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Apart from the mesons that the constituent quark model predicts, QCD allows for additional states beyond the qq system. Previous experiments have performed partial wave analysis on pion-production data and claim observation of an exotic JPC = 1-+ state decaying via p-[pi]. The g12 experiment took place at Jefferson Lab using the CLAS spectrometer, a liquid hydrogen target was used and a tagged photon beam. By studying the reactions [gamma]p → n-[pi]+[pi]+[pi]- and [gamma]p → [Delta]++[pi]+[pi]-[pi]-, the photoproduction of mesons decaying to 3-pi was studied using two different but complimentary channels. Events are selected with low four-momentum transfer to the baryon, in order to enhance one pion exchange production. For both 3-pi systems the data exhibit two intermediate decays, p-pi and f2[pi]. For the [gamma]p → n-[pi]+[pi]+[pi]- reaction over 600k events were acquired resulting in the largest 3 photoproduction dataset to date. The exotic JPC = 1-+ partial wave does not show resonant behavior and more so it is strongly consistent with a non-resonant non-interfering wave relative to a resonant [pi]2(1670). Furthermore, the partial wave analysis shows production of the a2(1320) and [pi]2(1670) mesons. For the first time we report observation of a photoproduced a1(1260) meson. For the [gamma]p → [Delta]++[pi]+[pi]-[pi]- reaction nearly 350k events were analyzed. A partial wave analysis was performed for the first time on this channel. The a1(1260), a2(1320), and the 2(1670) mesons were observed. Observation of the a1(1260) confirms the result first reported in [gamma]p → n-[pi]+[pi]+[pi]- reaction.
An amplitude analysis of the reaction .pi.p .-->. .pi.pi.−.pi.°.delta./sup + +/ at 7 GeV/c was performed using the isobar model for the 3.pi. system. The 3.pi. mass covers the range 0.82 to 1.9 GeV. No significant A1 production can be seen. The spin-parity of the .omega.*(1700) is determined to 3−. Properties of A2 and .omega.* production are determined and compared with theoretical models. The background is similar to that seen in analyses of charged 3.pi. systems.
We performed an event-based partial wave analysis (PWA) of the reaction [gamma] p -> p [omega] on a high-statistics dataset obtained using the CLAS at Jefferson Lab for center-of-mass energies from threshold up to 2.4 GeV. This analysis benefits from access to the world's first high precision spin density matrix element measurements, available to the event-based PWA through the decay distribution of omega-> [pi]+ [pi] - [pi]0. The data confirm the dominance of the t-channel [pi]0 exchange amplitude in the forward direction. The dominant resonance contributions are consistent with the previously identified states F[15](1680) and D[13](1700) near threshold, as well as the G[17](2190) at higher energies. Suggestive evidence for the presence of a J(P)=5/2+ state around 2 GeV, a "missing" state, has also been found. Evidence for other states is inconclusive.