Session HK: Mini-Symposium on Photoproduction Physics with GlueX at JLab I

The GlueX Experiment and Data Analysis

The GlueX experiment in Jefferson Lab's Hall D had its first full production run in spring of 2017, which reached a luminosity of 20 pb$^{-1}$ in the coherent peak of its 9 GeV polarized photon beam. The collaboration is working on its primary goal of exploring the light meson spectrum. The spring 2016 commissioning run already provided a wealth of data leading to a recent first publication on the $\rho$ beam spin asymmetry $\Sigma$. Further analysis of this dataset will be shown in this symposium. This talk will set the scene for the GlueX analysis presentations by describing the GlueX detector and photon beam production, the subdetector parts, and their performance. In addition, the coordinated production and analysis common to all studies will be described, and physics highlights to be discussed as part of the symposium will be presented.   

Pseudoscalar Photoproduction at GlueX

I present theoretical prediction for observables (beam asymmetry and differential cross section) for the eta, eta' and pion (neutral and charged) photoproductions.   

Charged Pion Photoproduction Beam Asymmetries at GlueX

The production of single pseudoscalar mesons with a 9 GeV linearly polarized beam provides insight into the production mechanisms for exotic hybrids mesons, a key goal of the GlueX experiment. Single pseudoscalar production also provides a simple system in which comparisons between theory and experiment can be made as GlueX begins to explore this energy regime with unprecedented statistics. Asymmetry measurements have a history dating to bubble chamber-era experiments such as those at SLAC and DESY and can be modeled by quasiparticle $t$-channel exchange using Regge theory. In this talk, the $\Sigma$ asymmetries with charged pions will be considered in the reactions $\gamma p \rightarrow \pi^+ n$, $\gamma p \rightarrow \pi^+ \Delta^0$, and $\gamma p \rightarrow \pi^- \Delta^{++}$.   

Finite-Energy Sum Rules in Eta Photoproduction off a Nucleon

Pseudoscalar-meson photoproduction on the nucleon is of current interest for hadron reaction studies. At low energies it provides information about the nucleon spectrum, while at high energies it reveals details of the residual hadron interactions due to cross-channel particle (Reggeon) exchanges. These two regimes are analytically connected, a feature that can be used to relate properties of resonances in the direct channel to Reggeons in the cross channels. Even though photons couple to both isospin $I=0,1$ states, there are some notable differences between high energy photoproduction of the $\eta$ ($I=0$) and the $\pi^0$ ($I=1$). These differences are ill understood. We have analyzed $\gamma N \to \eta N$ within a FESR framework. Using these sum rules, one is able to obtain the t-dependence of the high-energy Regge residues using low-energy models. The predictions from this approach were tested against the first GlueX results, confirming the our interpretation: a photon beam asymmetry close to $\Sigma=+1$ within the range $-t\approx 0.5-0.6$ GeV${}^2$ indicate that the absence of a dip in eta photoproduction should be attributed to natural exchanges.   

Exclusive $\eta$ photoproduction and $\Sigma$ beam asymmetries at GlueX

GlueX is capable of making $\Sigma$ beam asymmetry measurements using a tagged, linearly-polarized 9 GeV photon beam incident on a hydrogen target. Measurements of the $\Sigma$ beam asymmetry for the exclusive reaction, $\gamma$$p$ $\rightarrow$ $\eta$$p$, will provide insight into the meson production mechanism. These measurements are the first beam asymmetry results for the $\eta$ in this energy range and are expected to further constrain Regge theory models for photoproduced pseudoscalar mesons. This talk will present preliminary results of the photon $\Sigma$ beam asymmetry as a function of the Mandelstam variable, $t$, for multiple $\eta$ decay modes using data from a recent run.   

Study of $\eta'$ photoproduction at the GlueX experiment

The GlueX experiment at Jefferson Lab studies the light meson spectrum and searches for hybrid and exotic mesons. Understanding the photoproduction of $\eta'$ is essential to be able to understand the more complicated exotic mesons. In this analysis preliminary yield and cross section estimates for photoproduced $\eta'$ mesons will be presented for beam energies greater than 7 GeV, which is the first measurement in this energy regime. The $\eta'$ mesons have been identified through two decay channels, $\pi^+ \pi^- \eta$, and $\pi^+ \pi^- \gamma$.   

Beam Asymmetry of the $\eta^\prime$(958) meson through multiple decay channels in the GlueX Experiment

The $\eta$(548) and $\eta^\prime$(958) mesons are two of the richest unflavoured light mesons readily available at GlueX energies for studying meson resonances. Many other light mesons have decay channels involving the $\eta$ mesons with significant branching ratios, and $\pi\eta$/$\pi\eta^\prime$ resonances are among the top contenders for possibly-accessible exotic and hybrid resonances which GlueX aims to study. As such, the ability to reconstruct pure $\eta$/$\eta^\prime$ samples and analyze their decays is of utmost importance for understanding future work on more complicated decay structures. One of the first observables we can measure is the $\Sigma$ beam asymmetry, which for the $\eta^\prime$ has never been measured at beam energies above a few GeV. In this talk, preliminary analysis results for the $\eta^\prime$ beam asymmetry will be presented.