Bulletin of the American Physical Society
5th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 63, Number 12
Tuesday–Saturday, October 23–27, 2018; Waikoloa, Hawaii
Session 1WEB: Proton Charge Radius Puzzle II |
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Chair: Ashot Gasparian, NC AT&T State University Room: Hilton King's 1 |
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Tuesday, October 23, 2018 11:00AM - 11:30AM |
1WEB.00001: Jefferson Lab Proton Radius (PRad) experiment Invited Speaker: Nilanga Liyanage Jefferson Lab Proton Radius (PRad) experiment collected data in 2016 for a high precision determination of the proton charge radius through the electron-proton elastic scattering process using a novel non-magnetic-spectrometer method. The experiment reached $ep$ scattering angles as low as 0.7$^{\circ}$, thus gaining access to a record low $Q^2$ range from $2 \times 10^{-4}$ to $6 \times 10^{-2}$ (GeV/c)$^2$. Scattered electrons were detected in a high resolution, large acceptance electromagnetic calorimeter, as well as in a pair of large area Gas Electron Multiplier (GEM) detectors which provided precision coordinate determination. A windowless hydrogen gas flow target was used to ensure that there was no target window background. Systematic uncertainty in the $ep$ cross section measurement was controlled by normalizing it to the simultaneously measured well known Moller cross section. The precision of the $ep$ cross section results from the experiment is expected to be better than 1\%. Data analysis is nearing completion now. The preliminary results will be presented in this talk. |
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Tuesday, October 23, 2018 11:30AM - 12:00PM |
1WEB.00002: Status of the Muon Scattering Experiment (MUSE) at PSI Invited Speaker: Michael Kohl The proton radius puzzle was established in 2010 when the results of a reduced size of the proton obtained with muonic hydrogen spectroscopy were first released. Previously, measurements with regular hydrogen spectroscopy were consistent with low-energy electron scattering, both giving a larger proton radius. The primary motivation of the Muon Scattering Experiment (MUSE), now under construction at Paul-Scherrer Institute (PSI), is to investigate whether the difference seen between muonic and regular hydrogen spectroscopy is also present in comparison of muon and electron scattering from the proton. Different explanations of the puzzle would favor different scenarios for the outcome of MUSE. The puzzle could be restricted to the understanding of the bound state only, in which case scattering should be species-independent. On the other hand, if the discrepancy is due to a fundamental difference between $e$ and $\mu$ beyond the Standard Model, differences may be found in the scattering case, too. The MUSE experiment has been designed to measure the proton radius, proton form factors and lepton-proton elastic cross sections with competitive uncertainties to clearly distinguish between explanation scenarios. The use of electron and muon beams of either charge allows for four independent measurements, to directly probe the differences between lepton species, and to simultaneously disentangle effects due to two-photon exchange, which constitute some of the largest uncertainties in the theory framework not yet validated by experimental data. In this presentation, I will review the motivation for MUSE, and describe the experiment under construction, its present status and timeline for completion, and possible impact. On behalf of the MUSE Collaboration. |
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Tuesday, October 23, 2018 12:00PM - 12:30PM |
1WEB.00003: Elastic electron-proton scattering with low-energy electron beam at ELPH Invited Speaker: Yuki Honda 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 (GE(Q2), GM(Q2)), where Q2 is the four momentum transfer. The charge radius is defined by the gradient of the GE(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 GE(Q2) and GM(Q2) experimentally. In our Q2 region, the change in the GE(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. |
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