Search for the lepton number violating process $Ξ^- \rightarrow Σ^+ e^- e^- +c.c.$

TL;DR

This work reports the first search for the lepton-number-violating decay $Ξ^-\rightarrowΣ^+ e^- e^- +c.c.$ using $J/\psi\rightarrowΞ^-\bar{Ξ}^+$ decays recorded by the BESIII detector, with a blind-analysis approach on a dataset of $(10087 \pm 44) \times 10^6$ $J/\psi$ events. A double-tag method is employed: single-tag reconstruction of $\bar{Ξ}^+$ followed by double-tag selection for the signal mode, incorporating stringent PID, photon, and kinematic-fit requirements and a recoil-mass constraint to suppress backgrounds. An unbinned likelihood fit to the $m_{p\pi^0}$ distribution shows no significant signal, and a Bayesian upper limit on the branching fraction is derived as $\mathrm{Br}(Ξ^-\rightarrowΣ^+ e^- e^-)<2.0\times10^{-5}$ at 90% C.L., after accounting for systematic uncertainties via likelihood convolution. The result, while far above the predicted rates in some models ($\sim10^{-35}$–$10^{-36}$), provides a valuable experimental constraint that complements fixed-target studies and informs theoretical efforts on LNV in the baryon sector.

Abstract

We present a search for the lepton number violating decay $Ξ^-\rightarrowΣ^+e^-e^- +c.c.$ with $(10087\pm44)\times10^6$ $J/ψ$ events collected by the BESIII detector at the BEPCII collider. Employing a blind analysis strategy, no significant signal is observed above the expected background yield. The upper limit on the branching fraction is determined to be ${\rm Br}(Ξ^-\rightarrowΣ^+e^-e^-+c.c.)< 2.0\times10^{-5}$ at the $90\%$ confidence level.

Figures (3)

• Figure 1: The fitting result of the recoiling mass of $\bar{\Xi}^{+}$. The black dots with error bars represent the data, the red line indicates the fitting result, the blue line shows the component of signal, and the green dashed line indicates the component of the background.
• Figure 2: The fit result on the $M_{p\pi^0}$ distribution. The black dots with error bars correspond to data samples, the red solid line indicates the fitting result, the blue dotted line represents the signal MC shape, the green dashed-dotted line corresponds to the component of the $\Xi^-\rightarrow\Lambda\pi^-$ background, and the pink dashed-dotted line illustrates the component of other backgrounds.
• Figure 3: The black dots are the result of likelihood scan values, the red curve is the convolved likelihood curve, and red arrow denotes the upper limit on $N_{\rm sig}$ at the 90% C.L.