This coming October we’ll mark the 50th anniversary of the first hike up Mt. Monadnock by the Physics Department. We plan to hike Saturday, October 8th. Because the park recommends reservations, we will make reservations for a large group. Alumni are welcome and should contact Tom Blum or Alex Kovner as soon as possible to secure a parking spot. We’re also collecting pictures from past hikes for a slide show during the colloquium on Friday, October 7th. We hope to see you come October!
The first two experiments using the newly constructed collection of apparatus known as the Super BigBite Spectrometer were completed from Oct. 2021-Feb. 2022 in Jefferson Lab’s Experimental Hall A. Data were collected that will determine the neutron’s magnetic form factor (GMN) in a previously unexplored regime of momentum transfer Q2 up to 13.6 (GeV/c)2 with unprecedented precision. Form factor measurements at these energies are sensitive to the structure of the neutron at the sub-femtometer scale, and can resolve features of the neutron’s charge and current distributions at length scales approximately 20 times smaller than the size of a proton. These two completed experiments were the first in a family of precision studies of proton and neutron form factors at high momentum transfer using the SBS apparatus that will occupy the floor of Hall A through 2024. Professor Andrew Puckett’s group plays a leading role in the SBS collaboration (and the group is looking for several new graduate students to work on this exciting and high-impact program!).
Precision high momentum-transfer nucleon form factor measurements are extremely technically challenging, requiring several major innovations in detector technology and high-performance data acquisition and analysis. The GMN set of experiments achieved the first large-scale deployment and operation of Gas Electron Multiplier (GEMs) detectors in the high-luminosity, high-radiation, high-background-rate environment in Hall A. The GEMs were used in this set of experiments for tracking high-energy electrons through the BigBite Spectrometer, which was designed for detecting, tracking, and identifying scattered electrons with large angular and momentum acceptance at high luminosity. Given the large channel count and the high occupancy of the BigBite GEMs (approximately 42,000 readout strips with up to 30-40% of these firing in every triggered event), the SBS GMN run produced 2 petabytes of raw data (or typically about 1 GB/s during beam-on conditions). This is roughly 5 times as much raw data produced in four months of beam time in Hall A as the previous 25 years of Hall A running combined. Charged particle tracking in this extreme high-background environment is also extremely challenging, and UConn developed the software infrastructure and algorithms to do so with high performance and efficiency. The UConn group was one of the most actively involved in the preparation and execution of the experiment, developed the Monte Carlo simulation, event reconstruction and data analysis software, and is now leading the analysis of the collected data using Jefferson Lab’s scientific computing facilities. Two UConn Ph.D. students, Provakar Datta and Sebastian Seeds, will write their doctoral dissertations on the analysis of the SBS GMN dataset.
Figure 1 shows the collected Q^2 points for the extraction of GMN and the projected accuracy based on the data obtained, compared to existing data, selected theoretical models, and the projected Q2 coverage and precision of a measurement in Hall B with similar physics goals, but with larger systematic uncertainties from qualitatively different sources. The measurement of neutron form factors in the SBS-GMN experiment is based on the so-called “Ratio Method”, in which quasi-elastic electron-neutron and electron-proton scattering are measured simultaneously in scattering on a deuterium target (a deuterium nucleus is a weakly bound state of a single proton and a single neutron). By simultaneously detecting electron-neutron and electron-proton coincidence events in elastic kinematics, the ratio of electron-neutron and electron-proton scattering cross sections is determined with very small uncertainties. Combined with the existing knowledge of the electron-proton scattering cross section, the free neutron cross section can be extracted rather precisely.
To carry out this measurement, the SBS Collaboration constructed a large-acceptance Hadron Calorimeter (HCAL) consisting of large modules of many alternating layers of iron and plastic scintillator, which detects both protons and neutrons in the momentum range of these measurements with very high (and nearly identical) efficiencies, leading to much smaller systematic uncertainties compared to previous measurements of this type. Scattered electrons are detected in the BigBite spectrometer, and the scattering angles and momentum of the electron, as well as the location of the interaction vertex, are reconstructed from the precisely measured tracks of ionization they leave in the BigBite GEMs. Under the assumption of elastic scattering on quasi-free protons or neutrons, the scattered neutron or proton must carry all the energy and momentum transferred from the electron in the hard collision, allowing us to predict the location where the protons or neutrons should be detected in HCAL. To identify whether the scattering occurred on a proton or neutron, the scattered protons are given a small vertical deflection by the SBS dipole magnet so that they are well separated from the scattered neutrons by the time they are detected in HCAL.
Figure 2 shows a comparison between real data from the SBS-GMN experiment obtained at Q2 = 3 GeV2 and the Monte Carlo simulation of the experiment, which includes the full details of the detector geometry and response, and the physics of quasi-elastic scattering of electrons by bound protons and neutrons in the liquid deuterium target, showing the clear separation between protons and neutrons based on magnetic deflection of the protons before they are detected by the SBS Hadron Calorimeter (HCAL), and the low level of background (at this Q2) from processes other than quasi-elastic scattering, demonstrating a very good understanding of the detector at such an early stage of the analysis.
The example event distributions shown below were obtained at an incident electron beam energy of 6 GeV and Q2 = 4.5 GeV2:
Figure 2 (above) shows the invariant mass distributions for reconstructed electrons in BigBite, from the hydrogen (left) and deuterium (right) targets, before and after applying cuts on the angle between the reconstructed momentum transfer direction and the reconstructed scattering angle of the nucleon (proton or neutron) detected in the SBS hadron calorimeter (HCAL). The hydrogen distribution shows a clear peak at the proton mass corresponding to elastic scattering, and the angular correlation cut removes most of the inelastic background, while keeping most of the events in the elastic proton peak. The deuterium distribution is “smeared” by the Fermi momentum of the bound nucleons in deuterium, and the distributions of events passing the angular correlation cut under the hypothesis that the detected nucleon is a proton (red) or neutron (blue) illustrate the relatively clean selection of quasi-elastic scattering and rejection of most inelastic events using the SBS dipole magnet and hadron calorimeter.
Figure 3 (above) illustrates the method for nucleon charge identification using the SBS dipole magnet and the SBS hadron calorimeter. The plot shows the difference in vertical position between the detected nucleon at HCAL and the expected position predicted from the reconstructed electron kinematics assuming elastic (or quasi-elastic) scattering. The distributions are shown for hydrogen and deuterium targets for three different SBS magnetic field settings (magnet off, 70% of maximum field, 100% of maximum field). The hydrogen distributions show a single peak corresponding to elastic electron-proton scattering, that moves as the SBS magnetic deflection is varied. The deuterium distribution with field off shows a single nucleon (proton plus neutron) peak, smeared by Fermi motion. The deuterium distributions with SBS field on show a clear separation into proton (deflected) and neutron (undeflected) peaks, with protons undergoing the same average deflection as seen with the hydrogen target.
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!
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.