Partial wave analysis of Neutral b1 Meson at GlueX

Abstract

A long-standing goal of hadron physics has been to understand how the quark and gluon degrees of freedom that are present in the fundamental QCD Lagrangian manifest themselves in the spectrum of hadrons. The GlueX experiment at JLab contributes to the global spectroscopy program through studies of the mesons produced using a beam of 8-9 GeV linearly polarized photons. This experiment focuses on the exploration of the light-quark domain, potentially accessing hybrid mesons with exotic JPC quantum numbers in photoproduction reactions. Recent Lattice QCD calculations predict that the lightest exotic π1(1600) decays primarily to a b1 system and can be experimentally accessed through the dominant decay b1 → ωπ0. In this dissertation, direct production of the b1 meson γp → b1p where the axial-vector meson b1(1235) decays to ωπ0, is studied in detail. Results of Partial Wave Analysis (PWA) on the photoproduction of the b1(1235) meson system are shown, with emphasis on the extraction of D-wave to S-wave ratio (termed dsratio), which is a characteristic property of this meson and this coupling has been predicted by Lattice QCD calculations. The dsratio does not depend on the production mechanism nor on features such as the momentum transfer (|t|) or the polarization of the photon beam. PWA in the mass range between 1.0 − 2.0 GeV was carried out in bins of 80 MeV, and three momentum transfer, |−t|, ranges. The analysis procedure was validated using Monte Carlo simulated data to be free of any potential biases. Fits describe the five characteristic angles in the two-plane b1 → ωπ0 → π+π−π0π0 decay. An advanced model selection strategy used the metrics of Likelihood Ratio Test and information criteria Akaike Information Criterion and Bayesian Information Criterion, and demonstrated that the [JP ](ϵ) state of [1+](+) is the most dominant wave which concludes that the photoproduction of b1 is found to proceed through unnatural parity π0 exchange. Additionally, the [1−](+) wave without resonant structure was found to be the second strongest wave. This is consistent with results in the literature from a complimentary experiment that was run more than two decades ago, with the current work exceeding the statistical precision by at least an order of magnitude and systematic uncertainty being competitive with previous measurements. Evidence of the need for higher waves in the higher mass region was found and is in agreement with past results. Contributions from individual m projections for the photoproduction of ωπ0 corresponding to the [1+] as a function of |t| are extracted and presented for the first time in this dissertation. The dsratio was extracted under the condition of a fixed D − S phase (termed dphase) and was shown to be constant as a function of variables. This was expected, although with a lower value than theoretical calculations for the highest |t| bin, which will be investigated in future by others. A thorough set of systematic variations were performed to evaluate the systematic uncertainties. The study concluded that the largest error comes from the choice of the waveset used to describe the data. Future studies should include the variation of dphase. Finally, service contributions were made towards the gain calibration of the Barrel Calorimeter detector built by the Regina group. Additionally, for a period of eight months, a pioneering study was carried out using Artificial Intelligence towards the dimensional and positional optimization of a detector, for the Electron-Ion Collider (EIC) facility under the proto-collaboration named ECCE. The results of this study are summarized in this dissertation.