Elastic electron-proton scattering with low-energy electron beam at ELPH*

  Electron scattering is a very powerful tool to study the structure of nuclei. The proton, which is the most fundamental nucleus, has also been investigated by electron scattering for more than half century. In recent years, it becomes clear that the proton charge radius determined by muonic hydrogen spectroscopy is 4% (7 SD) smaller than that determined from electron scattering and hydrogen spectroscopy. This is called the proton radius puzzle.

  The electron elastic scattering cross section depends on the charge and magnetic form factors (G_E(Q²), G_M(Q²)), where Q² is the four momentum transfer. The charge radius is defined by the gradient of the G_E(Q²) at Q² = 0. We will carry out the e+p elastic scattering experiment in ultra-low Q² region (Q²=0.0003-0.008 (GeV/c)²) at ELPH, Tohoku University. The accelerator at ELPH provides a low-energy (E=20-60 MeV) electron beam. It makes possible to do the Rosenbluth separation which separates the G_E(Q²) and G_M(Q²) experimentally. In our Q² region, the change in the G_E(Q²) is only about 2 %, thus, we aim at performing the measurements with systematic uncertainty around 10^-3. To cancel the ambiguity in luminosity and solid angle, the e+p and e+C scatterings will be measured simultaneously by using a polyethylene ([CH₂]_n) target. Because the e+C has been measured very precisely, the absolute cross sections for the e+p can be determined from the ratio of those.

  A spectrometer for the low-energy electrons and a new beam line for the experiment are under construction. In my presentation, I will talk about the details and preparation status of our experiment.