| We perform a partial wave analysis on the reaction γp → pπ+π− for photon energies of 0.5–2.4 GeV (W = = 1.35–2.35 GeV/c2). The data was collected using the CLAS detector located at Jefferson Laboratory in Newport News, VA. We are searching for baryon states produced in γp → B and decaying by B → pπ+π − through quasi-two body intermediate states such as Δπ and pρ. Our partial wave decomposition allows us to accurately calculate the total and differential cross section. We also calculate the cross section for γp → Δ++π −, γp → Δ 0π+, and γp → pρ. We identify the D13(1520), P33(1600) and F15(1680) states in the decomposition. We do not see evidence for the baryon state decaying to Δπ at 1700 MeV/c2 proposed by Ripani, et al. [R+03]. We see no strong evidence for the positive parity missing baryons, although there are signals in the data which warrant further investigation.; The constituent quark model does an excellent job of predicting the hadron spectrum. Capstick, Cutkosky, Forsythe, Isgur and Koniuk [KI80a, CR93, CR94, CI86] have augmented the quark model for baryons, including decays, with QCD inspired corrections and get very good agreement with experiment. It has been known however, since the 1960's that there are many predicted baryons which are not observed experimentally [FH68, FH69]. Many of the models use a harmonic oscillator basis, and it is found that these missing states all fall in the N = 2, positive parity band. This prompted Lichtenberg [Lic69] to propose the diquark model, where two of the three quarks become tightly bound. This constraint leads to a spectrum devoid of the missing resonances of the full model. There is nothing in QCD however, which would imply any sort of diquark coupling. Later calculations [KI80a, FC83, CR93] suggest that these missing states may couple more strongly to Nππ final states than Nπ final states. Previous analysis, such as those performed by Manley and Saleski [MS92], have focused on Nπ scattering, where most of the experimental data lies. |
