Bulletin of the American Physical Society
2016 Fall Meeting of the APS Division of Nuclear Physics
Volume 61, Number 13
Thursday–Sunday, October 13–16, 2016; Vancouver, BC, Canada
Session HC: Proton Radius and Polarizabilities |
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Chair: Nikolaos Sparveris, Temple University Room: Junior Ballroom B |
Saturday, October 15, 2016 8:30AM - 8:42AM |
HC.00001: How to resolve the proton radius puzzle? Gil Paz In 2010 the first measurement of the proton charge radius from spectroscopy of muonic hydrogen was found to be five standard deviations away from the regular hydrogen value. Six years later, this "proton radius puzzle" is still unresolved. One of the most promising avenues to test the muonic hydrogen result is a new muon-proton scattering experiment called MUSE. We describe how effective field theory methods will allow to directly connect muonic hydrogen spectroscopy to muon-proton scattering. [Preview Abstract] |
Saturday, October 15, 2016 8:42AM - 8:54AM |
HC.00002: The MUon Scattering Experiment (MUSE) at PSI Michael Kohl The proton is not an elementary particle but has a substructure governed by the interaction of quarks and gluons. The size of the proton is manifest in the spatial distributions of the electric charge and magnetization, which determine the response to electromagnetic interaction. Recently, contradictory measurements of the proton charge radius between muonic hydrogen and electronic probes have constituted the proton radius puzzle, which has been challenging our basic understanding of the proton. The MUon Scattering Experiment (MUSE) in preparation at the Paul-Scherrer Institute (PSI) has the potential to resolve the puzzle by measuring the proton charge radius with electron and muon scattering simultaneously and with high precision, including any possible difference between the two, and with both beam charges. The status of the MUSE experiment will be reported. [Preview Abstract] |
Saturday, October 15, 2016 8:54AM - 9:06AM |
HC.00003: Simulation for Proton Charge Radius (PRad) Experiment at Jefferson Lab Li Ye The “Proton Charge Radius Puzzle” refers to $7\sigma$ discrepancy between the proton charge radius extracted from muonic hydrogen Lamb shift measurements and that from the atomic hydrogen Lamb shift and e-p elastic scattering measurements. In order to get a better understanding of this puzzle, the PRad experiment (E12-11-106\footnote{Spokespersons: A. Gasparian (contact), H. Gao, M. Khandaker, D. Dutta}) was proposed and recently performed with 1.1 and 2.2 GeV unpolarized electron beam in Hall B at Jefferson Lab. The experiment aims to extract the electric form factor and the charge radius of proton by simultaneously measuring the $e-p$ elastic scattering cross section and the M$\o$ller cross section at very low $\rm{Q}^2 (2\times10^{-4} \sim 10^{-1} \rm{(GeV/c)}^2$) region, with sub-percent precision. A windowless hydrogen gas flow target was used to better control the background. A high-efficiency and high-resolution calorimeter (HyCal) and a pair of Gas Electron Multiplier (GEM) chambers were used in the experiment. This talk will focus on comparing the detailed simulation of PRad experiment and its background with preliminary spectra from the data. [Preview Abstract] |
Saturday, October 15, 2016 9:06AM - 9:18AM |
HC.00004: Proton Charge Radius (PRad) Experiment at Jefferson Lab Weizhi Xiong, Chao Peng The PRad experiment (E12-11-106) was recently performed with 1.1 and 2.2 GeV unpolarized electron beam on a windowless H$_2$ gas flow target in Hall B at Jefferson Lab. The experiment aims to investigate the proton radius puzzle by extracting the electric form factor of proton in an unprecedented low four-momentum transfer squared region, $Q^2 = 2\times10^{-4}-0.1~\rm{(GeV/c)}^2$, with a sub-percent precision. The PRad experiment utilizes a non-magnetic and calorimetric method with a high efficiency and high resolution calorimeter (HyCal) and two Gas Electron Multiplier (GEM) chambers. Its systematics are well controlled by two main advantages of this experiment: (1) The scattered electrons from M$\o$ller and $e-p$ elastic scattering are measured simultaneously, and the $e-p$ cross section will be normalized to the well-known M$\o$ller process; (2) The windowless gas flow target has no cell windows at both up- and downstream, which was one of the primary background sources in the previous $e-p$ elastic scattering experiments. Thus the PRad experiment has systematic uncertainties totally different from the previous magnetic spectrometric $e-p$ elastic scattering experiments. In this talk, we will present the details of the experimental method and preliminary analysis of the data. [Preview Abstract] |
Saturday, October 15, 2016 9:18AM - 9:30AM |
HC.00005: Calibration Study and Preliminary Results of PRad Experiment Maxime Levillain The latest measurements of the proton radius through muonic hydrogen Lamb shift show a discrepancy of $7\sigma$ from a global analysis of standard hydrogen Lamb shift and elastic $ep$-scattering. In order to understand this \textit{proton radius puzzle}, the PRad experiment\footnote{The PRad experiment is supported in part by NSF MRI award PHY-1229153} successfully took in last June some elastic $ep$-scattering data at very low $Q^2$ ($2 \cdot 10^{-4}$ to $10^{-1}$~GeV$^2$) with very accurate angle and energy measurements to minimize the systematic uncertainties. % Before measuring the cross-sections that will be used to extract the electromagnetic form factor $G_E(Q^2)$ and the proton radius, a very careful calibration of the electromagnetic calorimeter (\textit{HyCal}) must be performed to get a good energy resolution and separate $ep$-events from M$\o$ller events especially at low angle. We will present an extended study of the electromagnetic calorimeter calibration of this experiment as well as some preliminary results on $ep$- and $ee$-scattering processes extracted from the data\footnote{This work is supported in part by NSF award PHY-0855543}. [Preview Abstract] |
Saturday, October 15, 2016 9:30AM - 9:42AM |
HC.00006: Understanding the ``Proton Radius Puzzle'': Nuclear Polarizability Correction in $\mu$D Oscar J. Hernandez, Nir Nevo Dinur, Chen Ji, Sonia Bacca, Nir Barnea The accuracy of the proton radius was improved ten-fold by new spectroscopic measurements in muonic hydrogen~[1] but it differs by $7 \sigma$ from hydrogen determinations. This discrepancy, has been coined the ``proton radius puzzle". The results of new high-precision experiments on muonic deuterium indicate a new deuterium radius puzzle~[2]. The accuracy of the nuclear charge radius determination from these measurements is limited by the uncertainty in the nuclear structure effects. We have calculated this correction in Ref.~[3] including the first estimate of the nuclear-model dependence. Due to the importance of constraining the uncertainty, we will determine the statistical and systematic uncertainties of the $\chi$EFT potentials by determining the co-variance matrices of our predictions. I will also discuss an alternate method that may reduce the theoretical uncertainty.\\ [][1] R.~Pohl {\it et al.}, Nature 466, 213 (2010).\\ [][2] R.~Pohl {\it et al.}, to appear; ECT* workshop, June 20-24 (2016).\\ [][3] O.J.~Hernandez, {\it et al.}, Phys. Lett. B (2014). [Preview Abstract] |
Saturday, October 15, 2016 9:42AM - 9:54AM |
HC.00007: Polarizability Measurements at MAMI David Hornidge A central problem of modern physics research is the solution to QCD in the non-perturbative regime. One method of testing QCD in this low-energy region is by measuring certain structure constants of hadrons---called polarizabilities---that show particular promise of allowing a direct connection to the underlying quark/gluon dynamics through comparison to modern QCD-inspired model calculations, and to solutions of QCD done computationally on the lattice. This talk will give an overview of recent and upcoming measurements at the Mainz Microtron to obtain the polarizabilities of both the proton and neutron. [Preview Abstract] |
Saturday, October 15, 2016 9:54AM - 10:06AM |
HC.00008: Nuclear Structure Observable with Polarized Target and Polarized Real Photon Beam at Mainz Microtron Dilli Paudyal The nucleon polarizabilities are fundamental structure observables, like the nucleon mass or charge. While the electric ($\alpha_{E1}$) and magnetic ($\beta_{M1}$) scalar polarizabilities of the nucleon have been measured, little effort has been made to extract the spin dependent polarizabilities. These nucleon polarizabilities, $\gamma_{E_{1}E_{1}},\gamma_{M_{1}M_{1}},\gamma_{M_{1}E_{2}}$ and $\gamma_{E_{1}M_{2}}$ describe the spin response of a proton to electric and magnetic dipole and quadrupole interactions. We plan to extract them using polarized photon beam and polarized target at the MAMI tagged photon facility in Mainz, Germany. This requires precise measurement of the double polarization observable $\sum_{2z}$ which is sensitive to these polarizabilities. The $\sum_{2z}$ is measured via a circularly polarized photon beam on a longitudinally polarized butanol target in the resonance region ($E=250-310$ MeV). Together with constraints from $\alpha_{E1}$ and $\beta_{M1}$, the forward spin polarizability $(\gamma_{0})$, and QCD based models, should allow us to extract all four spin polarizabilities. This presentation will be focused on the preliminary experimental results for the measurement of $\sum_{2z}$ at different energies and angles. [Preview Abstract] |
Saturday, October 15, 2016 10:06AM - 10:18AM |
HC.00009: High Precision Measurement of the Neutron Polarizabilities via Compton Scattering on Deuterium at HI$\gamma$S Mark Sikora The electric ($\alpha_n$) and magnetic ($\beta_n$) polarizabilities of the neutron are fundamental properties arising from its internal structure which describe the nucleon's response to applied electromagnetic fields. Precise measurements of the polarizabilities provide crucial constraints on models of Quantum Chromodynamics (QCD) in the low energy regime such as Chiral Effective Field Theories as well as emerging ab initio calculations from lattice-QCD. These values also contribute the most uncertainty to theoretical determinations of the proton-neutron mass difference. Historically, the experimental challenges to measuring $\alpha_n$ and $\beta_n$ have been due to the difficulty in obtaining suitable targets and sufficiently intense beams, leading to significant statistical uncertainties. To address these issues, a program of Compton scattering experiments on the deuteron is underway at the High Intensity Gamma Source (HI$\gamma$S) at Duke University with the aim of providing the world's most precise measurement of $\alpha_n$ and $\beta_n$. We report measurements of the Compton scattering differential cross section obtained at incident photon energies of 65 and 85 MeV and discuss the sensitivity of these data to the polarizabilities. [Preview Abstract] |
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