SBS Installation in Hall A at Jefferson Lab, July 2021

Professor Andrew Puckett’s research group is currently leading, as part of a collaboration of approximately 100 scientists from approximately 30 US and international institutions, the installation in Jefferson Lab’s Experimental Hall A of the first of a series of planned experiments known as the Super BigBite Spectrometer (SBS) Program, with beam to Hall A tentatively scheduled to begin in early September of 2021. In addition to Professor Puckett, the UConn group’s involvement in this effort includes Postdoctoral Research Associate Eric Fuchey, and Graduate Research Assistants Sebastian Seeds and Provakar Datta. The first set of experiments is focused on the measurement of neutron electromagnetic form factors at very large values of the momentum transfer Q2, which essentially probe the spatial distributions of electric charge and magnetism inside the neutron at very small distance scales, on the order of 20 times smaller than the charge radius of the proton.

Electrons from Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), with energies of up to 10 GeV (=10 billion electron-volts), will scatter elastically from protons and neutrons in a liquid deuterium target in Hall A. Scattered electrons will be detected in the upgraded BigBite Spectrometer, located on the left side of the beam, while the high-energy protons and neutrons recoiling from the “hard” collisions with the beam electrons will be detected in the SBS by the newly constructed Hadron Calorimeter (HCAL), located on the right side of the beam. The SBS dipole magnet will provide a small vertical deflection of the scattered protons, which allows HCAL to distinguish them from scattered neutrons, which are undeflected by the magnetic field, but produce otherwise identical signals in HCAL.

The first group of experiments will answer several important questions about the “femtoscopic” structure of the neutron, including:

  • What is the behavior of the neutron’s magnetic form factor GM at large momentum transfers? The SBS experiment will dramatically expand the Q2 reach compared to all previously existing neutron data, from approximately 4 –> 14 (GeV/c)2. See original experiment proposal here. Figure 1 shows the projected SBS data together with existing data and selected theoretical predictions for the high-Qbehavior of this form factor.
  • Projected SBS program results on the neutron magnetic form factor, compared to existing data and selected theoretical predictions
    Figure 1: Projected SBS program results on the neutron magnetic form factor, compared to existing data and selected theoretical predictions
  • How is the charge and magnetism of the proton shared among its “up” and “down” quark constituents as a function of Q2? The proton magnetic form factor has been measured over a much wider range of Q2 than the neutron, and combined proton and neutron measurements can be used to disentangle the contributions of “up” and “down” quarks (and diquark correlations) to the proton’s structure, under the assumption of charge symmetry of the strong interactions (see, e.g.,

    Projected results from SBS form factor program
    Projected results on nucleon electromagnetic form factors at large momentum transfers (Q^2) from the upcoming SBS program in Hall A at Jefferson Lab. See for more details.
  • How important and/or significant is the contribution of two-photon-exchange to elastic electron-neutron scattering? The first SBS experiment group will perform measurements of the electric/magnetic form factor ratio for the neutron using two different techniques known as “Rosenbluth Separation” and “Polarization Transfer”, at a Q2 where these two techniques have shown significant disagreement for the proton. Both measurements will be the first of their kind for the neutron at such large Q2 values (see, e.g., Polarization Transfer Proposal and Rosenbluth Separation Proposal).

    neutron two-photon exchange measurement projected precision
    Projected results of the planned Rosenbluth Separation measurement of the neutron form factor ratio, to be compared with the companion polarization transfer measurement of the same ratio at identical kinematics. The experiment will provide a precise test for the neutron of theoretical calculations of two-photon-exchange contributions thought to explain the discrepancy observed for the proton.