The PRAD collaboration recently published their results on very low Q² electron-proton scattering in Hall B at Jefferson Lab, extracting a proton charge radius of r_p = 0.831 ± 0.007 (stat) ± 0.012 (syst) fm, consistent with the precise extraction of this quantity from measurements of the Lamb shift in muonic hydrogen, first published in 2010.

Nature article

It appears that after nearly a decade of additional investigation, the “proton radius puzzle” that emerged after the original publication of the muonic hydrogen Lamb shift results has been more or less resolved in favor of the smaller proton charge radius of about 0.84 fm, as opposed to the pre-2010 consensus value of around 0.88 fm.

While our group was not directly involved in the measurement, we were very interested in the results, as we are focused on studying the same process (elastic electron-proton scattering) at very large values of Q².

 

Professor Puckett is attending and presenting an invited talk at the workshop “Diquark Correlations in Hadron Physics” at the European Center for Theoretical Studies in Nuclear Physics and Related Areas (ECT*) in Trento, Italy, during the week of Sept. 23-27. The webpage and program of the workshop can be found at the following link.

The first paper from the recently completed family of tritium target experiments in JLab’s Hall A is now published in Physics Letters B. The final published version of the paper can be found online at https://doi.org/10.1016/j.physletb.2019.134890

The INSPIRE-HEP database entry for the paper can be found at http://inspirehep.net/record/1720567

The paper reports the first measurement of the cross section ratios between Helium-3 and tritium for quasi-elastic (QE) proton knockout in electron scattering at large values of the missing momentum, in kinematics for which the contributions of non-QE reaction mechanisms are suppressed (large Q², x > 1). The measured ³He/³H cross section ratios agree with state-of-the-art few-body nuclear theory calculations up to missing momenta of about 250 MeV/c, equivalent to the Fermi momentum, but exceed all available calculations by 20-50% for missing momenta above the Fermi momentum of these A = 3 mirror nuclei. The high-missing-momentum data are sensitive to the dynamics of short-range-correlated two-nucleon pairs that dominate the high-momentum tail of the nuclear wave-function.

Current and former group members including Prof. Puckett, Eric Fuchey, and Freddy Obrecht are co-authors on the publication, having contributed to the data taking for the tritium experiments in the form of shifts and/or run coordination.

The 2010 publication of Professor Puckett’s doctoral dissertation research in Physical Review Letters recently became a “famous paper” (250-499 citations) in the INSPIRE High Energy Physics publication database.

The INSPIRE database entry for the paper can be found here.

The final published version of the paper at the Physical Review Online Archive (PROLA)

The recent, more detailed archival publication of the GEp-III and GEp-2gamma experiments.

Professor Puckett’s doctoral dissertation on which both papers are based.

Effective August 23, 2019, Professor Puckett has been promoted to the rank of Associate Professor and tenured.

 

Experiment E12-07-109, a measurement of the proton electromagnetic form factor ratio to Q²=12 GeV² using the polarization transfer method, of which Professor Puckett is a spokesperson, was recently re-evaluated by the Jefferson Lab Program Advisory Committee at its 47th meeting in July (PAC47), under JLab’s jeopardy review process. Experiments that have been previously approved but are not yet scheduled after a certain time period must be periodically reviewed by the PAC to determine whether they are still relevant, and re-evaluated in terms of both beam time allocations and scientific rating and priority.

The PAC re-approved E12-07-109 with no changes in beam time allocation (45 PAC days) or scientific rating (“A-“). E12-07-109 is projected to run in 2022 based on current informal internal Hall A schedule planning.

The proposal update document submitted for the jeopardy review can be found here.

The figure below shows the projected accuracy of the proton form factor ratio measurements from E12-07-109, compared to existing world data and a selection of theoretical models:

 

Starting August 5, 2019, Professor Puckett was elected by the current membership of the Super BigBite Spectrometer Collaboration Coordinating Committee (CC) to serve a one-year term as the chair of the committee.

The CC is the governing body of the SBS collaboration, formed to ensure the efficient preparation and execution of the experimental program, by identifying tasks which need to be carried out and identifying volunteers from the collaboration to carry out those tasks which the CC chooses to delegate, and by ensuring adequate communication between groups working on different aspects of the project, organizing informal weekly phone meetings and formal collaboration meetings twice a year.

Professor Puckett and the graduate student and postdoc members of the group are at Jefferson Lab for the currently running “PREX-II” experiment in Hall A, ongoing detector assembly and cosmic ray commissioning work for the upcoming Super BigBite installation in Hall A, and for the SBS Collaboration Meeting on August 5-6. The PREX-II experiment is a precise measurement of the parity-violating asymmetry in low-Q² elastic electron scattering from the ²⁰⁸Pb (Lead) nucleus. This measurement is highly sensitive to the “neutron skin thickness” of the lead nucleus, and constitutes a precise test of state-of-the-art nuclear theory and also has significant implications for the equation of state of neutron matter, relevant to the structure of neutron stars.

UConn graduate students Sebastian Seeds and Provakar Datta have been at JLab this summer assembling and cabling the detector package for the BigBite spectrometer, the main electron detector for the upcoming SBS family of experiments in Hall A, in preparation for cosmic ray testing and commissioning prior to installation in Hall A, currently scheduled for spring 2020.

Second-year graduate student group members Sebastian Seeds and Provakar Datta are participating in the 2019 Hampton University Graduate Studies (HUGS) summer school at Jefferson Lab.

Organized annually since 1986, The Hampton University Graduate Studies (HUGS) Summer Program at Jefferson Lab is an internationally renowned summer school in Nuclear Physics designed to train the nuclear workforce’s next generation of researchers. More details of the 2019 program can be found here.

Dr. Eric Fuchey, current postdoc in the group, presented an invited talk at the 8th Workshop of the APS Topical Group on Hadronic Physics in Denver, CO. The subject of the talk was the Jefferson Lab experimental program on Generalized Parton Distributions (GPDs).

The slides from the talk can be viewed at the following link:

EFuchey_GHP2019_final