Search for a massless particle beyond the Standard Model in the $Ξ^0\toΛ+ \text{invisible}$ decay

Abstract

A search for a massless beyond-standard-model particle is performed in the decay $Ξ^{0}\toΛ+\text{invisible}$ using $(1.0087 \pm 0.0044)\times 10^{10}$ $J/ψ$ events collected with the BESIII detector at the BEPCII collider. No significant signal is observed and the upper limit on the branching fraction $\mathcal{B}(Ξ^{0}\toΛ+\text{invisible})$ is set to be $2.3 \times 10^{-4}$ at the $90\%$ confidence level. This is the first search for a flavor-changing neutral current process with missing energy in $Ξ^0$ decays. Throughout this paper, charge-conjugate processes are always implied.

Figures (6)

• Figure 1: The possible Feynman diagrams for decay $\Xi^{0}\to\Lambda+\text{invisible}$ in NP models: (a) tree-level Feynman diagram (such as QCD axion) and (b)(c) one-loop Feynman diagrams (such as dark photon).
• Figure 2: The fit on the $M_{\rm{bc}}$ from the ST side. The black dots with error bars represent the data and the red line represents the total fit, whose signal component is shown with the green dashed line and the background component with the blue dashed line. The bottom panel shows the pull values, defined as $Pull^{i}=\frac{N_{\text{data}}^{i}-N_{\text{fit}}^{i}}{\sigma_{\text{data}}^{i}}$, where $N_{\text{data}}^{i}$ is the number of data events in the $i$-th bin, $N_{\text{fit}}^{i}$ is the corresponding total number of fitted events, and $\sigma_{\text{data}}^{i}$ is the statistical uncertainty of the data.
• Figure 3: The $E_{\text{extra}}^{\text{other}}$ distribution. The red histogram represents the Signal MC before the correction, the green histogram represents the Signal MC after the correction, and the black dots with error bars represent the $J/\psi\to\bar{\Xi}^{0}(\to\bar{\Lambda}\pi^{0})\Xi^{0}(\to\Lambda\pi^{0})$ control sample.
• Figure 4: Fit on the $E_{\text{extra}}$ distribution. The black dots with error bars represent the data sample, the red line represents the fitted curve, the green dashed line represents the signal component, and the blue dashed line represents the background component. The green histogram represents the signal MC sample (scaled according to the obtained upper limit of its BF), the blue histogram represents the inclusive MC sample, and the violet histogram represents the $\Xi^0\to\Lambda\gamma$ MC sample. The bottom panel shows the pull values defined as $Pull^{i}=\frac{N_{\text{data}}^{i}-N_{\text{fit}}^{i}}{\sigma_{\text{data}}^{i}}$, where $N_{\text{data}}^{i}$ is the number of data events in the $i$-th bin, $N_{\text{fit}}^{i}$ is the corresponding total number of fitted events, and $\sigma_{\text{data}}^{i}$ is the statistical uncertainty of the data.
• Figure 5: The distributions of the likelihood curves. The dotted-dashed black line is the initial curve, the dotted-dashed red line is the fit curve with a Gaussian function, and the solid blue line is the fit curve convolved with a Gaussian for systematic uncertainties. The blue arrow indicates the UL on BF at the 90% C.L.