image: On the left, the bands shown in the Dalitz plot correspond to states decaying into a pair of pions or one pion and one J/ψ. On the right is the Zc(3900) signal appears as a peak at around 3.9 GeV/c2. view more
Credit: ©Science China Press
Charmonium is a bound state of a charmed quark and a charmed antiquark, and a charmoniumlike state is a resonant structure that contains a charmed quark and antiquark pair but has properties that are incompatible with a conventional charmonium state. The study of these states deepen our understanding of the strong force governing the matter world around us, namely, how the quarks make proton and neutron, and how proton and neutron make nucleus.
The Beijing Spectrometer III (BESIII) experiment at the Beijing Electron–Positron Collider II (BEPCII) storage ring, which for the past decade has been studying particles produced in electron–positron collisions in the tau-charm energy region at center-of-mass energies from 2 to 5 GeV, and has by now accumulated the world’s largest dataset that enables searches for conventional and exotic hadrons, and many other physics.
The BESIII experiment can access a wide mass range of charmonium and charmoniumlike states, and has contributed significantly in this field. The most significant BESIII results include those related to conventional charmonium states, including the first observation of M1 transition ψ(2S) → γηc(2S) and the discovery of the X(3823); and the studies of charmoniumlike states, including the discoveries of the Zc(3900) and Zc(4020) tetraquark candidates, the resolution of the fine structure of the Y(4260), the discovery of the new production process of electron–positron annihilate into γX(3872), and the uncovering of strong evidence for the commonality among the X(3872), Y(4260), and Zc(3900) states.
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See the article:
Charmonium and charmoniumlike states at the BESIII experiment
https://doi.org/10.1093/nsr/nwab182
Journal
National Science Review