Open Access

Mathilde Himmelreich

• In the last twenty years, a variety of unexpected resonances had been observed within the charmonium mass region. Although the existence of unconventional states has been predicted by the quantum chromodynamics (QCD), a quantum field theory describing the strong force, a clear evidence was missing. The Y(4260) is such an unexpected and supernummerary state, first observed at BaBar in 2005, and aroused great interest, because it couples much stronger to hidden charm decays (charm-anticharm states like J/Psi or h_c) instead of open charm decays (D meson pairs). This is unusual for states with masses above the D anti-D threshold. Furthermore, it decays into a charged exotic state Y(4260)->Z_c(3900)^+- pi^-+. The charge of the Z_c(3900)^+- is an indication that it comprises of two more quarks than the charm-anticharm pair, and could therefore be assumed to be a four-quark state. Due to these still not understood properties of these QCD-allowed states, they are referred to as exotic XYZ states to emphasize their particularity. In 2017, the collaboration of the Beijing Spectrometer III (BESIII) investigated the production reaction of the Y(4260) resonance based on a high-luminosity data set. This significantly improved precision of the measurement of the cross-section sigma(e+e- -> J/Psi pi^+ pi^-) permitted a resolution into two resonances, the Y(4230) and the Y(4360). The Z_c(3900)^+- had been discovered by the BESIII collaboration in 2013, thus this experiment at the Beijing Electron-Positron Collider II (BEPCII) is a top-performing facililty to study exotic charmonium-like states. In this work, an inclusive reconstruction of the strange hyperon Lambda in the charmonium mass region is performed to study possible decays of Y states in order to provide further insight into their nature. Finding more states or new decay channels may provide crucial hints to understand the strong interaction beyond nonperturbative approaches. Three resonances are observed in the energy dependent cross-section: the first with a mass of (4222.01 +- 5.68) MeV and a width of (154.26 +- 28.16) MeV, the second with a mass of (4358.88 +- 4.97) MeV and a width of (49.58 +- 13.54) MeV and the third with a mass of (4416.41 +- 2.37) MeV and a width of (23.88 +- 7.18) MeV. These resonances, with a statistical significance Z > 5sigma, can be interpreted as the states Y(4230), Y(4360) and psi(4415). Additionally, a proton momentum-dependent analysis strategy has been used in terms of the inclusiveness of the reconstruction and to address the momentum discrepancies between generic MC and measured data.