Nucleon Form Factors – A Jefferson Lab Perspective

Novel Measurement of the Neutron Magnetic Form Factor from A=3 Mirror Nuclei

The electromagnetic form factors of the proton and neutron encode information on the spatial structure of their charge and magnetization distributions. While measurements of the proton are relatively straightforward, the lack of a free neutron target makes measurements of the neutron's electromagnetic structure more challenging and more sensitive to experimental or model-dependent uncertainties. Various experiments have attempted to extract the neutron form factors from scattering from the neutron in deuterium, with different techniques providing different, and sometimes large, systematic uncertainties. We present results from a novel measurement of the neutron magnetic form factor using quasielastic scattering from the mirror nuclei ^{3}H and ^{3}He, where the nuclear effects are larger than for deuterium but expected to largely cancel in the cross-section ratios. We extracted values of the neutron magnetic form factor for low-to-modest momentum transfer, 0.6 Collapse

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Affiliation(s)

S N Santiesteban University of New Hampshire, Durham, New Hampshire 03824, USA
S Li University of New Hampshire, Durham, New Hampshire 03824, USA Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
D Abrams University of Virginia, Charlottesville, Virginia 22904, USA
S Alsalmi Kent State University, Kent, Ohio 44240, USA King Saud University, Riyadh 11451, Kingdom of Saudi Arabia
D Androic University of Zagreb, Zagreb, Croatia
K Aniol California State University, Los Angeles, California 90032, USA
J Arrington Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Argonne National Laboratory, Lemont, Illinois 60439, USA
T Averett William and Mary, Williamsburg, Virginia 23185, USA
C Ayerbe Gayoso William and Mary, Williamsburg, Virginia 23185, USA
J Bane University of Tennessee, Knoxville, Tennessee 37966, USA
S Barcus William and Mary, Williamsburg, Virginia 23185, USA
J Barrow University of Tennessee, Knoxville, Tennessee 37966, USA Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
A Beck Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
V Bellini INFN Sezione di Catania, Italy
H Bhatt Mississippi State University, Mississippi State, Mississippi 39762, USA
D Bhetuwal Mississippi State University, Mississippi State, Mississippi 39762, USA
D Biswas Hampton University, Hampton, Virginia 23669, USA
A Camsonne Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
J Castellanos Florida International University, Miami, Florida 33199, USA
J Chen William and Mary, Williamsburg, Virginia 23185, USA
J-P Chen Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
D Chrisman Michigan State University, East Lansing, Michigan 48824, USA
M E Christy Hampton University, Hampton, Virginia 23669, USA Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
C Clarke Stony Brook, State University of New York, New York 11794, USA
S Covrig Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
R Cruz-Torres Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
D Day University of Virginia, Charlottesville, Virginia 22904, USA
D Dutta Mississippi State University, Mississippi State, Mississippi 39762, USA
E Fuchey University of Connecticut, Storrs, Connecticut 06269, USA
C Gal University of Virginia, Charlottesville, Virginia 22904, USA
F Garibaldi INFN, Rome, Italy
T N Gautam Hampton University, Hampton, Virginia 23669, USA
T Gogami Tohoku University, Sendai, Japan
J Gomez Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
P Guèye Hampton University, Hampton, Virginia 23669, USA Michigan State University, East Lansing, Michigan 48824, USA
T J Hague Kent State University, Kent, Ohio 44240, USA
J O Hansen Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
F Hauenstein Old Dominion University, Norfolk, Virginia 23529, USA
W Henry Temple University, Philadelphia, Pennsylvania 19122, USA
D W Higinbotham Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
R J Holt Argonne National Laboratory, Lemont, Illinois 60439, USA
C Hyde Old Dominion University, Norfolk, Virginia 23529, USA
K Itabashi Tohoku University, Sendai, Japan
M Kaneta Tohoku University, Sendai, Japan
A Karki Mississippi State University, Mississippi State, Mississippi 39762, USA
A T Katramatou Kent State University, Kent, Ohio 44240, USA
C E Keppel Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
P M King Ohio University, Athens, Ohio 45701, USA
L Kurbany University of New Hampshire, Durham, New Hampshire 03824, USA
T Kutz Stony Brook, State University of New York, New York 11794, USA
N Lashley-Colthirst Hampton University, Hampton, Virginia 23669, USA
W B Li William and Mary, Williamsburg, Virginia 23185, USA
H Liu Columbia University, New York, New York 10027, USA
N Liyanage University of Virginia, Charlottesville, Virginia 22904, USA
E Long University of New Hampshire, Durham, New Hampshire 03824, USA
A Lovato Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Computational Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA INFN-TIFPA Trento Institute for Fundamental Physics and Applications, 38123 Trento, Italy
J Mammei University of Manitoba, Winnipeg, MB R3T 2N2, Canada
P Markowitz Florida International University, Miami, Florida 33199, USA
R E McClellan Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
F Meddi INFN, Rome, Italy
D Meekins Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
R Michaels Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
M Mihovilovič Jožef Stefan Institute, 1000 Ljubljana, Slovenia Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, DE-55128 Mainz, Germany
A Moyer Christopher Newport University, Newport News, Virginia 23606, USA
S Nagao Tohoku University, Sendai, Japan
D Nguyen University of Virginia, Charlottesville, Virginia 22904, USA
M Nycz Kent State University, Kent, Ohio 44240, USA
M Olson Saint Norbert College, De Pere, Wisconsin 54115, USA
L Ou Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
V Owen William and Mary, Williamsburg, Virginia 23185, USA
C Palatchi University of Virginia, Charlottesville, Virginia 22904, USA
B Pandey Hampton University, Hampton, Virginia 23669, USA
A Papadopoulou Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
S Park Stony Brook, State University of New York, New York 11794, USA
T Petkovic University of Zagreb, Zagreb, Croatia
S Premathilake University of Virginia, Charlottesville, Virginia 22904, USA
V Punjabi Norfolk State University, Norfolk, Virginia 23529, USA
R D Ransome Rutgers University, New Brunswick, New Jersey 08854, USA
P E Reimer Argonne National Laboratory, Lemont, Illinois 60439, USA
J Reinhold Florida International University, Miami, Florida 33199, USA
S Riordan Argonne National Laboratory, Lemont, Illinois 60439, USA
N Rocco Theoretical Physics Department, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
V M Rodriguez División de Ciencias y Tecnología, Universidad Ana G. Méndez, Recinto de Cupey, San Juan 00926, Puerto Rico
A Schmidt Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
B Schmookler Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
E P Segarra Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
A Shahinyan Yerevan Physics Institute, Yerevan, Armenia
S Širca Jožef Stefan Institute, 1000 Ljubljana, Slovenia Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia
K Slifer University of New Hampshire, Durham, New Hampshire 03824, USA
P Solvignon University of New Hampshire, Durham, New Hampshire 03824, USA
T Su Kent State University, Kent, Ohio 44240, USA
R Suleiman Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
L Tang Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
Y Tian Syracuse University, Syracuse, New York 13244, USA
W Tireman Northern Michigan University, Marquette, Michigan 49855, USA
F Tortorici INFN Sezione di Catania, Italy
Y Toyama Tohoku University, Sendai, Japan
K Uehara Tohoku University, Sendai, Japan
G M Urciuoli INFN, Rome, Italy
D Votaw Michigan State University, East Lansing, Michigan 48824, USA
J Williamson University of Glasgow, Glasgow, G12 8QQ Scotland, United Kingdom
B Wojtsekhowski Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
S Wood Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
Z H Ye Argonne National Laboratory, Lemont, Illinois 60439, USA Tsinghua University, Beijing, China
J Zhang University of Virginia, Charlottesville, Virginia 22904, USA
X Zheng University of Virginia, Charlottesville, Virginia 22904, USA

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Measurements of the Electric and Magnetic Form Factors of the Neutron for Timelike Momentum Transfer

We present the first measurements of the electric and magnetic form factors of the neutron in the timelike (positive q^{2}) region as function of four-momentum transfer. We explored the differential cross sections of the reaction e^{+}e^{-}→n[over ¯]n with data collected with the BESIII detector at the BEPCII accelerator, corresponding to an integrated luminosity of 354.6  pb^{-1} in total at twelve center-of-mass energies between sqrt[s]=2.0-2.95  GeV. A relative uncertainty of 18% and 12% for the electric and magnetic form factors, respectively, is achieved at sqrt[s]=2.3935  GeV. Our results are comparable in accuracy to those from electron scattering in the comparable spacelike region of four-momentum transfer. The electromagnetic form factor ratio R_{em}≡|G_{E}|/|G_{M}| is within the uncertainties close to unity. We compare our result on |G_{E}| and |G_{M}| to recent model predictions, and the measurements in the spacelike region to test the analyticity of electromagnetic form factors.

New Insights into the Nucleon's Electromagnetic Structure

We present a combined analysis of the electromagnetic form factors of the nucleon in the space- and timelike regions using dispersion theory. Our framework provides a consistent description of the experimental data over the full range of momentum transfer, in line with the strictures from analyticity and unitarity. The statistical uncertainties of the extracted form factors are estimated using the bootstrap method, while systematic errors are determined from variations of the spectral functions. We also perform a high-precision extraction of the nucleon radii and find good agreement with previous analyses of spacelike data alone. For the proton charge radius, we find r_{E}^{p}=0.840_{-0.002}^{+0.003} _{-0.002}^{+0.002}  fm, where the first error is statistical and the second one is systematic. The Zemach radius and third moment are in agreement with Lamb shift measurements and hyperfine splittings. The combined dataset of space- and timelike data disfavors a zero crossing of μ_{p}G_{E}^{p}/G_{M}^{p} in the spacelike region. Finally, we discuss the status and perspectives of modulus and phase of the form factors in the timelike region in the context of future experiments, as well as the onset of perturbative QCD.

Electromagnetic Structure of the Neutron from Annihilation Reactions

The investigation of the fundamental properties of the nucleon is one of the most important topics in the modern hadron physics. Its internal structure and dynamics can be studied through the measurement of electromagnetic form factors which represent the simplest structure observables and serve as a test ground for our understanding of the strong interaction. Since the first attempt to measure the time-like form factors of the neutron, only four experiments published results on its structure from annihilation reactions. Due to the lack of statistics and experimental challenges, no individual determination of the form factors of the neutron has been possible so far. Modern developments of electron-positron colliders and the associated detectors allow to measure the effective FF of the neutron with the process e+e−→nn¯ with unprecedented precision at the BESIII experiment, which is based at the BEPCII collider in Beijing, China. In this report, we review the published results of the form factors on the neutron in the time-like regime, describe the experimental setup, and discuss their impact on our understanding of the strong interaction. Future works at BESIII will help to improve the precision of the neutron FFs and, combined with theoretical progress in this field, help to illuminate the properties of the neutron structure.

Towards a resolution of the proton form factor problem: new electron and positron scattering data

There is a significant discrepancy between the values of the proton electric form factor, G(E)(p), extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of G(E)(p) from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (ϵ) and momentum transfer (Q(2)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing ϵ at Q(2)=1.45  GeV(2). This measurement is consistent with the size of the form factor discrepancy at Q(2)≈1.75  GeV(2) and with hadronic calculations including nucleon and Δ intermediate states, which have been shown to resolve the discrepancy up to 2-3  GeV(2).