BESIII Experiment at the Beijing Electron Positron Collider in China began a series of specialized studies aiming at an understanding of the anomalous "Y(4260)" particle. As a striking and unexpected first observation from these new studies, the Y(4260) particle decays to a new, and perhaps even more mysterious, particle named the "Zc(3900)", the collaboration reported on Mar. 31th.
While commonly known subatomic particles, such as the proton and the neutron, are comprised of the relatively lightweight up- and down- quarks. The anomalous particles of charmonium, such as the Y(4260) and, now, the Zc(3900), appear to be members of a new class of recently discovered subatomic particles, called the XYZ mesons, that are adding new dimensions to the study of the strong forces that quarks and antiquarks exert on each other. The new observed particle "Zc(3900).", which decays into an electrically charged pi-meson plus a neutral J/psi and, thus, must itself carry an electric charge. Since it decays to a J/psi, the Zc(3900), which has a mass slightly higher than that of a helium atom, must contain a charmed quark and an anti - charmed quark. Different theoretical models have been proposed that attempt to explain how this could come about. The positively charged Zc(3900) particle could be a tightly bound four quark composite of a charmed and anti-charmed quark plus an additional up quark and anti-down quark. Or, perhaps, the Zc(3900) is a molecule-like structure comprised of two mesons, each of which contain a charmed quark (or anti-charmed quark) bound to a lighter anti-quark (or quark). Another scenario is that the Zc(3900) is an artifact of the interaction between these two mesons. In any case, the appearance of such an exotic state in the decay of another exotic state was unanticipated by most researchers.
At present, the ball is clearly in the experimenters' court and there is much hope, by theorists and experimenters alike, that with more experimental data, the veil that continues to shroud these mysterious particles can be lifted. BESIII is (will) accumulating a lot more data that will permit more comprehensive investigations of the nature of this unusual, electrically charged charmonium state. When all of these results are used as inputs to theory, it will open the door toward a fuller understanding of the XYZ particles discovered in recent years.
The Beijing Spectrometer (BESIII) experiment at the Beijing Electron Positron Collider is composed of about 350 physicists from 50 institutions in 11 countries. University of Science and Technology , China (USTC) is the one of the most major Cooperation beside IHEP. The team made large contributions on Zc(3900) discovery directly. it also got great achievement in physics analysis in last few years. At present, the team is highly involved in the searching for Zc(3900) particle in the different decay modes and its properties studies. Beside the data analysis, the USTC BESIII team has made significant contribution on the detector construction and operation, included one third electronics, End-cap Time of Flight, and luminosity monitor detector.
(School of Physical Sciences,ZHU Yingchun)
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