1−+ light exotic mesons in QCD

We systematically re-examine the extraction of the masses and couplings of the 1−+ hybrid, four-quark and molecule mesons from QCD spectral sum rules (QSSR). To NLO for the perturbative and power corrections, the hybrid mass is MH=1.81(6)GeV and MH⩽2.2(2)GeV from the positivity of the spectral function. In the same way, but to LO, the four-quark state mass is M4q=1.70(4)GeV and M4q⩽2.4(1)GeV, while the molecule mass is 1.3(1)GeV. The observed π1(1400) and π1(1600) could be explained by a two-component mixing with the set of input masses (1.2-1.3; 1.70-1.74) GeV and with a mixing angle θ≃−(11.7±2.2)°, which slightly favours a molecule/four-quark mixing, and which eventually suggests that the π1(2015) is mostly an hybrid meson. Isospin and non-exotic partners of the previous states and some of their radial excitations are also expected to be found in the energy region around 2 GeV. Further tests of this phenomenological scenario are required.