Status of the PrimEx $η$ experiment at Jefferson Lab

TL;DR

The PrimEx η experiment in Hall D uses the Primakoff effect with tagged photons on a ${}^4\mathrm{He}$ target to extract the η→γγ decay width, addressing tensions between Primakoff and collider measurements and probing low-energy QCD via η–η' mixing and chiral dynamics. The study combines a detailed Primakoff framework with GlueX detector capabilities, including a Compton cross section program and an ongoing Primakoff analysis, supported by a three-phase data run (2019, 2021, 2022) and an upcoming forward calorimeter upgrade. Extensions to heavier targets and a possible 22 GeV energy upgrade aim to improve Primakoff separations, enable η' width measurements, and refine the overall Primakoff program. The work lays groundwork for high-precision η and η' radiative widths, tests of chiral perturbation theory, and enhanced experimental capabilities for forward-angle photoproduction at JLab.

Abstract

The GlueX detector in the experimental Hall $D$ at Jefferson Lab offers a unique opportunity to perform a measurement of the decay width of eta mesons through the Primakoff effect. The PrimEx $η$ experiment complements the physics program at Jefferson Lab on measuring the decay width of light pseudoscalar mesons via the Primakoff process. The goal of PrimEx $η$ is to measure differential cross sections of $η$ mesons at forward angles using a beam of tagged photons incident on a liquid ${}^{4}{\rm He}$ target. The data will be used for the extraction of the decay width. This measurement is vital for understanding fundamental properties like the ratios of the light quark masses and the $η$-$η^\prime$ mixing angle, and will provide an important test of chiral symmetry breaking in QCD. Our experimental results will help reduce uncertainties on partial widths of all other $η$ decays. The experiment collected data during three physics runs between 2019 and 2022. We will give an overview of the PrimEx $η$ experiment and the current status of our data analyses. We will also discuss the feasibility of conducting future Primakoff measurements in light of the recent upgrade of the GlueX forward calorimeter and the potential accelerator energy upgrade to 22 GeV.

Figures (12)

• Figure 1: Measurements of the radiative decay width of $\eta$ meson and the projected result from the PrimEx $\eta$ experiment. Note that the actual uncertainties in the PrimEx measurement may be 2 to 2.5 times larger due to the beamline background observed during the experiment.
• Figure 2: Photoproduction of an $\eta$ meson by a photon in the Coulomb field of a nucleus (Prmakoff effect).
• Figure 3: Theoretical calculations of the differential cross section for $\eta$ meson Primakoff production and associated backgrounds on a ${}^4$He target primex. The long dashed line corresponds to the Primakoff process, the dashed line is the nuclear coherent process, the dash-dotted line is the incoherent process, the dotted line is the interference between the Primakoff and nuclear coherent, and the solid line is the total.
• Figure 4: Schematic view of the GlueX forward spectrometer and the tagger system.
• Figure 5: Schematic view of the PrimEx Compton calorimeter consisting of an array of $12 \times 12$ PbWO$_4$ scintillating crystals with a beam hole of $2 \times 2$ crystals in the middle ccal.