Session Y07: Light Mesons and Baryons

Measurement of the t-dependence for the Beam Asymmetry of Photoproduced η Mesons at GlueX

We report on the photoproduction of η mesons studied at the GlueX experiment at Thomas Jefferson National Laboratory in Newport News, VA. A linearly polarized photon beam at energies between 8.2 and 8.8 GeV, incident on a liquid hydrogen target, produces η dominantly through t-channel exchanges. Azimuthal angular distributions of the η, with respect to the direction of the polarized photon facilitate the extraction of the beam asymmetry Σ for the reaction γp→ηp. We can understand the production mechanisms of the η by analyzing Σ_η, which is sensitive to the particles exchanged. Compared to previous GlueX results [1,2], we have access to five times the statistics, thereby allowing us to extend these measurements to values beyond the previous limitation of -t ≤1.1 (GeV/c)². Preliminary results will be shown for events reconstructed from the decay of η→π⁺π^-π⁰ and η→γγ.

[1] S. Adhikari et al. [GlueX Collaboration], "Beam Asymmetry ∑ for the Photoproduction of η and η' Mesons at E_γ = 8.8 GeV,'' Phys. Rev. C, no. 5, 052201 (2019)

[2] H. Al Ghoul et al. [GlueX],"Measurement of the beam asymmetry ∑ for π⁰ and η  photoproduction on the proton at E_γ = 9 GeV,'' Phys. Rev. C, no.4, 042201 (2017)   

Measurement of t-Dependence of the Beam Asymmetry using a reaction $\gamma$ p $\rightarrow$ $\eta'$ p off the Proton at GlueX

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]. Use of higher statistics will allow us to study whether this holds true at higher momentum transfer. We will  present the  preliminary results from ongoing analysis  in particular azimuthal angular distributions, yield asymmetries and extracted beam asymmetries as a function of  $-t$ for  different  $\eta'$ decay modes such as  $\eta'$   $ \rightarrow$  $\eta$$\pi^{0}$$\pi^{0}$ \  \ and  \  \ $\eta'$ $\rightarrow$  $\eta$ $\pi^{+}\pi^{-}$.    

The $\eta$-meson photoproduction off a helium nucleus and off quasi-free nucleons at GlueX.

The PrimEx-eta experiment at Jefferson Lab is aiming to measure the radiative decay width of the $\eta$-meson with a 3.2$\%$ precision via the Primakoff effect from the $\eta$-meson photoproduction

  off a helium nucleus. The produced $\eta$-meson can be reconstructed by detecting either $\eta \to 2\gamma$ or $\eta \to 3\pi$ decays. The particularly long distance between the target and the Forward Calorimeter gives rise to an irreducible background produced along the beam-line for the decay $\eta \to 2\gamma$

  which is strongly suppressed or not present for the decays $\eta \to 3\pi$. Therefore, reconstructing $\eta$-meson from these two decay channels will help controlling the experimental systematics of

  the differential cross-section measurement for the coherent $\eta$-meson photoproduction off the He target at very forward polar angles where the Primakoff peak is expected. In addition, a large

  $\eta$-meson data sample from photoproduction off a helium nucleus and off quasi-free nucleons will allow a precision measurement of the $\eta \to 3\pi$ Dalitz distribution, which will improve

  light-quark mass ratio determination and search for new CP violation beyond the standard model. The first run was performed with the GlueX experimental setup in Hall D in 2019 and the second run in

  fall 2021 with the same setup but with the GlueX solenoid turn on. We will discuss the status of experiment and data analysis.   

The pion and kaon distribution amplitudes at the physical point from quasi-DA matrix elements

We present our lattice QCD results on the light-cone distribution amplitude (LCDA) of the Pion and the Kaon from their quasi-DA, the moving-pion-to-vacuum matrix element of a bilocal quark bilinear.   We determine the first few Gegenbauer moments of Pion and Kaon from a leading-twist OPE-based analysis and reconstruct their x-dependence based on fits to phenomenological Ansatze. We also investigate the dependence of the asymmetric part of Kaon DA on the dialed amount of SU(3) breaking.   

Calculation of Pion Partonic Distribution at Large x

Within recently developed residual mean field model we calculated pion partonic distribution in the range of Bjorken 0.1 < x < 1. Special emphasize is given to the peaking structure of valence quark distribution in the pion which allowed us to evaluate the parameters of residual system in the pion. With these parameters and by inclusion of quark-quark short range correlation through the hard gluon exchange we calculated high x partonic distribution of the pion and compared with the recently extracted  phenomenological pion PDF from experimental data.   

Update of the Jefferson Lab Eta Factory and the Search for BSM Physics

The planned Jefferson Lab Eta Factory (JEF) experiment relies on the construction of an updated electromagnetic calorimeter in the forward region of the GlueX detector in Hall D, to detect photons from eta and eta’ meson rare neutral decays. Focus on these decay channels is motivated by the search for signatures beyond the Standard Model, probing portals that couple the SM sector to the dark sector via new gauge bosons (e.g., dark scalar (S) and lepto-phobic dark vector (B) bosons) in the invariant mass region below 1 GeV. In addition, these decays will allow access to certain C-violating/P-conserving reactions, as well as test chiral perturbation theory to O(p6) expansion. Updating the inner region of the Forward Calorimeter (FCAL) of the GlueX detector to higher granularity PbWO₄ crystals will improve the reconstruction of multi-photon final states, minimize shower overlaps and optimize the energy and position resolutions. Machine Learning algorithms are being employed to improve shower classification and separation capabilities of the FCAL and will be compared to conventional reconstruction algorithms.   

Prospects for Backward Angle ($u$-channel) Physics at the Electron-Ion Collider

Backward ($u$-channel) production of vector mesons in $ep$ collisions is characterized by a final state consisting of a nearly-stopped proton and a vector meson with a large forward momentum. In conventional (forward, $t$-channel) production, the momentum transfer from the proton to the meson is small and is modeled by pomeron-exchange trajectories. The less well-understood $u$-channel production is modeled by a baryon-exchange trajectory and can provide unique insight into the nucleon wave function and inform contributions to baryon stopping in heavy-ion collisions. The Electron-Ion Collider presents the nuclear physics community with an opportunity to measure $u$-channel production at higher center-of-mass energies than previously available. Measurements of backwards-production events require far-forward detectors for observing high-rapidity mesons. eSTARlight, a code for modeling vector-meson production, is used to simulate backward-production events at the EIC. Several production channels will be analyzed at the known EIC collision energies, and detection capabilities for proposed detector designs will be assessed.   

Pion and kaon distribution amplitudes in the continuum limit

Meson distribution amplitudes (DAs)  hold the key to understanding how light-quark hadron masses emerge from QCD. Meson DAs are also important inputs in many hard exclusive processes at large momentum transfers. In this work, we present a lattice-QCD calculation of the pion, kaon and η_s distribution amplitudes using large-momentum effective theory (LaMET). The calculation is carried out on three lattice ensembles with 2+1+1 flavors of highly improved staggered quarks (HISQ), generated by MILC collaboration, at 310-MeV pion mass with 0.06, 0.09 and 0.12 fm lattice spacings. The resulting lattice matrix elements are renormalized in a hybrid scheme to cancel the linear divergence introduced by the Wilson line without introducing extra non-perturbative effects at large distance, thus a perturbative matching to MS bar scheme is valid. The renormalized matrix elements are extrapolated to the continuum, and then, with a modeling of the longtail, are inverse Fourier transformed to momentum space. The x-dependence of the DA is then extracted through the inverse perturbative matching.   

Global analysis of pion parton distributions using experimental and lattice QCD data

We perform fits of pion parton distribution functions (PDFs) to reduced pseudo Ioffe time distributions and current-current correlator "good lattice cross sections" generated from lattice QCD simultaneously with experimental data. The complementarity of the lattice and experimental data unveils more precise information about the pion PDFs as well as reliably quantifying the systematic effects associated with the lattice data. In this talk, I show the impacts of each of these types of lattice QCD observables as well as the consequences of the simultaneous Monte Carlo analysis.