Breakthrough in the search for exotic hadronic bound states
Observation of the charged charmonium-like Zc(3900) at BESIII
As part of an international team of scientists that operate the BESIII Experiment at the Beijing Electron Positron Collider in China scientists from GSI and the Helmholtz Institute Mainz have made a striking and unexpected first observation from new studies on the mysterious Y(4260) particle. The collaboration has reported that this particle in fact decays to a new, and perhaps even more enigmatic, particle that they named the "Zc(3900)".
Since its 2005 discovery by the BaBar experiment at the SLAC National Laboratory in Stanford California, the Y(4260) particle has continued to mystify researchers. While other particles that share certain similarities to the Y(4260) have long been successfully explained as examples of a charmed quark and anti-charmed quark paired together by the strong force of particle physics, attempts to incorporate the Y(4260) into this model have failed, and its underlying nature remains unknown.
In late December of 2012, the BESIII team embarked on a program of research to produce large numbers of Y(4260) particles by annihilating electrons and antielectrons (positrons) with a total energy that corresponds to the mass of the Y(4260). Once produced, the Y(4260) quickly decays, and its decay products, like the Zc(3900), are measured with the BESIII particle detector.
The anomalous particles of charmonium such as the Y(4260) and recently 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. In the most widely accepted theory of these forces, quantum chromodynamics (QCD), there are in fact more possibilities for charmonium mesons than simply a charmed quark bound to an anti-charmed quark. Some theories predict that more than just a charmed and anti-charmed quark may be bound together to form "tetraquark" or molecule-like mesons. This is the preferred explanation fort he Zc states.
The unique energy resolution of the PANDA experiment at FAIR will allow to finally unravel the structure of exotic mesons such as Zc(3900) and so improve our fundamental understanding of QCD and the diversity of particles in this theory.
Since the Zc(3900) has almost the same mass as another mysterious neutral particle, the X(3872), this new observation may be the first manifestations of a multiplet of exotic mesons and is therefore an experimental breakthrough for the understanding of exotic hadronic states. The Beijing Spectrometer (BESIII) experiment at the Beijing Electron Positron Collider is composed of about 350 physicists from 50 institutions in 11 countries.