GMn/nTPE
GMn is the magnetic form factor of the neutron. We collected scattering data from the Fall of 2021 until the end of Winter 2022 using fixed liquid hydrogen and liquid deuterium targets in Hall A at Jefferson Lab to extract this parameterization of the internal structure of the neutron. The Continuous Electron Beam Accelerator Facility (CEBAF) supplies continuous wave electrons up to 11 GeV to the experimental hall to scatter from our fixed targets. The Super BigBite spectrometer consists of an electron arm which measures scattered electrons and forms the single arm trigger for the experiment and a hadron arm which measures the energy and momentum of corresponding scattered protons and neutrons from many scattering events. From many events we are able to make tight cuts on only those events which correspond to elastic collisions and evaluate the ratio of the neutron to proton elastic cross sections. Along with world data and after judicious accounting for systematic errors, we can calculate GMn at higher inverse momentum transfer than any other experiment and with unprecedented precision.
nTPE is the contribution of two-photon exchange to the elastic electron-neutron cross section. In the Born approximation, one photon exchange (OPE) is expected to dominate e-p and e-n scattering. Using the Rosenbluth technique to separate Sachs form factors, we can extract nucleon cross sections at different values of the virtual photon polarization. With these data, the relative contribution of two photon exchange (TPE) to e-n scattering can be obtained. The discrepancy between elastic scattering measurements which determine form factors using nucleon polarization transfer and the Rosenbluth method appear to show a large contribution of two photon exchange to e-n scattering. At Jefferson National Laboratory during the Fall and Winter of 2021/2022 we collected precision data on the magnetic form factor of the neutron (GMn) at two kinematic points with the same inverse momentum transfer (4.5 GeV squared), but different virtual photon polarizations, to extract nTPE with good precision.
SBS Hadron Calorimeter (HCal)
A hadron calorimeter (HCal) is a device that is designed to measure the position and energy of scattered protons and neutrons (hadrons). The SBS HCal is uniquely designed to measure these quantities with an emphasis on timing and position resolution. For the GMn run group, we fully characterized the performance of the SBS HCal, designed and implemented all front end electronics and logic, set up and tested an independent hadron-arm trigger, configured and characterized dedicated time-to-digital (TDC) and analog-to-digital (ADC) converter channels, cabled and installed the subsystem for use across all further SBS experiments.
Legacy information on HCal including design parameters.
Other Projects
DAC Design, Fabrication, and Installation: The electron and hadron arms of the SBS spectrometer constitute two independent triggers. For each of these arms, it is necessary to set a remotely controllable threshold to the front end electronics to judiciously select real scattering events from the target chamber. We accomplish this by passing a DC voltage to a NIM PS706 discriminator which sets the threshold for signals to be considered for the trigger.
Optics Simulations: Many monte carlo simulations across the various configurations of both the electron arm and hadron arm of the SBS spectrometer informed the beginning optics models which are used to reconstruct electron momentum vectors from electron arm detector signals. We employed a Geant4 package developed specifically for SBS (G4SBS) to this end and the many simulations provided enough statistics to populate the matrix elements of our various optics models in preparation for the GMn run group.
RICH Quantum Efficiency: The SBS Ring Imaging CHerenkov (RICH) detector was set to be deployed for Semi-Inclusive Deep Inelastic Scattering (SIDIS). The 2000+ photomultiplier tubes (PMTs) necessary to achieve the necessary position resolution of the detector were all characterized using a custom PMT testing stand located at Professor Andrew Puckett's lab at UConn. This characterization entailed measuring the gain curves and quantum efficiency of each PMT with a pulsed LED array.
The SBS Coincidence Trigger: In order to properly implement an overlapping regions energy-over-threshold trigger in the electron arm and the hadron arm, the various signal transit times and expected particle flight times were assessed. These data informed the necessary delays to align signals from many detector subsystems with the opening of the necessary triggering windows for the coincidence trigger implemented during the GMn run group.