Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session FM: Electromagnetic Form Factors |
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Chair: Narbe Kalantarian, VUU |
Friday, October 30, 2020 2:00PM - 2:12PM |
FM.00001: How Analytic Choices Can Affect the Extraction of Electromagnetic Form Factors Douglas Higinbotham, Scott Barcus Scientists often try to incorporate prior knowledge into their regression algorithms, such as a particular analytic behavior or a known value at a kinematic endpoint. Unfortunately, there is often no unique way to make use of this prior knowledge, and thus, different analytic choices can lead to very different regression results from the same set of data. These choices can included items such as the function that is being used to fit the data or even the binning of the data. We will show for the Mainz proton elastic scattering data, with its 1422 data points and 31 normalization parameters, how these types of choices can affect the extraction of the proton's electromagnetic form factors. These results also demonstrate why it is critical when using regression algorithms to have either a physical model in mind or a firm mathematical basis. [Preview Abstract] |
Friday, October 30, 2020 2:12PM - 2:24PM |
FM.00002: Precise determination of proton magnetic radius from electron scattering data Jose Manuel Alarcon, Douglas Higinbotham, Christian Weiss We report about a novel extraction of the proton magnetic radius from the high-precision electron-proton elastic scattering cross section data. Our theoretical framework combines dispersion analysis and chiral effective field theory and implements the dynamics governing the shape of the low-$Q^2$ form factors. It allows us to use data up to $Q^2\sim$ 0.5 GeV$^2$ for constraining the radii and overcomes the difficulties of empirical fits and $Q^2 \rightarrow 0$ extrapolation. We obtain a magnetic radius $r_M^p$ = 0.850 $\pm$0.001 (fit 68\%) $\pm$0.010 (theory full range) fm, significantly different from earlier results obtained from the same data, and close to the extracted electric radius $r_E^p$ = 0.842 $\pm$0.002 (fit 68\%) $\pm$0.010 (theory full range) fm. [Ref: Alarcon, Higinbotham, Weiss, arXiv:2002.05167] [Preview Abstract] |
Friday, October 30, 2020 2:24PM - 2:36PM |
FM.00003: First determination of the charge-averaged $e^\pm$-p cross section Jan Bernauer, Axel Schmidt Proton form factors are a touchstone for our understanding of nucleon structure. While new experiments are advancing the precision frontier, their interpretation is hampered by the uncertainty in two-photon exchange corrections. We present here the first high-precision determination of the charge-averaged $e^\pm$-proton cross section, where two-photon exchange is suppressed in first order. The data were taken by the OLYMPUS collaboration to directly measure two-photon-exchange corrections via the cross section ratio, but careful analysis allowed us to extract the cross sections themselves. The data cover a highly interesting kinematic region where $G_M/G_\rm{std.\ dip.}$ exhibits a threshold/turn-over behavior in current fits and will have significant impact on form factor extractions. [Preview Abstract] |
Friday, October 30, 2020 2:36PM - 2:48PM |
FM.00004: Monte Carlo Simulation for a Future Two-Photon Exchange Experiment at DESY Patrick Moran Recent elastic lepton-proton measurements of the proton form factor ratio, $\mu^{p}\,G_{E}^{p}/G_{M}^{p}$, have yielded dramatically different results using polarized and unpolarized techniques. The most obvious explanation for this discrepancy is the contribution of the hard two-photon exchange corrections. A proposed experiment at DESY aims to measure this contribution by measuring the ratio of positron-proton to electron-proton elastic scattering with lepton energies of 2 and 3 GeV. This talk will focus on the Monte Carlo simulation that has been developed to model the elastic scattering and detector response. The performance of the lead tungstate calorimeter arrays and subsequent analysis will be discussed. [Preview Abstract] |
Friday, October 30, 2020 2:48PM - 3:00PM |
FM.00005: Measurement of the Two-Photon Exchange contribution to the electron-neutron elastic scattering cross section Eric Fuchey, Sheren Alsalmi, Bogdan Wojstekhowski We propose to make a high precision measurement of the two-photon exchange contribution (TPE) in elastic electron-neutron scattering at a four-momentum transfer $Q^2$~= 4.5 GeV$^2$. While significant efforts to study the two-photon-exchange have focused around elastic electron-proton scattering, the impact of TPE on neutron form factors was never examined experimentally. This experiment will provide the very first assessment of the two-photon exchange in electron-neutron scattering, which will be important for understanding the nucleon form factor physics.\\ The proposed experiment will be performed in Hall A using the BigBite (BB) spectrometer to detect the scattered electrons and Super-BigBite (SBS) to detect the protons and neutrons. This experimental setup is identical to the one of E12-09-019 $G_M^n$, expected to run in 2021, with which this experiment should run.\\ The ratio method, already used to measure the neutron magnetic form factor at JLab and Mainz, will be extended to extract the electric form factor of the neutron $G_E^n$ by scattering unpolarized electrons from deuterium quasi-elastically at two beam energies 4.4 and 6.6 GeV. This approach greatly reduces systematic errors compared to single electron arm configuration. [Preview Abstract] |
Friday, October 30, 2020 3:00PM - 3:12PM |
FM.00006: Vector Analyzing Power Measurements During PREX-II Ryan Richards I report on measurements of the vector analyzing power $A_{n}$ on various spin-0 nuclei during PREX-II at $< Q^{2} >$ = 0.0064 $(\frac{GeV}{c})^{2}$ and $<\theta>$ = $5^{\circ}$. The PREX-II experiment at Jefferson Lab measures the parity-violating (PV) asymmetry of longitudinally polarized electrons elastically scattering off an unpolarized ${^{208}}{Pb}$ target. The analyzing power couples to any residual transverse beam polarization thus making it a potential source of systematic error. Dedicated $A_{n}$ measurements took place during PREX-II highlighted by the first $A_{n}$ measurement on an intermediate Z nuclei, ${^{40}}{Ca}$. The ${^{40}}{Ca}$ measurement is interesting, providing additional input in better understanding the Z dependence of the analyzing power. In addition, $A_{n}$ measurements were taken on ${^{208}}{Pb}$ and ${^{12}}{C}$. The ${^{12}}{C} A_{n}$ is required due to the lead target being sandwiched between two diamond foils. [Preview Abstract] |
Friday, October 30, 2020 3:12PM - 3:24PM |
FM.00007: Vector Analyzing Power Measurements During CREX Robert Radloff In elastic electron nucleus scattering where the electron's polarization is perpendicular to its momentum, a vector analyzing power is proportional to the imaginary component of the two photon exchange process. The analyzing power was measured for $^{40}$Ca, $^{48}$Ca, $^{208}$Pb, and $^{12}$C using a 2.2 GeV electron probe. Calcium provides a useful insight into intermediate nuclei to fill the gap in experimental data between Aluminum and Lead. The measurement of two isotopes of calcium gives information about any Z independent effects that may exist. Preliminary results will be shown. [Preview Abstract] |
Friday, October 30, 2020 3:24PM - 3:36PM |
FM.00008: Electromagnetic proton form factors for neutrino physics and atomic spectroscopy Oleksandr Tomalak, Kaushik Borah, Richard Hill, Gabriel Lee We provide a parametric representation of nucleon form factors, uncertainties, and correlations at momentum transfers below a few GeV^2 incorporating all our knowledge about electromagnetic nucleon structure from electron scattering data and precise charge radii measurements as well as recent advances in radiative corrections and fit procedure. The proton magnetic form factor measurements of A1 Collaboration are strikingly different to older data resulting in a 3-5% difference for the central value of quasielastic cross sections and sizable shifts of energy levels in ordinary and muonic hydrogen. A successful realization of precise neutrino measurements and first measurements of the ground-state hyperfine splitting in muonic hydrogen call for further experimental investigations of the proton magnetic form factor at low momentum transfer \lesssim 1-3 GeV^2 [Preview Abstract] |
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