Bulletin of the American Physical Society
APS April Meeting 2017
Volume 62, Number 1
Saturday–Tuesday, January 28–31, 2017; Washington, DC
Session C12: Hadron Structure and Form Factors |
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Sponsoring Units: DNP GHP Chair: Rolf Ent, Jefferson Lab Room: Roosevelt 4 |
Saturday, January 28, 2017 1:30PM - 1:42PM |
C12.00001: Overview of high-Q$^2$ nucleon form factor program with Super BigBite Spectrometer in JLab's Hall A Andrew Puckett The elastic electromagnetic form factors (EMFFs) of the nucleon describe the impact-parameter-space distributions of electric charge and magnetization in the nucleon in the infinite momentum frame. The form factors are among the simplest and most fundamental measurable dynamical quantities describing the nucleon's structure. Precision measurements of the nucleon form factors provide stringent benchmarks testing the most sophisticated theoretical models of the nucleon, as well as \emph{ab initio} calculations in lattice QCD and continuum non-perturbative QCD calculations based on the Dyson-Schwinger equations. Measurements at momentum transfers $Q$ in the few-GeV range probe the theoretically challenging region of transition between the non-perturbative and perturbative regimes of QCD. The recent upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF) to a maximum electron beam energy of 11 GeV will facilitate the measurement of the nucleon helicity-conserving ($F_1$) and helicity-flip ($F_2$) form factors of both proton and neutron to $Q^2 > 10$ GeV$^2$, In this talk, I will present an overview of the Super BigBite Spectrometer, currently under construction in CEBAF's experimental Hall A, and its physics program of high-$Q^2$ nucleon EMFF measurements. [Preview Abstract] |
Saturday, January 28, 2017 1:42PM - 1:54PM |
C12.00002: Separated exclusive kaon production cross sections up to Q$^2$=2.1 GeV$^2$ and the kaon form factor Marco Carmignotto, Tanja Horn Electromagnetic form factors are a key observable in probing hadronic structure, providing us with important information about underlying physical quantities related to nonperturbative QCD. Light mesons composed of a valence quark-antiquark pair can be described by a single electric form factor and have been shown to be a great laboratory for these studies. Using electroproduction experiments, a successful program was developed at Jefferson Laboratory for probing the charged pion form factor in the regime of Q$^2$ up to 2.45 GeV$^2$. This provided a first glimpse at a possible transition from the nonperturbative to the perturbative regime, and also information on the structure of the pion. The kaon is the next lightest existing hadron, providing an interesting channel for assessing the strangeness degree of freedom with mesons. Although the kaon is relatively unexploited to date, there are promising results from experiments of the 6 GeV era of Jefferson Laboratory with potential for kaon form factor extractions. In this talk we will present the recent analysis of the t-channel kaon cross section and discuss the relative contribution of longitudinal and transverse photons to the cross section up to Q$^2$ values of 2.1 GeV$^2$ and prospects for form factor extractions. [Preview Abstract] |
Saturday, January 28, 2017 1:54PM - 2:06PM |
C12.00003: Kaon Transverse Charge Density from Space- and Time-like Data Nicholas Mecholsky, Johann Meija-Ott, Marco Carmignotto, Tanja Horn, Gerald Miller, Ian Pegg, Lorenzo Resca Measurements of electromagnetic form factors play an important role in our understanding of the structure and interactions of hadrons based on the principles of QCD. Transverse charge densities provide a framework for the interpretation of these form factors in terms of the physical charge and magnetization densities. They are obtained as two-dimensional Fourier transforms of the elastic form factors and describe the distribution of charge and magnetization in the plane transverse to the propagation direction of a fast moving nucleon. They are related to the Generalized Parton distributions (GPDs), which are expected to provide a universal (process-independent) description of the nucleon. The simplest hadronic system that also includes a heavier strange quark is the kaon, whose valence structure is a bound state of a quark and an antiquark. Its elastic electromagnetic structure is parameterized by a single form factor. Recent calculations suggest that strange quarks play a large role in, e.g., the shape of the parton distribution amplitude, making studies of the kaon’s internal structure of the kaon even more important. I will present the first extraction of the kaon transverse charge density from timelike and spacelike data including new data at high center of mass energies. [Preview Abstract] |
Saturday, January 28, 2017 2:06PM - 2:18PM |
C12.00004: Frequentist Analysis of SLAC Rosenbluth Data Douglas Higinbotham, Evan McClellan, Stephen Shamaiengar Analysis of the SLAC NE-11 elastic electron-proton scattering data typically assumes that the 1.6 GeV spectrometer has a systematic normalization offset as compared to the well-known 8 GeV spectrometer, yet such an offset should have been observed globally. A review of doctoral theses from the period finds that analysis with high statistics, inelastic data saw no significant normalization difference. Moreover, the unique kinematics utilized to match the two spectrometers for normalization required the 8 GeV to be rotated beyond it's well-understood angular range. We try to quantify the confidence level of rejecting the null hypothesis, i.e. that the 1.6 GeV spectrometer normalization is correct, and will show the result of simply analyzing the cross section data as obtained. This is a critical study, as the 1.6 GeV spectrometer data drives the epsilon lever arm in Rosenbluth extractions, and therefore can have a significant impact on form factor extractions at high momentum transfer. [Preview Abstract] |
Saturday, January 28, 2017 2:18PM - 2:30PM |
C12.00005: Reanalysis of Rosenbluth measurements of the proton form factors Alexander Gramolin, Dmitry Nikolenko We have reanalyzed\footnote{A. V. Gramolin and D. M. Nikolenko, Phys. Rev. C \textbf{93}, 055201 (2016)} the elastic electron-proton scattering data from SLAC experiments E140\footnote{R. C. Walker \textit{et al.}, Phys. Rev. D \textbf{49}, 5671 (1994)} and NE11\footnote{L. Andivahis \textit{et al.}, Phys. Rev. D \textbf{50}, 5491 (1994)}. This work was motivated by recent progress in calculating the corresponding radiative corrections and by the apparent discrepancy between the Rosenbluth and polarization transfer measurements of the proton electromagnetic form factors. New, corrected values for the scattering cross sections are presented, as well as a new form factor fit in the $Q^2$ range from $1$ to $8.83~\mbox{GeV}^2$. Our reanalysis brings the combined results of the SLAC experiments into better agreement with the polarization transfer data, but a significant discrepancy remains for $Q^2 > 3~\mbox{GeV}^2$. [Preview Abstract] |
Saturday, January 28, 2017 2:30PM - 2:42PM |
C12.00006: Results from the OLYMPUS Two-Photon Exchange Experiment Colton O'Connor Measurements of the proton's electric-to-magnetic form factor ratio obtained by different methods disagree significantly in a way that depends on $Q^2$. The OLYMPUS experiment was designed to empirically quantify two-photon exchange in lepton-proton scattering, an effect that, in some models, can fully account for this disagreement. This was achieved at the DORIS storage ring at DESY by measuring the ratio of the elastic cross-sections for positron-proton and electron-proton scattering with alternating 2.01 GeV lepton beams incident on an internal hydrogen gas target. Data were collected with an integrated luminosity of over 4.0 fb$^{-1}$ using a large-acceptance toroidal spectrometer and multiple luminosity monitoring systems, allowing for precise results ($<$1\% uncertainty) over the range of 0.6$\leq Q^2\leq $2.2 (GeV/$c$)$^2$. [Preview Abstract] |
Saturday, January 28, 2017 2:42PM - 2:54PM |
C12.00007: The Data Quality and Analysis Status of the Proton Charge Radius (PRad) Experiment at JLab Weizhi Xiong In order to investigate the proton radius puzzle, the PRad experiment (E12-11-106\footnote{Spokespersons: A. Gasparian (contact), H. Gao, M. Khandaker, D. Dutta}) was performed in 2016 in Hall B at Jefferson Lab. The experiment aims to extract the electric form factor of proton in an unprecedented low ${Q}^2$ region ($2\times10^{-4} - 0.1~\rm{(GeV/c)}^2$), with a sub-percent precision. The PRad experiment utilizes a non-magnetic calorimetric method with a high efficiency and high resolution calorimeter (HyCal), and two Gas Electron Multiplier (GEM) detectors. The systematic uncertainties are well controlled by two main advantages of the experiment: (1) The electron-proton ($e-p$) elastic scattering cross section is normalized to the well-known M$\o$ller scattering process, which is measured simultaneously during the experiment; (2) The H$_2$ gas flow target has no cell windows at both ends, which created primary backgrounds in the previous $e-p$ elastic scattering experiments. Thus the PRad experiment largely suppresses the two major systematic uncertainties in the previous magnetic spectrometric $e-p$ elastic scattering experiments. In this talk, we will discuss the data quality and analysis status, and present the first preliminary results from the current analysis process. [Preview Abstract] |
Saturday, January 28, 2017 2:54PM - 3:06PM |
C12.00008: The New Proton Radius Experiment (PRad) at Jefferson Lab Ashot Gasparian The proton charge radius (Rp) is one of the most fundamental quantities in physics. Precise knowledge of its value is critically important for both nuclear and atomic physics -- especially for the spectroscopy of atomic hydrogen. Recent high precision measurements of Rp using the muonic hydrogen atom demonstrated up to eight standard deviations smaller value than the accepted average from all previous experiments performed with different methods. This fact triggered the well known ``\textit{proton charge radius puzzle}'' in hadronic physics. The PRad collaboration at Jefferson Lab for the last four years developed a novel magnetic-spectrometer-free electron-proton scattering experiment to address this puzzle. The PRad experiment successfully performed in May and June of this year at Jefferson Lab collecting a large statistical and high quality experimental data set. The specifics of the method, the experimental characteristics of the setup together with the first preliminary results from the current data analysis process will be presented in this talk. [Preview Abstract] |
Saturday, January 28, 2017 3:06PM - 3:18PM |
C12.00009: Measurements and prospects on the Generalized Polarizabilities of the proton Nikos Sparveris The Generalized Polarizabilities (GPs) are fundamental quantities of the nucleon, sensitive to both the role of the quark and pion degrees of freedom, and as such they are extremely valuable for a complete understanding of the nucleon structure. The GPs can be explored through Virtual Compton Scattering and, contrary to the elastic form factors, are sensitive to all the excited spectrum of the nucleon. Recent results from MAMI as well as future prospects at JLab will be discussed. [Preview Abstract] |
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