Exotic Meson Production in the $f_{1}(1285)\pi^{-}$ System observed in the Reaction $\pi^{-} p \to \eta\pi^{+}\pi^{-}\pi^{-} p$ at 18 GeV/c

This Letter reports results from the partial wave analysis of the π−π−π+η final state in π−p collisions at 18GeV/c. Strong evidence is observed for production of two mesons with exotic quantum numbers of spin, parity and charge conjugation, JPC=1−+ in the decay channel f1(1285)π−. The mass M=1709±24±41MeV/c2 and width Γ=403±80±115MeV/c2 of the first state are consistent with the parameters of the previously observed π1(1600). The second resonance with mass M=2001±30±92MeV/c2 and width Γ=333±52±49MeV/c2 agrees very well with predictions from theoretical models. In addition, the presence of π2(1900) is confirmed with mass M=2003±88±148MeV/c2 and width Γ=306±132±121MeV/c2 and a new state, a1(2096), is observed with mass M=2096±17±121MeV/c2 and width Γ=451±41±81MeV/c2. The decay properties of these last two states are consistent with flux tube model predictions for hybrid mesons with non-exotic quantum numbers.