Z_c(3900)

Z_c(3900)

Z_c(3900) is an electrically charged charmonium‑like resonance reported near 3.9 GeV/c^2. It attracted rapid attention from collaborations at Belle (experiment), BESIII, and CLEO and prompted theoretical work at institutions such as CERN, Fermilab, and Brookhaven National Laboratory. The state has been studied in contexts involving the J/ψ meson, the ψ(2S), and open‑charm mesons like the D meson and D* meson.

Discovery and experimental observation

The first signals consistent with the resonance were reported by the BESIII Collaboration in 2013 in e+e− collisions at center‑of‑mass energies near the Y(4260), with contemporaneous evidence from the Belle (experiment) Collaboration using the KEKB accelerator and data from the BABAR experiment at SLAC National Accelerator Laboratory. Observations were based on peaks in invariant‑mass distributions of J/ψ π± combinations and were reinforced by subsequent analyses by CLEO and combined studies involving LHCb. Experimental papers and conference presentations from groups at IHEP, Beijing, University of Tokyo, University of California, Berkeley, and Purdue University contributed to confirmation and cross‑checks. Results were presented at venues such as the International Conference on High Energy Physics and the Rencontres de Moriond.

Properties and decay modes

Measured properties include a pole mass near 3.9 GeV/c^2 and a width of order tens of MeV, with values reported by BESIII and Belle (experiment) differing within experimental uncertainties. The state has charged partners (±) implying isospin multiplet structure, and decays prominently to J/ψ π± and has been searched for in channels involving ψ(2S), η_c, and combinations of open‑charm mesons such as D D* and D* D*. Branching fractions and partial widths were constrained by analyses performed at KEK, IHEP, Beijing, and CERN detectors, while spin‑parity assignments were investigated using angular distributions and amplitude analyses reported by groups at BESIII and Belle (experiment). Measurements constrain possible J^P assignments; analyses often favor J^P = 1^+ but alternative assignments were considered by teams including researchers from Indiana University Bloomington and Columbia University.

Theoretical interpretations

Interpretations span compact tetraquark models inspired by work at DESY and IHEP, hadronic molecule descriptions motivated by near‑threshold dynamics of D meson–D* meson systems, cusp or kinematic‑threshold effects discussed in seminars at CERN and Perimeter Institute, and hybrid or rescattering scenarios explored by theorists at MIT, Princeton University, and Los Alamos National Laboratory. Tetraquark models often invoke diquark–antidiquark correlations building on frameworks influenced by Nambu–Jona-Lasinio model ideas and studies from Rutgers University and University of Glasgow. Molecular pictures exploit effective field theory approaches related to Heavy Quark Effective Theory commonly used at Yale University and University of Washington. Lattice QCD studies by groups at Brookhaven National Laboratory and University of Tokyo have provided constraints but face computational challenges. Competing explanations were debated at workshops hosted by IHEP, Beijing and the European Centre for Theoretical Studies in Nuclear Physics and Related Areas.

Production mechanisms and related states

Production has been observed in e+e− annihilation near the Y(4260) resonance, initial‑state radiation processes explored by BABAR at SLAC National Accelerator Laboratory, and in association with other charmoniumlike candidates such as Z_c(4020), X(3872), and Y(4260). Comparisons were drawn with charged bottomoniumlike states such as Z_b(10610) and Z_b(10650) observed by Belle (experiment), suggesting possible heavy‑quark symmetry patterns discussed by theorists at CERN and Fermilab. Production cross sections and line shapes have been measured by collaborations including BESIII, Belle (experiment), and LHCb, and studies at KEK and SLAC National Accelerator Laboratory examined associated production with light mesons and radiative transitions. Searches for neutral partners and isospin multiplets were undertaken by BESIII and Belle (experiment) in multi‑channel analyses.

Measurement techniques and experimental challenges

Measurements rely on high‑statistics e+e− collision datasets from facilities such as KEKB, BEPCII, and PEP-II and detector systems like the Belle (experiment) detector, the BESIII detector, and the BABAR detector. Experimental challenges include disentangling resonance signals from continuum backgrounds studied at SLAC National Accelerator Laboratory, modeling final‑state interactions discussed in workshops at CERN, and performing amplitude analyses requiring complex fits developed at University of Bonn and IPPP (Institute for Particle Physics Phenomenology). Systematic uncertainties arise from mass calibration using resonances like ψ(2S) and from efficiency corrections benchmarked with control samples from J/ψ decays. Combined experimental and theoretical efforts at institutions including University of Oxford, University of Cambridge, and Stanford University continue to refine measurements and resolve ambiguities between competing interpretations.

Category:Exotic hadrons