Session MH: Nucleon Structure III

Results of the Proton Charge Radius Experiment (PRad) at JLab

In order to investigate the proton charge radius puzzle, the PRad experiment (E12-11-106) was performed in 2016 in Hall B at Jefferson Lab, with both 1.1 and 2.2 GeV unpolarized electron beams. The experiment aims to measure the $e-p$ elastic scattering cross section in an unprecedented low values of momentum transfer squared region ($\rm{Q}^2 = 2\times10^{-4} - 0.06~\rm{(GeV/c)}^2$), with a sub-percent precision. The PRad experiment utilizes a calorimetric method that is magnetic spectrometer free. Its detector setup includes a large acceptance and high resolution calorimeter (HyCal), and two large area, high spatial resolution Gas Electron Multiplier (GEM) detectors. To have a better control over the systematic uncertainties, the absolute $e-p$ elastic scattering cross section is normalized to that of the well-known M$\o$ller scattering process, which was measured simultaneously within similar kinematics and detector acceptances. The windowless H$_2$-gas-flow target utilized in the experiment largely removes a typical background source, the target cell windows. In this talk, we will present results of the experiment.   

Unravelling the $^3$He Electromagnetic Form Factors

New global fits of the $^3$He elastic cross section world data will be presented along with extractions of both electric and magnetic form factors and charge densities. The updated $^3$He fits were motivated by new high $Q^2$ data. The resultant $^3$He first magnetic form factor minimum is found to have shifted up in $Q^2$ by several fm$^{-2}$. Further, large discrepancies exist between theory predictions of the magnetic form factor and those determined by elastic electron scattering. To address this discrepancy a new experiment has been proposed for Jefferson Lab's Hall C to measure the double-polarization asymmetry of $^3$He. This would be the first extraction of $^3$He form factors using polarization observables. The advantage of this double-polarization measurement is that, unlike traditional Rosenbluth methods, the extraction is sensitive to the signs of the form factors. As a result, the sign of the asymmetry flips at each form factor minima. Double-polarization experiments have found large disagreement, particularly at high $Q^2$, between proton form factors extracted via polarization observables and unpolarized Rosenbluth separations. This experiment will determine if such a disagreement exists for $^3$He, while also allowing for hypothesis testing of theoretical models.   

Measuring the electric form factor of the proton at high momentum transfer in Hall A at Jefferson Lab.

For more than 50 years elastic electron scattering has provided a wealth of information about the structure of protons and neutrons through the extraction of nucleon form factors. These, in turn, have been the subject of intense theoretical scrutiny using various techniques ranging from first principles QCD calculations to several phenomenogical models. Moreover, as the first moments of the generalized parton distributions are related to the elastic Dirac form factors of the nucleon through model independent sum rules, elastic electron scattering studies help sharpen our 3D picture of a nucleon. Here we will give an update/status report on a Jefferson Lab experiment aiming extend, with good statistical and systematic precision, the measurements of the electric form form factor of the proton to four momentum transfers up to 12 $GeV/c^2$ using the JLab 11 GeV electron beam and a super big bite spectrometer in Hall A in conjunction with a highly segmented electromagnetic calorimeter. The experimental technique as well as the potential impact of such measurement on the field will be discussed.   

Neutral Pion Electroproduction in the Deeply Virtual Regime at 12 GeV Jefferson Lab

Deep exclusive processes can allow access to Generalized Parton Distributions (GPDs), a concept that lies at the root of 3D imaging of the proton's quark-gluon substructure, as GPDs contain information about the transverse spatial distribution of quarks and their longitudinal momentum inside hadrons. The key to extracting GPDs from experiments are the Quantum Chromodynamics (QCD) factorization theorems. Deeply Virtual Compton Scattering (DVCS) is the cleanest way to study GPDs. While DVCS data have given hints of the factorization regime being attained, such hints have not been observed for Deeply Virtual Meson Production (DVMP) data. Exclusive $\pi^{\mathrm{0}}$ electroproduction has been measured by experiment E12-06-114 in Hall A of JLab in order to test factorization in DVMP processes. Cross sections have been measured at three fixed Bjorken-$x$ ($x_{B})$: 0.36, 0.48 and 0.6 in the Q$^{\mathrm{2\thinspace }}$range 3 to 9 GeV$^{\mathrm{2}}$. High statistical measurements of polarized and unpolarized cross sections of H(e,e'$\gamma )$p could allow mapping and extraction of GPD information from the nucleon. In this talk, I will show the experimental setup, calibration and preliminary results of the neutral pion electroproduction cross sections for $x_{B}$ \textgreater 0.3 from this experiment.   

Extraction of Twist-3 Observables from Deeply Virtual Compton Scattering

Imaging the 3D partonic structure of the nucleon is a fundamental goal of every major nuclear experimental program, including the EIC. Ji first proposed Deeply Virtual Compton Scattering (DVCS) as a probe for understanding the spatial distribution of the partons by fourier transform of the exchanged momentum transfer between the initial and final proton. The extraction of observables from Deeply Virtual Exclusive Reactions in a clear and concise formalism, such that the various twist components and angular dependencies can be untangled, is key. We present a completely covariant description of the DVCS process that can be extended to any kinematics, either fixed target or collider. In our helicity formalism, we extract observables such that the dependence on $Q^{2}$ is clear and kinematic suppressions are not confused with higher twist observables. We have extended our formalism to other Exclusive Reactions, such as Timelike Compton Scattering (TCS), which we demonstrate is key in the extraction of Twist-3 observables used to study the orbital angular momentum of the proton.   

Science opportunities with a Neutral Particle Spectrometer in Hall C at Jefferson Lab.

The two-arm combination of a high-resolution neutral-particle spectrometer (NPS) and a magnetic spectrometer offers unique scientific capabilities for studies of the transverse spatial and momentum structure of the nucleon in Hall C. It makes possible measurements of the basic semi-inclusive neutral-pion cross section to validate QCD factorization, a cornerstone of 3D transverse momentum imaging. It enables precision measurements of the deeply-virtual Compton scattering cross section at different beam energies to extract the real part of the Compton form factor without any assumptions. The combination of high precision calorimetry with NPS allows measurements to push the energy scale of real Compton scattering, the process of choice to explore factorization in a whole class of wide-angle processes, and its extension to neutral pion photo-production. The combination of high precision calorimetry with NPS and a novel compact high intensity photon sources greatly enhances scientific benefit to exclusive processes like wide-angle and time-like Compton scattering with transverse polarized targets. In this talk I will give an overview of the science program and discuss the status of the NPS construction including data from recent prototype tests.   

Source for Compton Scattering from Solid Polarized Targets

Wide angle Compton scattering (WACS) from polarized protons holds great promise: access to the generalized parton distribution functions $\tilde{H}$ and $\textit{E}$ with different weighting and moments than in other hard exclusive processes, emphasizing the $\textit{u}$-quarks and the valence region. Previously, experiments were proposed using bremsstrahlung from polarized electrons striking a radiator. Unfortunately, the mixed electron-$\gamma$ beam limits the polarized target performance due to radiation damage and restricted luminosity owing to the heat load. We will present the technical design concept of a compact, high intensity photon source (CPS) to be used with dynamically nuclear polarized targets. The novel CPS technique opens access to physics processes with very small scattering probabilities, not possible with currently existing facilities. Capable of producing 10$^{12}$ equivalent $\gamma$/sec, the CPS will result in a large gain in polarized experiment figure-of-merit (by a factor of $\sim$ 30). Compared to a traditional bremsstrahlung photon source the CPS will present several advantages, including much lower radiation levels, both prompt and post-operational due to the beam line elements radio-activation.