Observation of a Charged Charmoniumlike Structure ine+e−→π+π−J/ψ

Abstract

We study the process e + e − → π + π − J/ψ at a center-of-mass energy of 4.260 GeV using a 525 pb −1 data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be (62.9 ± 1.9 ± 3.7) pb, consistent with the production of the Y (4260). We observe a structure at around 3.9 GeV/c 2 in the π ± J/ψ mass spectrum, which we refer to as the Zc(3900). If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the π ± J/ψ invariant mass spectrum, neglecting interference, results in a mass of (3899.0 ± 3.6 ± 4.9) MeV/c 2 and a width 3 of (46 ± 10 ± 20) MeV. Its production ratio is measured to be R = σ(e + e − →π ± Zc(3900) ∓ →π + π − J/ψ)) σ(e + e − →π + π − J/ψ) = (21.5 ± 3.3 ± 7.5)%. In all measurements the first errors are statistical and the second are systematic. PACS numbers: 14.40.Rt, 14.40.Pq, 13.66.Bc Since its discovery in the initial-state-radiation (ISR) process e + e − → γ ISR π + π − J/ψ [1], and despite its subsequent observations [2][3][4][5], the nature of the Y (4260) state has remained a mystery. Unlike other charmonium states with the same quantum numbers and in the same mass region, such as the ψ (4040) A similar situation has recently become apparent in the bottomonium system above the BB threshold, where there are indications of anomalously large couplings between the Υ(5S) state (or perhaps an unconventional bottomonium state with similar mass, the Y b (10890)) and the π + π − Υ(1S, 2S, 3S) and π + π − h b (1P, 2P ) final states [14,15]. More surprisingly, substructure in these π + π − Υ(1S, 2S, 3S) and π + π − h b (1P, 2P ) decays indicates the possible existence of charged bottomoniumlike states [16], which must have at least four constituent quarks to have a non-zero electric charge, rather than the two in a conventional meson. By analogy, this suggests there may exist interesting substructure in the Y (4260) → π + π − J/ψ process in the charmonium region.In this Letter, we present a study of the process e + e − → π + π − J/ψ at a center-of-mass (CM) energy of √ s = (4.260± 0.001) GeV, which corresponds to the peak of the Y (4260) cross section. We observe a charged structure in the π ± J/ψ invariant mass spectrum, which we refer to as the Z c (3900). The analysis is performed with a 525 pb −1 data sample collected with the BESIII detector, which is described in detail in Ref. [17]. In the studies presented here, we rely only on charged particle tracking in the main drift chamber (MDC) and energy deposition in the electromagnetic calorimeter (EMC).The GEANT4-based Monte Carlo (MC) simulation software, which includes the geometric description of the BE-SIII detector and the detector response, is used to optimize the event selection criteria, determine the detection efficiency, and estimate backgrounds. For the signal process, we use a sample of e + e − → π + π − J/ψ MC events generated assuming the π + π − J/ψ is produced via Y (4260) decays, and using the...

The hidden-charm pentaquark and tetraquark states

“…31. BESIII named this structure as the Z c (3885) since its measured mass (see Table 8) was slightly lower than that of the Z c (3900) measured in the J/ψπ channel by BESIII [64] and Belle [124]. However, the measured mass and width of the Z c (3885) [159] were consistent with those of the Z c (3900) state obtained by Xiao et al [61].…”

“…This upper limit was lower than the production cross section of Z c (3900) ± → π ± J/ψ obtained in Ref. [64], which is about 14 pb.…”

“…Z c (3900) and Z c (3885). Since 2013, the BESIII Collaboration announced several charged charmonium-like states Z c (3900) [64], Z c (3885) [159], Z c (4020) [160] and Z c (4025) [161] at √ s = 4.26 GeV. All these structures were observed in the process Y(4260) → π − Z + c (as shown in Table 8).…”

“…The Z c (3900) was observed in the J/ψπ ± invariant mass distribution of the e + e − → J/ψπ + π − process by the BESIII Collaboration [64]. As shown in Fig.…”

“…Thus, the intermediate f 0 (980) contribution to Y(4260) → π + π − J/ψ is not dominant [123]. Besides Y(4260) → J/ψ f 0 (980) → J/ψπ + π − , the Y(4260) → Z c (3900) ± π ∓ → J/ψπ + π − mode was also seen [64,124], where the charged charmonium-like structure Z c (3900) was observed in the corresponding J/ψπ ± invariant mass spectrum, which we will discuss in detail later.…”