Observation of a charged charmoniumlike structure Z_c(4020) and search for the Z_c(3900) in e+e- to pi+pi-h_c

Abstract: We study e+e- --> pi+pi-h_c at center-of-mass energies from 3.90 GeV to 4.42 GeV using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections are measured at 13 energies, and are found to be of the same order of magnitude as those of e+e- --> pi+pi-J/psi but with a different line shape. In the \pi^\pm h_c mass spectrum, a distinct structure, referred to as Z_c(4020), is observed at 4.02 GeV/c^2. The Z_c(4020) carries an electric charge and couples to charmonium. A fit to the \pi^\pm h_c invariant mass spectrum, neglecting possible interferences, results in a mass of (4022.9\pm 0.8\pm 2.7) MeV/c^2 and a width of (7.9\pm 2.7\pm 2.6) MeV for the Z_c(4020), where the first errors are statistical and the second systematic. No significant Z_c(3900) signal is observed, and upper limits on the Z_c(3900) production cross sections in \pi^\pm h_c at center-of-mass energies of 4.23 and 4.26 GeV are set.

• The paper reports the clear observation of the charged charmoniumlike Z_c(4020) at 4.02 GeV/c² using invariant mass analysis.
• It compares Z_c(4020) parameters with those of Z_c(4025), noting close alignment within 1.5 standard deviations.
• The study constrains Z_c(3900) production by establishing upper limits on its cross-section at key center-of-mass energies.

A Study on Charged Charmoniumlike Structures: Observation of $Z_c(4020)$

This paper presents an investigation centered on the observation of a charged charmoniumlike structure, identified as $Z_c(4020)$, and a subsequent search for the $Z_c(3900)$ in $e^+e^- \to h_c$ processes at various center-of-mass (CM) energies ranging from 3.90 to 4.42 GeV. Utilizing data gathered by the BESIII detector at the Beijing Electron Positron Collider, the authors measure the Born cross sections across 13 energy levels, which notably differ in their line shape compared to analogous reactions.

Key Observations

1. Observation of $Z_c(4020)$: Within the $\pi^\pm h_c$ mass spectrum, the study identifies a distinct charged structure at 4.02 GeV/$c^2$, designated as $Z_c(4020)$. This state exhibits a significant coupling to charmonium, with a measured mass of $(4022.9 \pm 0.8 \pm 2.7)$ MeV/$c^2$ and a width of $(7.9 \pm 2.7 \pm 2.6)$ MeV, as determined through a fitting process in which potential interferences were neglected.
2. Comparison with $Z_c(4025)$: The paper finds that the parameters of $Z_c(4020)$ closely align with those of the previously observed $Z_c(4025)$ in $D^{*}\bar{D}^{*}$ final states. Although the differences fall within 1.5 standard deviations, further investigation is necessary to conclusively establish whether these structures represent the same state.
3. Search for $Z_c(3900)$: The examination fails to yield a significant detection of the $Z_c(3900)$, leading to the establishment of upper limits on its production cross-sections at 4.23 and 4.26 GeV.

Experimental Approach and Results

The authors employ the BESIII detector's capabilities to scrutinize the invariant mass spectrum, focusing on $e^+e^- \to$ reactions with $h_c$ being reconstructed via its electric-dipole transition. A detailed analysis of the data at various CM energies reveals a pronounced structure known as $Z_c(4020)$, characterized by a mass and width determined through unbinned maximum likelihood fits.

In terms of numerical results, the cross sections for $Z_c(4020)$ production are reported as follows: $(8.7\pm 1.9\pm 2.8\pm 1.4)$ pb at 4.23 GeV, $(7.4\pm 1.7\pm 2.1\pm 1.2)$ pb at 4.26 GeV, and $(10.3\pm 2.3\pm 3.1\pm 1.6)$ pb at 4.36 GeV. Notably, systematic errors associated with the measurements are comprehensively addressed.

Implications and Future Directions

The findings presented in this paper have significant implications for theoretical models considering the existence of exotic charmoniumlike states. The coupling of $Z_c(4020)$ to charmonium and its electric charge distribution suggest a more complex quark composition than simple charm-anticharm configurations, potentially indicating states such as tetraquarks or molecular compositions.

The congruence of $Z_c(4020)$ parameters with those of $Z_c(4025)$ calls for further investigation to unravel the true nature of these states. Future studies might explore detailed spin-parity assignments and engage in cross-section measurements across additional energy points to furnish a more comprehensive understanding.

Ultimately, the paper contributes to ongoing discussions around the nature of charmoniumlike states and their implications for hadronic physics, offering a basis for subsequent experimental and theoretical exploration in the field.