Discovery of XYZ particles at the BESIII Experiment

TL;DR

This article surveys BESIII's contributions to the XYZ sector, focusing on the discovery of charged tetraquark candidates Z_c(3900), Z_c(4020), and Z_cs(3985) and the observation of multiple vector charmoniumlike Y states via energy scans. It highlights the experimental strategy of reconstructing charged final states and performing cross-section scans across a broad energy range, together with detailed Dalitz and invariant-mass analyses to extract resonance parameters. The findings indicate a rich tetraquark-like spectrum and a complex vector state landscape that cannot be fully explained by conventional charmonium, motivating coupled-channel analyses and theoretical models invoking tetraquarks, molecules, or hybrids. Looking ahead, the 2024 BEPCII upgrade expands the accessible energy range and luminosity, enabling a more comprehensive mapping of the XYZ spectrum and potential discovery of new multiplets and phenomena.

Abstract

Charmonium is a bound state of a charmed quark and a charmed antiquark, and a charmoniumlike state is a resonant structure that contains at least a charmed quark-antiquark pair but has properties that are incompatible with a conventional charmonium state. The charmoniumlike states are also called XYZ particles to indicate their underlying nature is still unclear. The BESIII experiment has contributed significantly in the study of the XYZ particles, and here we review the discoveries of the Zc(3900), Zc(4020), and Zcs(3985) tetraquark states and the observations of several new vector charmoniumlike states at the BESIII experiment.

Figures (6)

• Figure 1: Dalitz plot for selected $e^+e^-\to \pi^+\pi^- J/\psi$ events in the $J/\psi$ signal region (a, the inset show background events from the $J/\psi$ mass sidebands) and the $Z_c(3900)$ signal in the $M_{\mathrm{max}}(\pi J/\psi)$ (b) zc3900. Points with error bars are data, the curves are the best fit, the dashed histograms are the phase space distributions and the shaded histograms are the non-$\pi^+\pi^- J/\psi$ background estimated from the normalized $J/\psi$ sidebands.
• Figure 2: Dalitz plot ($M^2_{\pi^+h_c}$ vs. $M^2_{\pi^+\pi^-}$) for selected $e^+e^-\to \pi^+\pi^- h_c$ events (a) and the $Z_c(4020)$ signal observed in $\pi h_c$ invariant mass spectrum (b) zc4020. Points with error bars are data, the solid curves are the best fit, the shaded histograms are the non-$\pi^+\pi^- h_c$ background estimated from the normalized $h_c$ sidebands.
• Figure 3:
• Figure 4: The $Y(4230)$ signals observed in the BESIII data.
• Figure 5: Supernumerary vectors in the charmonium mass region may suggest existence of exotic states.