Session DJ: Hadronic Structure Measurements and Instrumentation I

Measurements of the Two-Photon Exchange Contributions to the $e-n$ Elastic Scattering Cross-section on the SBS at JLab and Related GEM Crosstalk Analysis

The nTPE experiment ran as part of the first segment of JLab's SBS program. nTPE is a pioneering measurement of the two-photon exchange contribution to the electron-neutron elastic scattering cross section. The experiment had a four-momentum transfer of $Q^2 = 4.5$\,(GeV/c)$^2$. Comparisons of proton form factor measurements acquired through polarization transfer and the Rosenbluth technique show discrepancies, especially as the four-momentum transfer, $Q^{2}$, increases above 2\,(GeV/c)$^2$. 

Two photon exchange is strongly favored as the reason for this discrepancy and the experiment assesses the contribution of two-photon exchange in $e$-$N$ elastic scattering processes by elastically scattering electrons off neutrons in a deuterium target. The scattered electrons were detected in the BigBite spectrometer --- which is equipped with a Gas Electron Multiplier (GEM) detector-based tracker --- while the scattered nucleons were detected in a hadron calorimeter located behind the SBS magnet. Measurement of the ratio D(e,e'n) over D(e,e'p) production rates allows us to reach required low systematics.

High background rates in BigBite's tracker (100 kHz/cm$^{2}$) were present during the experiment and require rejection and clean-track reconstruction techniques for proper extraction of physics results. This talk will discuss the nTPE experiment, GEM detector background rejection techniques, and some preliminary analysis results.    

Precision measurements of the Deuterium to Hydrogen F2 structure function ratio at large x

E12-10-002 was a commissioning experiment for the 12 GeV upgrade in

Hall C at Jefferson Lab which measured H(e,e’) and D(e,e’) inclusive cross

sections using the new Super High Momentum Spectrometer (SHMS).

The experiment focused on the intermediate Q² , large Bjorken x, region where

existing world data is sparse and/or the statistical uncertainites are large.

This new data will better constrain parton distribution function (PDF)

fits, test quark-hadron duality at higher Q² , improve DIS and resonance

modeling, and provide insight on the valence quark dynamics at large x.

New results of the deuterium to proton structure functions, F₂^D/F₂^P , will

be shown along with a brief discription of the analysis, its impact on PDF

fits and preliminary quark hadron duality studies.   

The Hypernuclear Spectroscopy Program at Jefferson Lab

The central question of nuclear science is the structure of nuclear matter. For a complete 

understanding of the evolution of nuclear matter in the universe, detailed 

knowledge is necessary not only for stable nuclei, but also increasingly for 

unstable and exotic nuclear matter. Nuclear matter containing strangeness is 

thought to play an important role in stellar objects like neutron stars. The 

knowledge of the underlying interaction between strange baryons (hyperons) and 

nucleons (protons and neutrons) is incomplete. A complete theory of 

baryon-baryon interactions in the context of the underlying QCD degrees of 

freedom, quarks and gluons, will ultimately also have to describe interactions 

with and between strange baryons. In the laboratory, these can be best studied 

by implanting hyperons in ordinary nuclei, thus converting them to hypernuclei. 

The high quality electron beam at Jefferson Lab allowed for the first time to 

study hypernuclei by (e,e'K$^+$) reaction spectroscopy. Six experiments on 

a number of light to medium heavy targets were conducted.

Currently, the hypernuclear spectroscopy system at Jefferson Lab is being 

upgraded to ensure compatibility with the higher beam energies after the 12 GeV 

upgrade. Four new experiments have already been approved.

They will measure the hyperhydrogen nuclei which will addresses charge symmetry 

breaking or isospin dependence. Spectroscopy 40K and 

48K hypernucllei will study the isospin dependence of the 3-body Lambda-nn 

force. Finally, a study on a lead target will provide data on an extended nucleus with large neutron excess; 

this is the closest approximation to neutron star matter than can be achieved 

in the laboratory.

In this talk, I will review the results from the previous program and conclude with an outlook 

on the future experiments.   

Measurements and Simulations of (e,e’n)/(e,e’p) in the Proton-Rich Nucleus 3He

Recent data mining analyses from the CLAS collaboration at Jefferson Lab have enabled detection of neutrons from the hard break up of Short-Range Correlated (SRC) pairs, leading to the observation that protons are disproportionately represented in high momentum states in neutron-rich nuclei. Here we seek to determine whether neutrons speed up in proton-rich nuclei by studying the proton-rich nucleus ³He using data from the CLAS e2a experiment. This talk will present preliminary measurements of ³He(e, e′n)/³He(e, e′p) in the mean field and short-range correlated pair regimes, indicating an enhancement of neutrons in high momentum states. We also present theoretical predictions of ³He(e,e′n)/³He(e,e′p) in the Plane-Wave Impulse Approximation using 3-body spectral functions.   

Two-Photon Exchange Contribution to the Electron-Neutron Elastic Scattering Cross Section and Data Calibrations for Gas Electron Multiplier Tracking Detectors

The neutron Two-Photon Exchange (nTPE) experiment in Hall A, which uses the 12 GeV electron accelerator at Jefferson Lab and is the part of the Super BigBite Spectrometer (SBS) program, is the first measurement of the two-photon exchange contribution in elastic electron-neutron scattering from a deuterium target at Q² = (GeV/c)². The two-photon exchange contribution to e-N elastic scattering will be extracted by measuring the ratio(neutron/proton) of quasi-elastic yields at a single Q², but at two different beam energies (and electron scattering angles). The experiment was performed using the BigBite Spectrometer which detects the scattered electrons, and the Super BigBite Spectrometer which detects the scattered nucleons using a large aperture dipole magnet and hadron calorimeter. A main component of the BigBite Spectrometer are Gas Electron Multiplier (GEM) tracking detectors. GEMs were manufactured by Istuito Nazionale di Fisica Nucleare (INFN) and the University of Virginia (UVA) and were designed to handle a handle a high rate of charged particle flux. The scope of this talk will be an overview of the physics goals for the nTPE experiment and a brief update about data calibrations for the GEM detectors from the ongoing analysis.

    

Exclusive production of a pair of high transverse momentum photons in pion-nucleon collisions for extracting generalized parton distributions

We show that exclusive massive photon-pair production in pion-nucleon collisions can be systematically studied in terms of QCD factorization approach if the photon's transverse momentum with respect to the colliding pion ??_?? ≫ Λ_{QCD}. We demonstrate that leading power non-perturbative contributions to the scattering amplitudes of this exclusive process are process-independent and can be systematically factorized into universal pion's distribution amplitudes (DAs) and nucleon's generalized parton distributions (GPDs), which are convoluted with corresponding infrared safe and perturbatively calculable short-distance hard parts. The correction to this factorized expression is suppressed by powers of 1/??_??. We also demonstrate quantitatively that this new type of exclusive processes is not only complementary to existing processes for extracting GPDs, but also capable of providing an enhanced sensitivity to the momentum fraction ??-dependence of both DAs and GPDs.   

Online Monitoring using Machine Learning for the SpinQuest Experiment at Fermilab

The SpinQuest experiment (E1039) is a transversely polarized fixed target experiment at Fermi National Accelerator Laboratory that will measure the azimuthal asymmetry of dimuon pair production via scattering of unpolarized protons from transversely polarized NH3 and ND3 targets. By using neural network machine learning, we can quickly filter triggered events and reconstruct the kinematics of the detected muons on a spill-by-spill basis, letting us check the quality of incoming data, detect false asymmetries as they arise, and monitor the overall health of the experiment. This system will also be able to assist in off-line analysis by identifying data that merits examination, allowing us to better utilize existing tools. An overview of this system will be presented, as well as its context in the E1039 experiment.   

Highly-segmented electron-positron telescope array

A new array to study electron-positron pair production in nuclear reactions is being constructed at the Cyclotron Institute, Texas A&M University. The array is suitable for a wide range of experiments. One of the first projects planned for the new setup is validation of the recent claim for observation of the hypothetical X17 "dark boson" particle in the decay of the highly excited state of ⁸Be [Ref1]. Investigation of the production of electron-positron pairs in light-ion collisions below the Coulomb barrier [Ref2] and studies of monopole transition probabilities is also envisioned. The design consists of four highly-segmented double-sided silicon strip detectors (DSSDs)  having 128x128 channels, each backed by 9 CsI(Tl) scintillator crystals (36 in total). A Monte Carlo simulation of the setup, combined with the experimental data obtained with the prototype with an electron source, will be presented, and the performance of the detector will be discussed.

[Ref1] A. J. Krasznahorkay et al. Phys. Rev. Lett. 116, 042501 (2016).

[Ref2] T. Settlemyre, H. Zheng and A. Bonasera, Nucl. Phys. A 1015, 122282 (2021).