1^{-+} light exotic mesons in QCD

Abstract

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 M_H=1.81(6) GeV and M_H < 2.2(2) GeV from the positivity of the spectral function. In the same way, but to LO, the four-quark state mass is M_{4q}= 1.70(4) GeV and M_{4q} < 2.4(1) GeV, while the molecule mass is about 1.3(1) GeV. The observed \pi_1(1400) and \pi_1(1600) might 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 \theta= -(11.7+- 2.2)^0, which slightly favours a molecule/four-quark mixing, and which eventually suggests that the \pi_1(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.