Search for a dark baryon in the $Ξ^-\rightarrowπ^-+{\rm invisible}$ decay

TL;DR

This work searches for a dark baryon in the two-body decay $\Xi^- \rightarrow \pi^- + {\rm invisible}$ using about $10^{10}$ $J/\psi$ events collected with the BESIII detector. The analysis employs a double-tag strategy and a data-driven treatment of electromagnetic energy to suppress backgrounds, finding no significant signal. Consequently, it sets 90%/95% confidence level upper limits on $\mathcal{B}(\Xi^- \rightarrow \pi^-+{\rm invisible})$ for several dark-baryon mass hypotheses and constrains the Wilson coefficients $C_{us,s}^L$ and $C_{us,s}^R$, providing stronger laboratory bounds than previous LHC results. These results tighten the viable parameter space for GeV-scale dark-baryon scenarios and demonstrate the power of hyperon decays as probes of the dark sector.

Abstract

A search for a dark baryon is performed for the first time in the two-body decay $Ξ^-\rightarrowπ^-+{\rm invisible}$ using $(10.087\pm0.044)\times10^{9}$ $J/ψ$ events collected at a center-of-mass energy of $\sqrt{s}=3.097\,\mbox{GeV}$ with the BESIII detector at the BEPCII collider. No significant signal is observed, and the 90% (95%) confidence level upper limits on the branching fraction $B(Ξ^-\rightarrowπ^-+{\rm invisible})$ are determined to be $4.2\times10^{-5}$ ($5.2\times10^{-5}$), $6.9\times10^{-5}$ ($8.4\times10^{-5}$), $6.5\times10^{-4}$ ($7.6\times10^{-4}$), $1.1\times10^{-4}$ ($1.3\times10^{-4}$) and $4.5\times10^{-5}$ ($5.5\times10^{-5}$), under the dark baryon mass hypotheses of 1.07$\,\mbox{GeV}/c^2$, 1.10$\,\mbox{GeV}/c^2$, $m_Λ$ (1.116$\,\mbox{GeV}/c^2$), 1.13$\,\mbox{GeV}/c^2$, and 1.16$\,\mbox{GeV}/c^2$, respectively. The constraints obtained on the Wilson coefficients $C_{u s, s}^L$ and $C_{u s, s}^R$ are more stringent than the previous limits derived from the LHC searches for the colored mediators.

Figures (4)

• Figure 1: The $M_{\bar{\Lambda}\pi^+}$ distribution of ST candidates. The black dots with error bars are data and the red solid curve is the fit result. The $J/\psi\rightarrow\gamma\bar{\Xi}^+\Xi^-$ peaking background, signal and other nonpeaking background are represented by the green shaded region, blue solid and orange dashed curves, respectively. The red arrows indicate the ST signal window.
• Figure 2: The fit results of the $E_{\rm EMC}$ distributions under the dark baryon mass hypotheses of (a) $m_\chi=1.07\,\hbox{GeV}/c^2$, (b) $m_\chi=1.10\,\hbox{GeV}/c^2$, (c) $m_\chi=m_\Lambda$, (d) $m_\chi=1.13\,\hbox{GeV}/c^2$, and (e) $m_\chi=1.16\,\hbox{GeV}/c^2$. The black dots with error bars are data points and the red solid lines show the fit results. The signal, signal shape, $\Xi^-\rightarrow\pi^-\Lambda(\rightarrow n\pi^0)$ background, and other background in the inclusive MC sample are represented by the green solid line, green shaded region, blue dashed line, and orange dashed line, respectively. The signal shape is normalized to BFs of (a) $5\times10^{-4}$, (b) $2\times10^{-3}$, (c) $1\times10^{-1}$, (d) $3\times10^{-3}$, and (e) $8\times10^{-4}$. The bottom panels show the fit residuals.
• Figure 3: The expected and observed ULs at the 95% C.L. on the BF of $\Xi^-\rightarrow\pi^-+{\rm invisible}$ under different $m_\chi$ hypotheses. The blue and red lines represent the maximum allowed BFs, where the Wilson coefficients are set to $C_{u s, s}^L<0.84\, \mathrm{TeV}^{-2}$ and $C_{u s, s}^R<0.19\, \mathrm{TeV}^{-2}$ derived from the LHC searches for the colored mediators dark_baryons.
• Figure 4: The 95% C.L. ULs on the Wilson coefficients $C_{u s, s}^L$ and $C_{u s, s}^R$ derived from the results under different $m_\chi$ hypotheses. The constraints derived from the LHC searches dark_baryons are also shown.