Search for the FCNC charmonium decay $J/ψ\to D^0 μ^+ μ^- + \text{c.c.}$

TL;DR

This paper tackles the rare FCNC transition in charmonium by searching for $J/ψ \to D^{0} μ^{+} μ^{-} + \text{c.c..}$ using BESIII data corresponding to approximately $10^{10}$ $J/ψ$ events. The analysis reconstructs $D^{0}$ via three hadronic modes, applies stringent muon and photon selections, and performs a simultaneous unbinned maximum-likelihood fit to the $M_{D^{0} μ^{+} μ^{-}}$ spectrum across all modes, with systematic effects incorporated through likelihood-smearing. No significant signal is observed, and an upper limit on the branching fraction of $1.1 \times 10^{-7}$ at 90% C.L. is established, representing the first muon-involved FCNC search in charmonium. The result constrains beyond-Standard-Model scenarios and provides a benchmark for future, more sensitive studies of FCNC processes in the charmonium sector, consistent with SM expectations of order $10^{-13}$ for such decays.

Abstract

Based on a data sample of $(10087 \pm 44) \times 10^6$ $J/ψ$ events taken with the BESIII detector, we search for the flavor-changing neutral current charmonium decay $J/ψ\to D^{0} μ^{+} μ^{-} + \text{c.c.}$. No significant signal above the background is observed, and the upper limit on its branching fraction is set to be $\mathcal{B}(J/ψ\to D^{0}μ^{+}μ^{-} + \text{c.c.} ) < 1.1 \times 10^{-7}$ at the 90% confidence level. This marks the first search for a flavor-changing neutral current charmonium decay involving muons in the final state.

Figures (4)

• Figure 1: Feynman diagram for the $J/\psi\to \bar{D}^{0}\mu^+\mu^-$ decay in the SM.
• Figure 2: The invariant mass $M_{D^{0}}$ distributions for $D^{0} \to K^{-}\pi^{+}$ (a), $D^{0} \to K^{-}\pi^{+}\pi^{0}$ (b), $D^{0} \to K^{-}\pi^{+}\pi^{+}\pi^{-}$ (c), along with the corresponding $\chi_{D^0}^2$ distributions for $D^{0} \to K^{-}\pi^{+}$ (d), $D^{0} \to K^{-}\pi^{+}\pi^{0}$ (e), $D^{0} \to K^{-}\pi^{+}\pi^{+}\pi^{-}$ (f). The green shaded histogram shows the signal MC sample scaled to $\mathcal{B}(J/\psi\to D^{0}\mu^{+}\mu^{-}) = 4.0\times10^{-2}$ for the $M_{D^{0}}$ distributions and $\mathcal{B}(J/\psi\to D^{0}\mu^{+}\mu^{-}) = 1.5\times10^{-2}$ for the $\chi_{D^0}^2$ distributions. The red histogram is the inclusive MC sample, and the black dots with error bars are data, and the blue arrows indicate the cut region or position. Due to the large amount of background, the error bars on the data are small.
• Figure 3: The distributions of $M_{D^{0}\mu^{+}\mu^{-}}$ for $J/\psi\to D^{0}\mu^+\mu^-$ of the selected candidates in data, signal MC sample, and inclusive MC sample. The simultaneous fit result is shown in the top left, and the individual fit result for each mode is shown in the other three sub-figures, respectively. The black dots with error bars are data, the magenta dotted-dashed line shows the shape of signal MC sample scaled to $\mathcal{B}(J/\psi\to D^{0}\mu^{+}\mu^{-}) = 1.0\times10^{-6}$ for the three tags. The blue shaded histogram is the inclusive MC sample, the red line is the fit result, and the blue solid lines is the fitted background.
• Figure 4: The distributions of normalized likelihoods versus signal yields $N_{\rm{sig}}$ or BF of $J/\psi\to D^{0}\mu^{+}\mu^{-}$. The black dashed curve represents the initial distribution, while the red dashed curve shows the fitted curve with a Gaussian function. The blue solid line illustrates the result convolved with a Gaussian function for systematic uncertainties. The blue arrow indicates the UL on its BF at the 90% C.L.