A High Precision Measurement of the Proton Charge Radius at JLab

Thesis/Dissertation · Mon Feb 03 00:00:00 EST 2020

OSTI ID:1673592

The elastic electron-proton (e-p) scattering and the spectroscopy of hydrogen atoms are the two traditional methods to determine the proton charge radius (r_p). In 2010, a new method using muonic hydrogen (µH)¹ spectroscopy reported a r_p result that was nearly ten times more precise but significantly smaller than the values from the compilation of all previous r_p measurements. This discrepancy is often referred to as the "proton charge radius puzzle". In order to investigate the puzzle, the PRad experiment (E12-11-106) was first proposed in 2011 and performed in 2016 in Hall B at the Thomas Jefferson National Accelerator Facility, with both 1.1 and 2.2 GeV electron beams. The experiment measured the e-p elastic scattering cross sections in an unprecedented low values of momentum transfer squared region (Q² = 2.1× 10^-4-0.06 (GeV/c)²), with a sub-percent precision. The PRad experiment utilized a calorimetric method that was magnetic-spectrometer-free. Its detector setup included a large acceptance and high resolution calorimeter (HyCal), and two large-area, high spatial-resolution Gas Electron Multiplier (GEM) detectors. To have a better control over the systematic uncertainties, the absolute e-p elastic scattering cross section was normalized to that of the well-known Møller scattering process, which was measured simultaneously during the experiment. For each beam energy, all data with different Q² were collected simultaneously with the same detector setup, therefore sharing the same integrated luminosity. The windowless H₂ gas-flow target utilized in the experiment largely removed a typical background source, the target cell windows. The proton charge radius was determined as r_p = 0.831±0.007_stat.±0.012_p fm, which is smaller than the average r_p from previous e-p elastic scattering experiments, but in agreement with the µH spectroscopic results within the experimental uncertainties.

• Research Organization: Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Nuclear Physics (NP)
• DOE Contract Number: AC05-06OR23177
• OSTI ID: 1673592
• Report Number(s): JLAB-PHY-20-3266; DOE/OR/23177-5059
• Country of Publication: United States
• Language: English