A High Precision Measurement of the Proton Charge Radius at JLab

<p>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 the muonic hydrogen (μH)\footnote{A muonic hydrogen has its orbiting electron replaced by a muon.} 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, creating the ``proton charge radius puzzle".</p><p>In order to investigate the puzzle, </p><p>the PRad experiment (E12-11-106\footnote{Spokespersons: A. Gasparian (contact), H. Gao, M. Khandaker, D. Dutta}) 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\times10^-4 - 0.06~\rm{(GeV/c)}²), with a sub-percent precision.</p><p>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 M$\o$ller 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 a common normalization parameter. 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 \pm 0.007_\rm{stat.} \pm 0.012_\rm{syst.}~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.</p>

Author dc:creator

• Xiong, Weizhi

Advisor dc:contributor.advisor

• Gao, Haiyan

• Particle physics
• Physics

Handle dc:identifier.uri

https://hdl.handle.net/10161/20844

OAI identifier oai:identifier

oai:dukespace.lib.duke.edu:10161/20844