Hadrons are fundamental particles bound together by strong interaction. Until recently, only two classes of hadrons states had been experimentally established: baryons consisting of three quarks and mesons made of quark-antiquark combinations. There is no a priori reason why only baryons and mesons should exist as bound states of quarks. EU-funded scientists of the BESIII-XYZ (Understanding the nature of exotic hadrons - charmonium spectroscopy at BESIII) project searched for evidence of the existence of exotic combinations of quarks using data from the Beijing Electron-Positron Collider II (BESIII) experiment in China. When the first generation experiment - BES began in 1989, the focus was set on the physics of charm and τ lepton that are accessible at the centre-of-mass energies attainable. Over the past two decades, thanks to the unprecedented high luminosity available, high-precision measurements became possible. This unique data set was used to investigate XYZ particles. The expression 'XYZ particles' is a synonym for hadrons containing a charm and an anti-charm quark. Scientists found a new particle named ψ(13D2) that lies exactly in the XYZ particle mass range and shows properties consistent with conventional charmonium states, suggesting that the theory works well. This discovery revealed that the unexpected behaviour of XYZ particles is not due to limitations in theoretical predictions. On the other hand, some XYZ particles appear not to fit in with the standard charmonium states. The BESIII-XYZ team accumulated for the first time overwhelming evidence that some XYZ particles are indeed of exotic nature and could also be related to each other. Specifically, by analysing data from the beginning of year 2014, scientists demonstrated that the particle X(3872) could be the decay product of another potentially exotic XYZ particle, Y(4260). The particle Zc(3900) was also found in the decay of Y(4260). These surprising observations served an important input to differentiate between various theoretical scenarios for XYZ particles. Moreover, the BESIII-XYZ results have helped to strengthen the position of the European hadron physics community in the physics programme of future electron-positron colliders.
BESIII, quarks, hadrons, BESIII-XYZ, XYZ particles