We report the observation of the X(3823) in the process e + e − → π + π − X(3823) → π + π − γχc1 with a statistical significance of 6.2σ, in data samples at center-of-mass energies √ s =4. 230, 4.260, 4.360, 4.420 and 4.600 GeV collected with the BESIII detector at the BEPCII electron positron collider. The measured mass of the X(3823) is (3821.7 ± 1.3 ± 0.7) MeV/c 2 , where the first error is statistical and the second systematic, and the width is less than 16 MeV at the 90% confidence level. The products of the Born cross sections for e + e − → π + π − X(3823) and the branching ratio B[X(3823) → γχc1,c2] are also measured. These measurements are in good agreement with the assignment of the X(3823) as the ψ(1 3 D2) charmonium state.PACS numbers: 13.20. Gd, 13.25.Gv, 14.40.Pq Since its discovery, charmonium -meson particles which contain a charm and an anti-charm quark -has been an excellent tool for probing Quantum Chromodynamics (QCD), the fundamental theory that describes the strong interactions between quarks and gluons, in the non-perturbative (low-energy, long-distance effects) regime, and remains of high interest both experimentally and theoretically. All of the charmonium states with masses that are below the open-charm threshold have been firmly established [1,2]; open-charm refers to mesons containing a charm quark (antiquark) and either an up or down antiquark (quark), such as D orD. However, the observation of the spectrum that are above the opencharm threshold remains unsettled. During the past decade, many new charmoniumlike states were discovered, such as the X(3872) [3], the Y (4260) [4,5] and the Z c (3900) [5][6][7]. These states provide strong evidence for the existence of exotic hadron states [8]. Although charged charmoniumlike states like the Z c (3900) provide convincing evidence for the existence of multi-quark states [9], it is more difficult to distinguish neutral candidate exotic states from conventional charmonium. Moreover, the study of transitions between charmonium(like) states, such as the Y (4260) → γX(3872) [10], is an important approach to probe their nature, and the connections between them. Thus, a more complete understanding of the charmonium(like) spectroscopy and their relations is necessary and timely. In this Letter, we report a search for the production of the ψ 2 state via the process e + e − → π + π − X, using 4.67 fb −1 data collected with the BESIII detector operating at the BEPCII storage ring [14] at center-of-mass (CM) energies that range from √ s = 4.19 to 4.60 GeV [15]. The ψ 2 candidates are reconstructed in their γχ c1 and γχ c2 decay modes, with χ c1,c2 → γJ/ψ and J/ψ → ℓ + ℓ − (ℓ = e or µ). A GEANT4-based [16] Monte Carlo (MC) simulation software package is used to optimize event selection criteria, determine the detection efficiency, and estimate the backgrounds. For the signal process, we generate 40,000 e + e − → π + π − X(3823) events at each CM energy indicated above, using an phase space model, with X(3823) → γχ c1,c2
