A new measurement of the neutron electric form factor with the Super Bigbite Spectrometer apparatus

Penman, Gary (2025) A new measurement of the neutron electric form factor with the Super Bigbite Spectrometer apparatus. PhD thesis, University of Glasgow.

Abstract

Protons and neutrons, collectively known as nucleons, make up the nuclei at the core of atoms which form our world. The nucleon has been under intensive study for over 100 years, and yet we still do not fully understand the internal dynamics which govern properties like its spin or its mass - which contributes to almost all of the visible mass in the universe. These dynamics are governed by quantum chromodynamics (QCD), the predictions of which are experimentally tested at high energy accelerator facilities such as Jefferson Lab. The GEN-II experiment (E12-09 016) is one such experiment.

GEN-II is part of the Super Bigbite Spectrometer (SBS) experimental form factor programme taking place in Hall A at Jefferson Lab, which aims to make precision measurements of the nucleon electromagnetic form factors (EMFFs) at record high values of squared four-momentum transfer Q2 . EMFFs describe the electric and magnetic moment distributions within the nucleon. They can be measured through elastic electron scattering off the nucleon, and describe the recoil response of the target nucleon at a given energy scale.

GEN-II is a double polarised semi-exclusive beam target asymmetry (BTA) experiment, seeking to measure the electric form factor of the neutron, GnE, at three new values of squared four-momentum transfer Q2 = 2.92,6.74 and 9.82 GeV². The latter two points being at record high Q2 . The form factor is determined through measuring the BTA of quasielastic scattering of a neutron from a polarised nuclear target. The experiment utilised the CEBAF accelerator to produce longitudinally polarised electrons up to ∼85% polarisation, which were scattered off neutrons within a novel polarised helium-3 (³He) target. This new polarised ³He target was employed by building on the technology of its precursors which existed in similar preceding experiments. This target was designed to operate at the high luminosities typical of Hall A, and reached a record breaking combination of polarisation and beam intensity known as figure of merit, three times larger than those predecessors.

The SBS collaboration designed and constructed two brand new high acceptance spectrometers for these experiments, an electron arm named Bigbite (BB) and a hadron arm named Super Bigbite. Both spectrometers featured a large acceptance EM dipole magnet, and complementary detector systems. The electron arm contained gaseous electron multipliers (GEMs) which were used for high precision tracking of the scattered electrons, a heavy gas cherenkov (GRINCH) which was used for PID between electrons and pions, a plastic scintillator timing hodoscope to provide high resolution timing of the start of events, and a pair of EM calorimeters (BBCal) which provided energy measurements of detected particles, and provided the experimental trigger. The hadron arm also contained a system of GEMs which will be utilised for future SBS experiments, and a hadron calorimeter designed to provide position, timing and energy measurements of the recoiling nucleon.

The calibration of all detector subsystems, beam and target data is discussed, with a focus on novel timing calibrations to the hodoscope and hadron calorimeter. An analysis of selecting quasielastic events and suppressing background contributions from a number of sources which contaminate the final event sample is given. The largest irreducible backgrounds are found to be from misidentified protons, timing accidentals and inelastic events. The physical asymmetry is measured and used to extract a value for the form factor ratio GnE/GnM. High precision Q2 data for GnM is used to then extract GnE. This work finds at Q2 = 2.92 GeV2 that GnE = 0.0129+0.0019 −0.0020. This result is in statistical agreement with existing fits to world data, and predictions from the constituent quark model and Dyson–Schwinger equations, in this region of Q2 .

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Montgomery, Dr. Rachel A. and Hamilton, Dr. David .
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85150
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 05 Jun 2025 07:15
• Last Modified: 05 Jun 2025 07:22
• Thesis DOI: 10.5525/gla.thesis.85150
• URI: https://theses.gla.ac.uk/id/eprint/85150