Session U10: Nucleon and Meson Structure

The radiative decay width measurement of the η-meson at GlueX

The PrimEx-eta experiment at Jefferson Lab is aiming to measure the radiative decay width of the η-meson via the Primakoff effect. It will provide a critical input to determine the η-η′ mixing angle and the light quark mass ratio model-independently. Three runs were performed with the GlueX experimental setup in Hall D in 2019, 2021, and 2022. We will discuss the status of experiment and how the radiative decay width is extracted from the η-meson photoproduction off a helium nucleus. We will also discuss the measurement of Compton scattering off an atomic electron, which is used to control experimental systematics, including the detection efficiency, the luminosity, and the measurement stability overtime.   

Beam Spin Asymmetry for Deeply Virtual $pi^0$ Electroproduction with CLAS12.

Deeply virtual exclusive meson electroproduction is a powerful tool to study the internal structure of the nucleon as the process amplitude contains Generalized Parton Distributions (GPDs).

Among these reactions, the pseudoscalar meson reactions provide access to the chiral-odd GPDs which gives it a special place in the extensive program of exclusive measurements.

In this talk, we present the new experimental measurements of beam spin asymmetry performed for the deeply virtual exclusive $pi^0$ production in a wide kinematic region with photon virtualities $Q^2$ up to 8 GeV$^2$.

The data were collected by the CEBAF Large Acceptance Spectrometer (CLAS12) at Hall B, Jefferson Lab with a longitudinally polarized 10.6 GeV electrons scattered on an unpolarized liquid hydrogen target.

Sizable asymmetry values indicate a substantial contribution from transverse virtual photon amplitudes to the polarized structure functions.

The interpretation of these measurements in terms of the GPDs demonstrates their sensitivity to the chiral-odd GPD $ar E_T$, which contains information on quark transverse spin densities in unpolarized and polarized nucleons and provides access to the proton's transverse anomalous magnetic moment.   

A New Measurement of the Neutron Elastic Magnetic Form Factor, GMn, up to Q2 = 13.5 GeV2 at Jefferson Lab.

A new experimental program of nucleon elastic form factor measurements at large Q² began in Hall A at Jefferson Lab last year.  The program is based on two large acceptance spectrometers, the BigBite for detecting electrons, and the SuperBigBite for detecting hadrons.  In this talk I will present the first experiment, a measurement of the elastic magnetic form factor of the neutron, G_Mⁿ, up to Q² = 13.5 GeV², with expected statistical precision better than all previous experiments above Q² = 1 GeV².  The measurement was made with beam energy up to 10 GeV using the "ratio” method where G_Mⁿ is extracted from the ratio of d(e,e’p) to d(e,e’n) events in quasi-elastic scattering.  By measuring a ratio, we are less sensitive so many systematics.  The first pass data analysis is now completed.  Preliminary results will be shown.

    

Exclusive Phi Electroproduction Cross Section Measurement off the Proton at CLAS12

One of the main research programs motivating the 12 GeV upgrade at Jefferson Lab is the imaging of the 3-dimensional internal structure of the nucleon at high resolution. The exclusive $phi$ electroproduction process off a hydrogen target probes the proton's gluon Generalized Parton Distribution in the valence region, encoding the correlations between transverse position and longitudinal momentum. Additionally, due to its strange content, the phi meson is a unique probe of the gluon content of the proton in the valence region. An update is given on the exclusive $phi$ meson electroproduction cross section measurement at 10.6 GeV by Jefferson Lab's CLAS12 detector.