Search for the photo-excitation of exotic mesons in the pi+pi+pi- system

A search for exotic mesons in the $\pi^{+}\pi^{+}\pi^{-}$ system photoproduced by the charge exchange reaction $\gamma p\to \pi^{+}\pi^{+}\pi^{-}(n)$ was carried out by the CLAS collaboration at Jefferson Lab. A tagged-photon beam with energies in the 4.8 to 5.4 GeV range, produced through bremsstrahlung from a 5.744 GeV electron beam, was incident on a liquid-hydrogen target. A Partial Wave Analysis (PWA) was performed on a sample of 83,000 events, the highest such statistics to date in this reaction at these energies. The main objective of this study was to look for the photoproduction of an exotic $J^{PC} = 1^{-+}$ resonant state in the 1 to 2 GeV mass range. Our PWA analysis, based on the isobar model, shows production of the $a_{2}(1320)$ and the $\pi_{2}(1670)$ mesons, but no evidence for the $a_{1}(1260)$, nor the $\pi_{1}(1600)$ exotic state at the expected levels. An upper limit of 13.5 nb is determined for the exotic $\pi_1(1600)$ cross section, less than 2% of the $a_2(1320)$ production.