Abstract
Proton is not a point-like particle; it has a finite size and an internal structure. The proton charge radius is usually measured through two different methods: using spectroscopy of hydrogen atoms, or through electron-proton (e − p) elastic scattering at low momentum transfer. In 2010, Phol et al [1] published a new measurement for proton charge radius obtained using the spectroscopy method from muonic hydrogen atoms. An ordinary hydrogen (H) atom is a bound state of an electron orbiting a proton. Likewise, a muonic hydrogen atom has a muon orbiting a proton. Since a muon has 200 times more mass than an electron, resulting in a much smaller orbiting radius, its orbit is much more sensitive to the proton charge distribution in space than the electron’s of an ordinary H-atom. Thus, the measurement from muonic hydrogen provides a 10 times more precise result than the previous methods. The radius from the muonic hydrogen measurement reported in [1] was significantly smaller than all the previous measurements combined. The difference between the two values is more than 5σ away. This large discrepancy triggered the so-called Proton Charge Radius Crisis. In order to investigate the proton charge radius, the PRad experiment was performed in experimental hall B at Jefferson Lab in June, 2016. PRad experiment used a novel e − p elastic scattering method. Considering the limitations introduced by magnetic spectrometers in all previous e − p elastic scattering experiments, PRad adopted a magnetic-spectrometer-free, calorimetric method for electron detection. Its detector system consisted of a high energy resolution hybrid calorimeter, closely installed around the beam line, and a pair of large area, high spatial resolution GEM detectors, which improved the experiment position resolution by a factor of 20. The absence of magnetic spectrometers enabled PRad to reach very forward electron scattering angles, and allowed collection of data in a very small momentum transfer region (Q^2 = 2 × 10^{-4} − 6 × 10^{-2} (GeV/c)^2 ), which had never been reached before. The PRad experiment used a windowless gas flow hydrogen target to remove the background from target cell walls, and a vacuum box to further reduce the background from the beam line. To better control the uncertainties, the e − p elastic cross section was normalized to the well-known Møller (electron-electron elastic scattering) cross section; the Møller scattered electrons were collected simultaneously with the e − p electrons within the same detector acceptance for both beam energy settings used for the experiment (1.1 GeV and 2.2 GeV). The result from PRad experiment is r_p = 0.833 ± 0.007 stat. ± 0.012 syst. fm, in agreement with the muonic hydrogen result within the experimental uncertainty.
