Search for the Photoexcitation 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 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 partial wave analysis 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.

Figure 1

Missing mass of the three-pion system. The distribution was fit to a Gaussian plus a linear background (solid curve). The inset shows a wider scale. The region between the vertical lines at 0.884 and 0.992 GeV identified the neutron.

Figure 2

Mass spectra. (a) $n{\pi }^{-}$ invariant mass, (b) $n{\pi }^{+}$ invariant mass [the left arrow indicates $N^{*}(1520/1535)$ resonances and the right arrow $N^{*}(1650/1675/1680)$ resonances], (c) ${\pi }^{+}{\pi }^{-}$ invariant mass, and (d) ${\pi }^{+}{\pi }^{+}{\pi }^{-}$ invariant mass. The hatched areas are the distributions of remaining events after the momentum transfer and ${\pi }^{+}$ laboratory angle selections. The histograms in (b) and (c) are filled twice for each event, once for each ${\pi }^{+}$.

Figure 3

(a) Peripheral meson production, (b) baryon resonance background processes.

Figure 4

PWA results: Combined intensities of waves included in the fit. The intensities shown are the sum of the intensities from both ranks of the spin-density matrix: (a) Total intensity, (b) Background intensity, (c) $2^{++}(\rho \pi {)}_D$ intensity, and (d) $2^{-+}$ total intensity. The dashed vertical line in (c) and (d) indicates the border of the two fitting regions. The drop in the isotropic, noninterfering wave (“background”) at the boundary, is a consequence of the reduced wave set used at low mass.

Figure 5

Partial wave decompositions: (a) $2^{-+}(\rho \pi {)}_{P,F}$ (the dashed vertical line indicates the border of the two fitting regions), (b) $2^{-+}(f_2\pi {)}_S$, (c) $1^{++}(\rho \pi {)}_S$, and (d) $1^{-+}(\rho \pi {)}_P$ (the vertical and horizontal arrows indicate the mass and width of the ${\pi }_1(1600)$ as reported by Ref. [10]).