to the global effort to constrain beyond-Standard-Model scenarios related to baryogenesis and Majorana neutrinos.

Abstract

The observed matter-antimatter asymmetry in the universe is a serious challenge to our understanding of nature. Baryon/lepton number violation (BNV/LNV) decays have been searched for in many experiments to understand this large-scale observed fact. We present the recent results from the BESIII experiment, including a search for BNV through $Λ-\barΛ$ oscillation in the decays $J/ψ\to pK^-\barΛ$ and $J/ψ\toΛ\barΛ$. We also present searches for LNV in $D_s^+\to h^+h^0e^+e^-$ and $ω/φ\toπ^+π^-e^+e^-$ decays, alongside probes into processes violating both baryon and lepton numbers simultaneously, such as $Ξ^0\to K^+e^-/K^-e^+$.

Figures (5)

• Figure 1: Distribution of $M_{p\pi^-}$ for (a) WS events in the signal region and (insert) over the full span, where the filled circle with error bar is from data, the pink filled histogram, normalized arbitrarily, stems from simulated WS signal events, and the arrows in the inset figure show the edges of the signal region; (b) RS events from data, where the filled circles with error bars are from data, the blue solid line represents the result of the fit and the dashed line shows the background contribution.
• Figure 2: The fits to the invariant mass of corresponding final state of all data samples for each signal channel. The blue solid, magenta dashed and blue dashed lines represent the total fit, signal and background shapes, respectively. The downward magenta peaks denote negative signal yields.
• Figure 3: The ULs of the BFs at the 90% CL as a function of $m_{\nu_{m}}$ for the $D_s^+ \to \phi e^+ \nu_{m}(\to\pi^-e^+)$ decay.
• Figure 4: The 2D distribution of $M_{\gamma\gamma}$ versus $M_{\pi^+\pi^+e^-e^-}$ of the candidate events from the $J/\psi$ data, where the red box shows the signal region. The left plot is for $\omega\to\pi^+\pi^+e^-e^-$, and the right plot is for $\phi\to\pi^+\pi^+e^-e^-$.
• Figure 5: Distributions of $M_{Ke}$ versus $M_{\bar{\Lambda}\pi^0}$ of the accepted candidates for (a) $\Xi^0\to K^-e^+$ and (b) $\Xi^0\to K^+e^-$ in data, respectively. The red box indicates the signal region.