The proton form factor measurements at Jefferson Lab, past and future
- Norfolk State Univ., VA (United States)
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{sup 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.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE SC Office of Nuclear Physics (SC-26) (United States)
- DOE Contract Number:
- AC05-06OR23177
- OSTI ID:
- 1114306
- Report Number(s):
- JLAB-PHY-13-1682; DOE/OR-23177-2470; FG02-89ER40525; TRN: US1400144
- Journal Information:
- Physics of Particles and Nuclei, Vol. 45, Issue 1; Conference: SPIN 2012, Dubna (Russian Federation), 17-22 Sep 2012; ISSN 1063-7796
- Country of Publication:
- United States
- Language:
- English
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