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 DF: Proton Puzzles I |
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Chair: Yuji Goto, RIKEN Nishina Center Room: Hilton King's 2 |
Thursday, October 25, 2018 9:00AM - 9:30AM |
DF.00001: The proton, at the heart of most visible matter Invited Speaker: Marc Vanderhaeghen To answer many questions at the forefront of particle physics, atomic physics, and nuclear astrophysics, a precise quantitative knowledge of the strong interaction in the non-perturbative domain of Quantum Chromo Dynamics is crucial. In this talk, several examples of the state of our understanding of precision hadron physics will be surveyed. In the interplay with atomic physics, I will give an update on the proton radius puzzle and show how new measurements of polarizabilities, combined with more refined theoretical analyses, can importantly improve on the limiting factors in the interpretation of high precision tests of the Lamb shift in muonic atoms. In improving the precision of proton elastic form factors, two-photon exchange (TPE) corrections between the lepton and hadron are at present understood as the largest source of hadronic uncertainty. I will review recent work within a dispersion relation framework to estimate such TPE corrections both in elastic electron-proton and muon-proton scattering, which will be tested in a new generation of experiments. Furthermore I will show that the dilepton production process on a proton is a versatile new tool in the study of non-perturbative hadron physics. At low dilepton masses, it allows to access the real part of the forward Compton amplitude, which enters the hadronic corrections in the Hydrogen atom Lamb shift, or the electromagnetic proton-neutron mass difference. At dilepton masses in the J/Psi region, it allows to access the s-wave J/Psi-p scattering length, indicating whether J/Psi-nuclear bound states may exist. Finally, I will discuss how deep inelastic scattering processes allow to perform a tomographic image of the proton in terms of its constituent quarks and gluons in both coordinate and momentum space. |
Thursday, October 25, 2018 9:30AM - 9:45AM |
DF.00002: Muonic Atom Lamb Shifts via Simple Means and the Proton Radius Problem John Paul Lestone A new simple method for understanding and calculating Lamb shifts to better than 1% is introduced and applied to muonic hydrogen and muonic deuterium systems. The hydrogen and deuterium Lamb shifts are due to an intrinsic fuzziness for the bound electron and muon probing the electric field near the proton, associated with their interaction with the vacuum, of 74 fm and 0.36 fm respectively. The smallness of the muon fuzziness suggests that the associated Lamb shifts need to be calculated including some aspects of the internal degrees of freedom of the proton. If the charge of the proton is assumed to be contained within a quasi-free p + for half of the time, then the calculated mp and md Lamb shifts are consistent with experiment without a need for a change in the proton radius. |
Thursday, October 25, 2018 9:45AM - 10:00AM |
DF.00003: Low-Q^2 proton form factor program in Mainz Jan Christopher Bernauer The proton radius puzzle, the many-sigma difference in the extracted values from scattering and electric hydrogen spectroscopy on one side and muonic spectroscopy on the other, is one of the most striking puzzles in physics today. In electron scattering experiments, the radius can be deduced from the slope of the electric form factor at $Q^2$=0, and the precise knowledge of the form factor evolution at small $Q^2$ is paramount for an accurate extraction. In the talk, I will describe the current state-of-the-art experiments and the future plans for measurements at the Mainz facilities MAMI and MESA and briefly discuss the challenges for next generation measurements |
Thursday, October 25, 2018 10:00AM - 10:15AM |
DF.00004: Preliminary Results of the Proton Charge Radius Experiment at JLab Chao Gu, Weizhi Xiong
In order to investigate the proton radius puzzle, the PRad experiment (E12-11-106) was performed in 2016 in Hall B at Jefferson Lab, with both 1.1 GeV and 2.2 GeV unpolarized electron beams to measure the ep elastic scattering cross section at very low four-momentum transfer squared, Q2, from 2×10−4 to 6×10−2 (GeV/c)2 region with a sub-percent precision. The experiment utilizes a non-magnetic calorimetric method with a high resolution calorimeter (HyCal), and two large area, high spatial resolution Gas Electron Multiplier (GEM) detectors. The experiment also uses a windowless H2 gas flow target to remove typical backgrounds from target cell windows. The systematic uncertainties of the absolute ep elastic scattering cross section is also controlled by the well-known Moller scattering process, which is measured simultaneously in this experiment within similar kinematics and acceptances. In this talk, the preliminary results from both beam energy settings will be presented. |
Thursday, October 25, 2018 10:15AM - 10:30AM |
DF.00005: Proton Charge Radius Measurement by Low-Energy Electron Scattering Toshimi Suda, ULQ2 Collaboration The proton size, the root-mean-square charge radius, has been so far determined by electron scattering and hydrogen spectroscopy of the normal atoms as well as the muonic-hydrogen spectroscopy. The latest determination of the proton radius using muons, 0.84 fm disagrees by 4 % with one determined by electrons, 0.88 fm. Intensive discussions on possible reasons this discrepancy, including critical reviews of past analysis of experimental data, have been going on, but the situation is not yet clear. It is known that the radius determination from electron scattering data as well as hydrogen spectroscopy is somewhat model dependent. In order to diminish model dependence as much as possible in the radius determination, we are going to carry out electron scattering under the kinematics of the lowest-ever momentum transfer at Tohoku University. It is worth emphasizing here that this is the only place in the world, where such a low-energy electron scattering experiment is possible. In my talk, I will discuss in details on the on-going electron-scattering project at our laboratory. |
Thursday, October 25, 2018 10:30AM - 10:45AM |
DF.00006: Two-Photon Exchange: The experimental side Douglas Hasell, Jan Christopher Bernauer Two-photon exchange is the expected explanation of the proton form-factor ratio puzzle. Three recent experiments at DESY, VEPP-3 and JLAB aimed to measure this effect directly. In the talk, I will present the results of these experiments with focus on the OLYMPUS experiment at DESY. I will discuss the current state of affairs and outline possible future measurements at DESY (near term) and JLAB (long term). |
Thursday, October 25, 2018 10:45AM - 11:00AM |
DF.00007: Two photon exchange corrections to the Lamb shift in muonic Helium-4 Carl E. Carlson We calculate the two-photon exchange corrections to the Lamb shift in muonic Helium-4 atom within a dispersion relation framework. These corrections are crucial to obtaining the Helium-4 radius from Lamb shift data, with an accuracy well beyond what has been obtained from electron scattering data. Part of the effort entailed making analytic fits to the electron-He4 quasielastic scattering data set, for purposes of doing the dispersion integrals. The results will be compared to results obtained from the very different calculations using a potential model and an effective field theory calculation. |
Thursday, October 25, 2018 11:00AM - 11:15AM |
DF.00008: ABSTRACT WITHDRAWN
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Thursday, October 25, 2018 11:15AM - 11:30AM |
DF.00009: Recent progress in theoretical calculations of two-photon exchange in electron-proton scattering Peter Blunden Two-photon exchange (TPE) effects are relevant for extractions of proton form factors at moderate to high $Q^2$, and of the proton radius at very low $Q^2$. Recent experiments to directly measure hard TPE effects by comparing elastic $e^+-p$ to $e^--p$ scattering have motivated further theoretical work in this area. In this contribution I give an overview of recent progress in theoretical calculations of TPE effects, focussing on the contributions of intermediate states in the resonance region $W\le 2$~GeV$^2$ using the CLAS parameterization. |
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