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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.
The 3[pi] system produced in the reaction[gamma]p[yields][pi][sup+][pi][sup+][pi][sup -] n4.8-5.4 GeV is investigated from the E01-017 (g6c) running of CLAS. This energy range allows for the study of excited mesons in the 1-2 GeV mass range, in their decay to 3[pi], proceeding through[rho][pi] and f[sub 2][pi] emissions. At these energies, there is significant overlap in phase space for events with[rho] and f[sub 2] production, recoiling off an excited baryon, such as the[Delta](1232), N*(1520) and N*(1680). We show that after few kinematic selections, events of the latter type are suppressed in the final data set, allowing us to perform a PWA on the 3[pi] system.
The pi 0 → gamma gamma width is determined to next to leading order in the combined chiral and 1/Nc expansions. It is shown that corrections driven by chiral symmetry breaking produce an enhancement of about 4.5% with respect to the width calculated in terms of the chiral-limit amplitude leading to Gamma{sub {pi}}° → [gamma][gamma] = 8.1 +/- 0.08 MeV. This theoretical prediction will be tested via pi 0 Primakoff production by the PRIMEX experiment at Jefferson Lab.
The pi 0 {yields} gamma gamma width is determined to next to leading order in the combined chiral and 1/Nc expansions. It is shown that corrections driven by chiral symmetry breaking produce an enhancement of about 4.5% with respect to the width calculated in terms of the chiral-limit amplitude leading to Gamma{sub {pi}}{sup 0} {yields} {gamma}{gamma} = 8.1 +/- 0.08 MeV. This theoretical prediction will be tested via pi 0 Primakoff production by the PRIMEX experiment at Jefferson Lab.
Two charged pion final states are studied for GeV photons incident on protons. The data come from Thomas Jefferson National Accelerator Facility using the CLAS detector. A tagged photon beam of 0.5-2.4 GeV/c was produced through bremsstrahlung radiation and was incident on a liquid H{sub 2} target. This analysis looks at the reaction {gamma}p {yields} p{pi}{sup +}{pi}{sup -} using a partial wave analyis to identify intermediate baryon resonances. Total cross section is compared to previous experiments and preliminary differential cross sections for intermediate baryon resonance quantum numbers are shown.
We have measured the differential cross section for the $\gamma$$^3$He$\rightarrow \pi^+ t$ reaction. This reaction was studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.50 to 1.55 GeV were incident on a cryogenic liquid $^3$He target. The differential cross sections for the $\gamma$$^3$He$\rightarrow \pi^+ t$ reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model.