Author: Andrew Puckett

50 Years of Quantum Chromodynamics Published in EPJ C

The “50 Years of Quantum Chromodynamics” invited review by the European Physical Journal C is now published online at the following link:

https://doi.org/10.1140/epjc/s10052-023-11949-2

Professor Puckett contributed section 10.1 on nucleon electromagnetic form factors. The review is a comprehensive history and introduction to research in QCD intended to serve as a useful reference for researchers in the field from beginning graduate students to senior physicists, as well as a snapshot of current research and future directions in the field.

UConn physics SBS collaborators at Hawaii DNP Meeting, 2023

Professor Puckett and UConn physics graduate students Provakar Datta and Sebastian Seeds were part of a strong contingent of Hall A SBS Collaborators at the recent APS DNP/JPS meeting on the big island of Hawaii, reporting on the progress of various data analyses and the preparations for upcoming experiments. This was the 6th Joint Meeting of the American Physical Society’s Division of Nuclear Physics (APS DNP) and the Physical Society of Japan (JPS). More information about the meeting, including the abstracts and authors, can be found at the meeting website.

SBS collaborators in attendance at the Hawaii DNP meeting pose for a photo following a contributed session with many SBS-related talks.
SBS collaborators in attendance at the Hawaii DNP meeting pose for a photo following a contributed session with many SBS-related talks. Professor Puckett (back row, left), Provakar Datta (front row, left), Sebastian Seeds (back row, third from left), and former UConn postdoc Eric Fuchey (back row, third from right), now at William and Mary.
UConn graduate student Sebastian Seeds starts his talk.
UConn graduate student Sebastian Seeds starts his talk.
SBS Collaboration Members attending the Hawaii DNP meeting, Nov. 2023
SBS Collaboration Members attending the Hawaii DNP meeting pose outside the conference venue.

SBS Collaboration Meeting, July 17-18, 2023

Professor Puckett and UConn graduate students Provakar Datta, Sebastian Seeds, and Kip Hunt participated in the Super BigBite Spectrometer (SBS) Collaboration Meeting at Jefferson Lab on July 17-18, 2023. In the two-day meeting, talks were presented on the status of ongoing experiments, SBS equipment developments, data analysis of completed experiments, preparations for upcoming experiments, and ideas for future experiments using SBS equipment. Additionally, on the first day, Dr. Anatoly Radyushkin gave a seminar on recent developments in the calculation of the nucleon’s quark distributions in lattice gauge theory. At the end of the meeting, an “end of run party” for the first two completed experiments was held at the SURA Residence Facility.

SBS Collaboration photo, July 2023
Attendees of the SBS Collaboration Meeting pose for a group photo at Jefferson Lab in Newport News, Va., on Monday, July 17, 2023. (Photo by Aileen Devlin | Jefferson Lab)

Hall A Winter Collaboration Meeting and High-Energy Workshop

Professor Puckett was at Jefferson Lab the week of January 23-27 to give an invited talk on the science that would be enabled by a potential future energy upgrade of CEBAF to 22 GeV, for the Jefferson Lab Users’ Organization Board of Directors Meeting, and for the Hall A Winter Collaboration Meeting.

The invited talk at the high-energy workshop can be viewed here.

At the Hall A Meeting, Professor Puckett and his graduate students Provakar Datta and Sebastian Seeds all gave invited talks, see below:

Professor Puckett’s talk

Provakar Datta’s talk

Sebastian Seeds’ talk

Winter Hall A Collaboration meeting held at Jefferson Lab on Thursday, January 26, 2023. (Photo by Aileen Devlin | Jefferson Lab)

UConn/SBS Graduate Students Win JLUO Poster Competition

Graduate students from the SBS collaboration won the first (Provakar Datta, UConn) and 3rd (Maria Satnik, College of William and Mary) prizes and an honorable mention (Sebastian Seeds, UConn) in the graduate student poster competition at the Jefferson Lab Users’ Organization annual meeting.

Links to the posters and short video overviews of the posters can be found at the following link:

https://www.jlab.org/jluo2022/posters

First two SBS experiments completed (Feb. 2022)

From Oct. 2021-Feb. 2022, experiments E12-09-019 and E12-20-008 were completed in Jefferson Lab’s Experimental Hall A. Data were collected that will determine the neutron’s magnetic form factor (GMN) in a previously unexplored Q2 regime up to 13.6 (GeV/c)2 with unprecedented precision. These experiments were the first large-scale deployment and operation of Gas Electron Multipliers (GEMs) in the high-luminosity, high-radiation, high-background-rate environment in Hall A. The GEMs were used in this experiment for charged-particle tracking through the BigBite Spectrometer. Given the large channel count and the high occupancy of the BigBite GEMs in Hall A (approximately 42,000 readout strips with up to 30-40% of these firing in every 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. 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. Two UConn Ph.D. students will write their doctoral dissertation on the analysis of the SBS GMN dataset.

Projected Q2 coverage and precision of actually collected SBS GMN data
Fig. 1: Projected Q^2 points and expected precision of the data for the neutron’s magnetic form factor obtained from the SBS GMN run during Oct.-Feb., 2021-2022

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 qualitatively different sources of systematic uncertainty.

The example event distributions shown below were obtained at an incident electron beam energy of 6 GeV and Q2 = 4.5 GeV2:

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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.

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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.

New high-Q^2 form factor results published in PRL

New precision cross section measurements for high-energy elastic electron-proton scattering from Jefferson Lab’s Hall A have recently been published in Physical Review Letters. The measurements significantly improve the precision of the world’s knowledge of the proton magnetic form factor at large momentum transfers and establish that the discrepancy between extractions of the proton’s electric/magnetic form factor ratio from cross section and polarization measurements persists to large values of Q2. The supplemental material for the publication can be found here. The preprint of the accepted manuscript, titled “Form Factors and Two-Photon Exchange in High-Energy Elastic Electron-Proton Scattering” can be found on the arXiv.

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., https://inspirehep.net/literature/1812076)

    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 https://inspirehep.net/literature/1812076 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.

 

SBS SIDIS experiment (E12-09-018) jeopardy proposal re-approved by PAC49

Experiment E12-09-018 (the SBS SIDIS experiment) in Jefferson Lab’s Hall A, first approved by JLab PAC38 for 64 beam-days with an “A-” scientific rating in 2011, was evaluated under the Jefferson Lab Program Advisory Committee’s jeopardy process, which periodically reconsiders the approval status, beam time allocation, and scientific rating of experiments which have been approved but not yet scheduled after a certain amount of time. The experiment is currently expected to run some time in 2023. PAC49, held during July 19-23, considered the continued science motivation for the experiment and reviewed the progress in the preparation of the experiment since it was first approved, and re-approved it with no change in beam time allocation or scientific rating.

The experiment will measure so-called “single-spin asymmetries” (SSAs) in the production of charged and neutral pions and kaons in “deep inelastic” collisions of CEBAF’s continuous electron beam with transversely polarized neutrons in Helium-3 nuclei (transversely polarized means that the spin-1/2 Helium-3 nuclei, and therefore the unpaired neutrons they contain, have their nuclear spins preferentially aligned perpendicular to the direction of the electron beam). These asymmetries are sensitive to the orbital motion and transverse polarization of the neutron’s elementary quark constituents, and can provide for three-dimensional “imaging” of quarks’ motion and spin inside the neutron. The kinematics of the collisions, known as “Semi-Inclusive Deep Inelastic Scattering (SIDIS)” are chosen such that the cross section for the observed reaction is dominated by hard scattering of electrons by quasi-free quarks in the target neutron, and independent “fragmentation” of the recoiling quarks into observable hadrons (pions, kaons, etc).

By using the BigBite and Super BigBite spectrometers in Jefferson Lab’s experimental Hall A, together with an advanced, high-pressure helium-3 gas target polarized via spin-exchange optical pumping, that can withstand very high electron beam currents, the SBS SIDIS experiment will measure the SSAs for production of pions and kaons in “hard” electron-quark collisions on a neutron target with statistical precision 10-100 times greater than any previous experiment on either a proton or deuterium target. Measurements of SSAs involving transverse polarization of the nuclear spins are extremely challenging from an experimental point of view, and as such, very little new data have been collected on these effects for roughly the last decade. The SBS SIDIS experiment represents the best near-future opportunity to make significant experimental progress on these observables.

Figure 1 shows an example of the projected results of the SBS SIDIS experiment for π+ production on the neutron compared to existing data on proton and deuteron targets.

Projected SBS SIDIS data compared to existing
Projected results, with statistical uncertainties, of the approved SBS SIDIS experiment for the so-called “Sivers asymmetry” in the production of positive pions in “deep inelastic” electron-neutron collisions, compared to existing data from the HERMES and COMPASS collaborations on proton and deuterium targets (see linked proposal above for details).
Statistical figure of merit of SBS SIDIS compared to HERMES and COMPASS
Projected statistical “figure of merit”, defined as the reciprocal asymmetry uncertainty squared, of the approved SBS SIDIS experiment (for positive pions only), per unit interval of Bjorken x, compared to existing data from the HERMES and COMPASS collaborations. Integrated over all x values, the SBS SIDIS experiment will reach a statistical figure-of-merit that is 10-30 times higher than any previously published measurement on a proton or neutron target, and 30-200 times higher for x > 0.1