Predictions regarding the excited baryon spectrum provided by symmetric quark models called Constituent Quark Models (CQMs) show good agreement with experimental measurements in the low-energy region (less than ≈ 1.8 GeV). The mass region above ≈ 1.8 GeV, however, contains many resonances which are predicted to exist by these models but have not been experimentally verified. This describes a well known problem in Baryon Spectroscopy, the issue of missing resonances. These resonances are considered missing as the mass measurements made regarding these resonances are either absent or fairly large in their uncertainties. This discrepancy between the theoretical predictions and the experimental measurements can be attributed to several sources. Firstly, the majority of the data regarding the excited baryon spectrum originates from pion-nucleon or kaon-nucleon scattering (which the missing resonances may only weakly couple to). Therefore, as suggested by recent quark model calculations, a study of reactions involving photoproduction (γp) may present a better opportunity for the production of these missing resonances. In addition, previous analyses involved unpolarized data. This absence of polarization leads to ambiguous analysis results, therefore a constraint such as the polarization of the photons can be used in order to further constrain the kinematics of the reaction(s). The analysis of polarized photoproduction data (involving polarization of the photon and/or target) in the low-energy region (< 1.8 GeV) presents the opportunity to further study previously observed resonances, possibly resolving currently unanswered questions about their properties. An analysis of polarized photoproduction data in the high-mass region (> 1.8 GeV) allows for a study of the resonances contributions, providing insight into the issue of the missing resonances.
The study of a photoproduced 3-body final state (such as γp → p π+π-) has been indicated as a promising method for detecting the effects of the missing resonances as this final state topology accounts for most of the cross section above ≈ 1 GeV. A study of double-meson final states very well may fill the holes in the experimental data as the majority of analyses regarding this issue have come from the analysis of quasi 2-body final states (such as Nπ, Nη, Nω, KΛ, and KΣ). It is also likely that these missing resonances decay to high mass intermediate states instead of directly into a meson and a ground state nucleon. Therefore the decay of these resonances is more of a chain (resulting in a two-meson-one-ground-state-nucleon state) than a direct decay.
The presence (or absence) of these missing resonances can be determined through quantities sensitive to resonance contributions called polarization observables. A study of a pseudoscalar single-meson final state produced via polarized photoproduction gives access to 7 of these polarization observables. The analysis of a double-meson final state however, allows access to a total of 15 polarization observables. These additional 8 polarization observables (which are unique to a double-meson final state) arise from the more complicated kinematics needed to describe the reaction. For example, the introduction of a second meson leads gives rise to a second frame in which the two meson are produced back-to-back as well as two additional angles describing the orientation of this plane. By measuring these sensitive quantities, a determination of what resonances contribute, or do not contribute, to the excited baryon spectrum can be determined.
Presented in this work are the first ever measurements of the polarization observable Is for a final state with two pions and the first ever measurements of Ic for a final state containing charged pions (let alone the first measurements of both observables for the specialized case of γp → p π+π- reactions). The presented measurements were made using the high-statistics data available in the CLAS g8b data set. This data were taken at the Thomas Jefferson National Accelerator Facility (JLab) from July 20th to September 1st of 2005 using linearly polarized photons, an unpolarized liquid hydrogen (LH2 target), and the CEBAF Large Acceptance Spectrometer (CLAS). The highly-polarized photons were produced via bremsstrahlung using an unpolarized electron beam provided by the Continuous Electron Beam Accelerator Facility (CEBAF) accelerator and a well-oriented diamond radiator. These polarized photons were produced at five different coherent edge energies: 1.3 GeV, 1.5 GeV, 1.7 GeV, 1.9 GeV, and 2.1 GeV. Considering the 200 MeV-wide window of highly polarized photons whose upper limit is the coherent edge energy, and the five different coherent edge energies used, highly polarized photons were produced covering an total energy range of 1 GeV. These data along with the utilized analysis tools have lead to clean, continuous, low-error measurements of Is and Ic which will aide the hadronic physics community in its search for the complete description of one of the most fundamental systems in nature, the baryon.