Author: Andrew Puckett

Professor Puckett’s Group Prepares New Measurements of “femtoscopic” Neutron Structure at Jefferson Lab

UConn group on the floor of Hall A
The UConn group on the floor of Hall A during the SBS installation. From left to right: Postdoctoral Research Associate Dr. Eric Fuchey, Professor Andrew Puckett, and Graduate Research Assistants Provakar Datta and Sebastian Seeds. Click the image for a slideshow of additional installation photos and for more details about the experiment.

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. Jefferson Lab, located in Newport News, Virginia, is a national user facility operated by the US Department of Energy, and is the world’s premiere laboratory for imaging the subatomic (and subnuclear) quark-gluon structure of protons, neutrons, and nuclei using its continuous, polarized electron beam. In addition to Professor Puckett, the UConn researchers involved in this effort are Postdoctoral Research Associate Eric Fuchey, and Graduate Research Assistants Provakar Datta and Sebastian Seeds. The first set of experiments in the SBS program, slated to run during Fall 2021, 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 of order 0.05-0.1 fm (1 fm = one femtometer = 10-15 m = 0.000 000 000 000 001 m), approximately 10-20 times smaller than the size of the proton and approximately 1 million times smaller than the size of a typical atom.

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 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 SBS experiments, collectively known as the “GMN run group”, will answer several important questions about the “femtoscopic” structure of the neutron, including:

  • What is the behavior of the neutron’s magnetic form factor at large momentum transfers? The SBS experiment will dramatically expand the Q2 reach of neutron magnetic form factor data compared to all previously existing measurements, from approximately 4 –> 14 (GeV/c)2. See original experiment proposal here.
  • 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., https://inspirehep.net/literature/1812076)
  • 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)

The GMN run group will start in early September and run through the fall of 2021. The broader SBS program will continue in Hall A through at least 2023, and will drastically improve our understanding of the femtoscopic quark-gluon structure of protons, neutrons, and atomic nuclei. Professor Puckett’s research in the SBS and Hall A Collaborations is supported by the US Department of Energy, Office of Science, Office of Nuclear Physics. Stay tuned!

Prof. Kyungseon Joo named Chair of CLAS Collaboration at Jefferson Lab

Kyungseon Joo, a professor of physics, has been named Chair of the CLAS Collaboration, one of the largest international collaborations in nuclear physics.  CLAS involves 50 institutions from 9 countries and has about 250 collaborators.  The collaboration recently completed the upgrade of the CEBAF Large Acceptance Spectrometer (CLAS12) for operation at 11 GeV beam energy in Hall B at Jefferson National Laboratory in Newport News, VA, funded by the United States Department of Energy.

CLAS12 is based on a dual-magnet system with a superconducting torus magnet that provides a largely azimuthal field distribution that covers the forward polar angle range up to 35°, and a solenoid magnet and detector covering the polar angles from 35° to 125° with full azimuthal coverage. Trajectory reconstruction in the forward direction using drift chambers and in the central direction using a vertex tracker results in momentum resolutions of 1% and 3%, respectively. Cherenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data-acquisition rates allow operation at a luminosity of 1035 cm−2s−1. These capabilities are being used in a broad scientific program to study the structure and interactions of nucleons, nuclei, and mesons, using polarized and unpolarized electron beams and targets for beam energies up to 11 GeV.

As Chair, Joo represents the collaboration in scientific, technical, and managerial concerns, while he closely works with the Lab management on scheduling experiments, organizing collaboration activities and expanding the reach of the collaboration. He currently focuses on collaboration-wide efforts to timely make first publications from CLAS12 with high impact science.

CLAS detector in Hall B at Jefferson Lab
The CLAS12 detector in the Hall B beamline. The beam enters from the right near the upstream end of the solenoid magnet and the cryogenic service tower, followed by the High Threshold Cherenkov Counter and the torus magnet with the drift chambers. The Low Threshold Cherenkov Counter, Forward Time-of-Flight, and the electromagnetic calorimeters are seen at the downstream end to the left.