skip to main content

Title: The proton form factor measurements at Jefferson Lab, past and future

Use of the double-polarization technique to obtain the elastic nucleon form factors has resulted in a dramatic improvement of the quality of two of the four nucleon electromagnetic form factors, G{sub Ep} and G{sub En}. It has also changed our understanding of the proton structure, having resulted in a distinctly different Q 2-dependence for both G{sub Ep} and G{sub Mp}, contradicting the prevailing wisdom of the 1990’s based on cross section measurements, namely that G{sub Ep} and G{sub Mp} obey a “scaling” relation {mu}G{sub Ep} ~ G{sub Mp}. A related consequence of the faster decrease of G{sub Ep} revealed by the Jefferson Lab (Jlab) polarization results was the disappearance of the early scaling F{sub 2}/F{sub 1} ~ 1/Q{sup 2} predicted by perturbative QCD. In three experiments, Gep(1), Gep(2) and Gep(3), in Halls A and C at Jlab, the ratio of the proton’s electromagnetic elastic form factors, G{sub Ep} /G{sub Mp} , was measured up to four momentum transfer Q{sup 2} of 8.5 GeV{sup 2} with high precision, using the recoil polarization technique. The initial discovery that the proton form factor ratio measured in these three experiments decreases approximately linearly with four-momentum transfer, Q{sup 2}, for values above ~ 1 GeV{supmore » 2}, was modified by the Gep(3) results, which suggests a slowing down of this decrease. There is an approved experiment, Gep(5), to continue these measurements to 15 GeV{sup 2}. A dedicated experimental setup, the super bigbite spectrometer (SBS), will be built for this purpose. It will be equipped with a new focal plane polarimeter to measure the polarization of the recoil protons. In this presentation, I will review the status of the proton elastic electromagnetic form factors, mention succinctly a number of theoretical approaches to describe results and show some features required for the future Gep(5) experiment.« less
  1. Norfolk State Univ., VA (United States)
Publication Date:
OSTI Identifier:
Report Number(s):
JLAB-PHY-13-1682; DOE/OR-23177-2470
Journal ID: ISSN 1063-7796; FG02-89ER40525; TRN: US1400144
DOE Contract Number:
Resource Type:
Resource Relation:
Journal Name: Physics of Particles and Nuclei; Journal Volume: 45; Journal Issue: 1; Conference: SPIN 2012, Dubna (Russian Federation), 17-22 Sep 2012
Research Org:
Thomas Jefferson National Accelerator Facility, Newport News, VA (United States)
Sponsoring Org:
USDOE SC Office of Nuclear Physics (SC-26) (United States)
Country of Publication:
United States