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Here we report a high precision measurement of electron beam polarization using Compton polarimetry. The measurement was made in experimental Hall A at Jefferson Lab during the CREX experiment in 2020. A precision of dP/P = 0.36% was achieved detecting the back-scattered photons from the Compton scattering process. This is the highest precision in a measurement of electron beam polarization using Compton scattering ever reported, surpassing the ground-breaking measurement from the SLD Compton polarimeter. Such precision reaches the level required for the future flagship measurements to be made by the MOLLER and SoLID Experiments.

The GlueX experiment at Jefferson Lab studies photoproduction of mesons using linearly polarized 8.5GeV photons impinging on a hydrogen target which is contained within a detector with near-complete coverage for charged and neutral particles. We present measurements of spin-density matrix elements for the photoproduction of the vector meson rho(770). The statistical precision achieved exceeds that of previous experiments for polarized photoproduction in this energy range by orders of magnitude. Here we confirm a high degree of s-channel helicity conservation at small squared four-momentum transfer t and are able to extract the t-dependence of natural and unnatural-parity exchange contributions to themore » production process in detail. We confirm the dominance of natural-parity exchange over the full t range. We also find that helicity amplitudes in which the helicity of the incident photon and the photoproduced rho(770) differ by two units are negligible for -t<0.5GeV²/c².« less

Here, we report the total and differential cross sections for $$J/\psi$$ photoproduction with the large acceptance GlueX spectrometer for photon beam energies from the threshold at 8.2 GeV up to 11.44 GeV and over the full kinematic range of momentum transfer squared, $$t$$. Such coverage facilitates the extrapolation of the differential cross sections to the forward ($t = 0$) point beyond the physical region. The forward cross section is used by many theoretical models and plays an important role in understanding $$J/\psi$$ photoproduction and its relation to the $$J/\psi$$-proton interaction. These measurements of $$J/\psi$$ photoproduction near threshold are also crucialmore » inputs to theoretical models that are used to study important aspects of the gluon structure of the proton, such as the gluon Generalized Parton Distribution (GPD) of the proton, the mass radius of the proton, and the trace anomaly contribution to the proton mass. We observe possible structures in the total cross section energy dependence and find evidence for contributions beyond gluon exchange in the differential cross section close to threshold, both of which are consistent with contributions from open-charm intermediate states.« less

The proton is one of the main building blocks of all visible matter in the Universe. Among its intrinsic properties are its electric charge, mass and spin. These properties emerge from the complex dynamics of its fundamental constituents—quarks and gluons—described by the theory of quantum chromodynamics. The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering. An example is the highly precise measurement of the electric charge radius of the proton. By contrast, little is known about the inner mass density of the proton, which is dominated by the energymore » carried by gluons. Gluons are hard to access using electron scattering because they do not carry an electromagnetic charge. Here we investigated the gravitational density of gluons using a small colour dipole, through the threshold photoproduction of the J/ψ particle. We determined the gluonic gravitational form factors of the proton from our measurement. Here we used a variety of models and determined, in all cases, a mass radius that is notably smaller than the electric charge radius. In some, but not all cases, depending on the model, the determined radius agrees well with first-principle predictions from lattice quantum chromodynamics. This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter.« less

In this work, we report a precise measurement of the parity-violating asymmetry $$A_{\rm PV}$$ in the elastic scattering of longitudinally polarized electrons from $$^{48}{\rm Ca}$$. We measure $$A_{\rm PV} =2668\pm 106\ {\rm (stat)}\pm 40\ {\rm (syst)}$$ parts per billion, leading to an extraction of the neutral weak form factor $$F_{\rm W} (q=0.8733$$ fm$$^{-1}) = 0.1304 \pm 0.0052 \ {\rm (stat)}\pm 0.0020\ {\rm (syst)}$$ and the charge minus the weak form factor $$F_{\rm ch} - F_{\rm W} = 0.0277\pm 0.0055$$. The resulting neutron skin thickness $$R_n-R_p=0.121 \pm 0.026\ {\rm (exp)} \pm 0.024\ {\rm (model)}$$~fm is relatively thin yet consistent with manymore » model calculations. The combined CREX and PREX results will have implications for future energy density functional calculations and on the density dependence of the symmetry energy of nuclear matter.« less

We report precision determinations of the beam-normal single spin asymmetries (A_n) in the elastic scattering of 0.95 and 2.18 GeV electrons off ¹²C, ⁴⁰Ca, ⁴⁸Ca, and ²⁰⁸Pb at very forward angles where the most detailed theoretical calculations have been performed. The first measurements of A_n for ⁴⁰Ca and ⁴⁸Ca are found to be similar to that of ¹²C, consistent with expectations and thus demonstrating the validity of theoretical calculations for nuclei with Z ≤ 20. We also report A_n for ²⁰⁸Pb at two new momentum transfers (Q²) extending the previous measurement. Our new data confirm the surprising result previously reported,more » with all three data points showing significant disagreement with the results from the Z ≤ 20 nuclei. These data confirm our basic understanding of the underlying dynamics that govern A_n for nuclei containing ≲ 50 nucleons, but point to the need for further investigation to understand the unusual A_n behavior discovered for scattering off ²⁰⁸Pb.« less

The beam-normal single-spin asymmetry, $$B_n$$, exists in the scattering of high energy electrons, polarized transverse to their direction of motion, from nuclear targets. To first order, this asymmetry is caused by the interference of the one-photon exchange amplitude with the imaginary part of the two-photon exchange amplitude. Measurements of $$B_n$$, for the production of a $$\Delta(1232)$$ resonance from a proton target, will soon become available from the Qweak experiment at Jefferson Lab and the A4 experiment at Mainz. The imaginary part of two-photon exchange allows only intermediate states that are on-shell, including the $$\Delta$$ itself. Therefore such data is sensitivemore » to $$\gamma\Delta\Delta$$, the elastic form-factors of the $$\Delta$$. This article will introduce the form-factors of the $$\Delta$$, discuss what might be learned about the elastic form-factors from these new data, describe ongoing efforts in calculation and measurement, and outline the possibility of future measurements.« less