Search for sub-GeV invisible particles in inclusive decays of $J/ψ$ to $φ$

Abstract

A search for an invisible particle, $X$, with a mass between 0 and 0.96 $\textrm{GeV}/\textit{c}^{2}$, is performed in the process $J/ψ\rightarrowφ+ X$ using $(8774.0\pm39.4)\times10^{6}$ $J/ψ$ events collected with the BESIII detector from 2017 to 2019. The $φ$ meson is fully reconstructed and an efficient veto of photons, neutral and charged hadrons up to twice the $K_L^0$ mass is applied to the rest of the event and the recoil mass against the $φ$ is obtained precisely from the kinematic constraint in the event. No significant signal over the expected background is observed in the investigated region and the upper limit on the inclusive branching fraction of $J/ψ\rightarrowφ+ X$ is determined to be $7.0\times10^{-8}$ at 90\% confidence level. Upper limits at a 90\% confidence level are also given for this branching fraction as a function of the invisible particle mass, varying from $4\times10^{-9}$ to $4\times10^{-8}$ over the investigated mass range. Additionally, a 90\% confidence level upper limit on the branching fraction of $η\rightarrow \rm{invisible}$ is determined to $2.4\times10^{-5}$, which improves the previous best results by more than four times. The analysis technique in this work offers a clean window to search for sub-GeV invisible particles, which can be adapted for other $J/ψ$ decays and direct $e^+e^-$ annihilation experiments in future studies, and improve the sensitivity by orders of magnitude.

Figures (3)

• Figure 1: The topology of $J/\psi\rightarrow\phi + X, \phi\rightarrow K^+ K^-$, where $X$ represents a sub-GeV invisible particle.
• Figure 2: The distributions of $M^{2}_{\rm{recoil}}(\phi)$ for data and MC samples. The dots with error bars are the data. The blue, orange, green, and purple dashed lines are the signal MC samples with $m_{X} =$ 0.2, 0.4, 0.6, and 0.92 $\textrm{GeV}/\textit{c}^{2}$, assuming a BF of $2 \times 10^{-8}$. The red solid line is the expected background estimated by the MC study and the $e^+e^-$ collision data samples at $\sqrt{s}=$ 3.080, 3.650, and 3.682 $\textrm{GeV}$. The grey box shows the total uncertainty of the expected background, including statistical and systematic uncertainties.
• Figure 3: The observed (blue points) and expected (red dashed line) upper limits on $\mathcal{B}(J/\psi\rightarrow\phi + X)$ as a function of $m_{X}$ in the accessible $m_{X}$ range at 90% C.L.. The expected upper limits and their uncertainties are obtained using the background model with no signal. The $\pm1\sigma$ and $\pm2\sigma$ expected upper limit bands are shown as the green and yellow bands, respectively.