Session MP: Hadrons: Photoproduction at GlueX

Photoproduction of $\eta$ Mesons with the \textsc{GlueX} Experiment

Studies of the production and decay of the $\eta$ meson in photoproduction offer a wide range of physics opportunities: these include constraining models of hadronic photoproduction processes, relevant to searches for novel states in the light meson spectrum, searches for BSM states, high precision tests of chiral perturbation theory, and tests of CP conservation in the QCD sector. \textsc{GlueX}, a high-intensity photoproduction experiment located at Hall~D of Jefferson Lab, offers a high-statistics sample of $\eta$ mesons off the proton and a unique degree of forward boosting in the lab frame to suppress background. However, the production mechanisms of the eta meson are poorly constrained at $E_\gamma=$\hspace*{0.03cm}~6-11~GeV by previous experiments. We present recent results from the \textsc{GlueX} experiment that allow greater insight into $\eta$ photoproduction using several $\eta$ decay modes. Such measurements inform future searches for exotic hybrid mesons and serve as benchmarks for the Jefferson Lab Eta Factory, a future upgrade to the existing \textsc{GlueX} facility.   

Differential $\eta$ meson photoproduction cross sections off the proton at center of mass energies up to 4.75 GeV from the GlueX experiment

The GlueX experiment studies the light meson spectrum and searches for hybrid and exotic mesons. As part of the GlueX program, photoproduction cross sections of $\eta$ mesons have been measured in the $\gamma + p \rightarrow \eta + p$ reaction in a new, previously unexplored regime of higher beam energy and smaller production polar angles. This will provide new information about the underlying reaction mechanisms. $\eta$ mesons have been identified through their $\gamma \gamma$ decay channel which has a branching ratio of 39.41\%. Besides the high energy data, an additional set of low energy photoproduction data has been collected that are overlapping with previously published data. We will present differential cross sections with high statistics for $\eta$-p center of mass energies between 2.52 and 4.75 GeV, which is equivalent to photon beam energies between 2.9 and 11.6 GeV. The results at the lower beam energies will be compared to previously published data and all results will be compared with recent model calculations.   

MP.00003:

The GlueX experiment is a photoproduction experiment which is located at Thomas Jefferson National Lab in Newport News, Virginia. We report on measurements of the beam asymmetry ($\Sigma$) in the reaction $\gamma p\rightarrow \eta' p$, using a tagged, linearly polarized 9 GeV photon beam incident on a liquid hydrogen target. A previous measurement, which was limited to momentum transfer up to $-$t=0.9 (GeV/c)$^2$, indicated that the reaction mechanism is dominated by $\rho$ and $\omega$ meson exchanges [1]. Newly collected data with 3-4 times larger statistics will allow us to study whether this holds true at larger momentum transfer. We will present the preliminary results of the ongoing analysis for azimuthal angular distributions, yield asymmetries and extracted beam asymmetries as a function of $-$t for different $\eta'$ decay modes.   

Beam Asymmetry in $\gamma$p $\rightarrow$ $\eta$ $\Delta^+$ at GlueX

The photo-production mechanism used in the \textsc{GlueX} experiment by impinging an 8.2-8.8 GeV linearly polarized photon beam on a liquid hydrogen target allows the mapping of light mesons in unprecedented detail with particular interest in exotic meson candidates. Polarization observables such as beam asymmetry $\Sigma$, extracted from azimuthal ($\phi$) angular distributions between the meson production plane and the polarized photon beam, help in understanding production mechanisms and quasi-particle exchange processes using Regge theory. We report preliminary results on the beam asymmetry measurements for $\eta$ in $\gamma$p $\rightarrow$ $\eta$ $\Delta^+$. The reaction $\gamma$p $\rightarrow$ $\eta$ $\Delta^+$ provides an opportunity for validation of previous $\eta$ asymmetry measurements and theoretical calculations.   

Measurement of the $t$-dependence for the Beam Asymmetry of Photoproduced $\eta$ Mesons at GlueX

We report on the photoproduction of $\eta$ mesons studied at the GlueX experiment at Thomas Jefferson National Laboratory in Newport News, VA. These particles are produced by a linearly polarized photon beam at energies between 8.2 and 8.8 GeV incident on protons in a liquid hydrogen target. Azimuthal ($\phi$) angular distributions with respect to the direction of the polarized photon facilitate the extraction of the beam asymmetry $\Sigma$ for the reaction $\vec\gamma p \rightarrow \eta p $. $\Sigma$ is derived as a function of four-momentum transfer $-t$. Compared with previous GlueX results [1,2,3], the 2018 run period produced approximately 3-4 times more statistics, thereby allowing us to extend these measurements to values beyond the previous limitation of $-t \leq$ 1.1 (GeV/$c$)$^2$. Preliminary results will be shown for events reconstructed from the decays of $\eta \rightarrow \pi^{+} \pi^{-} \pi^{0} $ and $\eta \rightarrow \gamma \gamma $. [1] S.~Adhikari {\it et al.} [GlueX Collaboration], Phys.\ Rev.\ C {\bf 100}, no. 5, 052201 (2019) [2] H.~Al Ghoul \textit{et al.} [GlueX],Phys. Rev. C \textbf{95}, no.4, 042201 (2017) [3] P.~Collins {\it et al.} [CLAS Collaboration],Phys.\ Lett.\ B {\bf 771}, 213 (2017)   

Partial wave analysis studies with simulated $\eta^{'}\pi^{0}$ events in GlueX

For conventional mesons composed of quark-antiquark pairs certain $J^{PC}$ quantum numbers are not allowed and are called exotic, where $J$ is the total angular momentum of the system and $P$ and $C$ correspond to parity and charge conjugation. Our understanding of how quarks form mesons has evolved within quantum chromodynamics (QCD) and we expect a richer spectrum of mesons by taking into account also gluonic degrees of freedom. This spectrum includes hybrid mesons that are quark-antiquark pairs coupled to gluonic field excitation that can have non-exotic $J^{PC}$ quantum numbers. There is experimental evidence for the $\pi_{1}$ hybrid meson with exotic $J^{PC}=1^{-+}$. The primary motivation of the Jefferson Lab GlueX experiment is the search for light hybrid mesons. Contributions of resonances with different spins in mass spectrum of the $\eta^{'}\pi^{0}$ system can be studied via partial wave analysis (PWA). We use a newly developed model for photo-production via linearly polarized beam to describe the decomposition of the reaction amplitude in terms of partial waves corresponding to different resonances. The goal of the present study is to develop a strategy for the search for the $\pi_{1}\;(1600)$ exotic meson via PWA with simulated data for the $\eta^{'}\pi^{0}$ system.   

Studies comparing $\pi^0 \eta$ and $\pi^0 \eta'$ systems in search for exotic hybrid mesons at GlueX

In the search for exotic hybrid mesons at the GlueX experiment, located at Thomas Jefferson National Laboratory, a strong interest has been placed on both the $\pi^0 \eta$ and $\pi^0 \eta'$ systems because of their possibility of containing non quark-antiquark (exotic) quantum numbers due to the systems' odd angular momentum $L$. By comparing both of these channels, the role of flavor symmetry could also be highlighted, allowing for a better understanding of meson production mechanisms. In this talk, preliminary results, including angular distribution analyses and double Regge exchange analyses (i.e. non-resonant Deck processes), for $\gamma p \rightarrow \pi^0 \eta^{(')}p \rightarrow 4\gamma \pi^+ \pi^- p$ will be shown utilizing all of the GlueX Phase-1 data and comparing the observations to previous experimental (COMPASS) and theoretical (JPAC) results of the same channels.   

Study of Double Regge Exchange in the $\pi^0\eta$ System at GlueX

Hybrid mesons are mesons that have valence quark and gluonic degrees of freedom. Establishing the spectrum of the hybrid mesons promises to provide unique insight into QCD and quark confinement. There is significant evidence for only the lightest exotic hybrid meson known as the $\pi_1$ with observations in the $\pi\eta$ system. For this system any odd-$\ell$ partial wave is exotic and can be seen as an asymmetry in the $cos(\theta)_{GJ}$ distribution. Baryon resonances and the non-resonant Deck process can produce a similar signature. The latter is expected to be a significant background around the $\pi_1$ mass region and therefore needs to be understood. The GlueX experiment, located at Thomas Jefferson National Laboratory, uses a linearly polarized photon beam on a proton target. Photoproduction of the $\pi^0\eta$ system off the proton into the 4 photon final state, i.e. $\gamma p\rightarrow \pi^0\eta p\rightarrow 4\gamma p$, is being analyzed. The non-resonant Deck process dominates the large $M(\pi\eta)$ region. Studies of events in this region will be shown with comparisons to a double-Regge exchange model being developed in collaboration with the Joint Analysis Physics Center(JPAC). A comparison will also be made to the $\gamma p\rightarrow \Delta^{++}\eta\pi^-$ channel.   

Analysis of the reaction $\gamma p \rightarrow \eta^{'} \pi^{0}p$ in GlueX in search of photoproduced exotic mesons

The GlueX experiment in Hall-D at Jefferson lab is dedicated to measure the light meson spectrum and search for hybrid and exotic mesons. The discovery of hybrid mesons will be important to our understanding of the gluonic field responsible for the binding of quarks in hadrons. The experiment uses a tagged linearly polarized photon beam created by coherent Bremsstrahlung and a polarization peaking at around 9 GeV which is incident on a 30 cm long liquid hydrogen target. The detector with an almost 4$\pi$ acceptance and good efficiency for both charged particles as well as photons allows for final states with high particle multiplicities . This analysis focuses on the study of the $\eta^{'}\pi^{0}$ system with the decays $\eta^{'} \rightarrow \pi^{+}\pi^{-}\eta (\eta \rightarrow \gamma\gamma)$ and $ \pi^{0} \rightarrow \gamma\gamma$. We will present initial analysis results of the GlueX Phase-I dataset focusing on the selection of $\eta^{'}$ events and the background studies.