Measurement of the Ratio GEP/GMP to Large Q2: Interesting New Results from Jefferson Lab
In two experiments at Jefferson Lab the ratio of the electromagnetic form factors of the proton, GEp=GMp, was obtained in the four-momentum transfer squared range of 0.5 to 5.6 GeV2 with a polarization transfer technique, by measuring Pt and Pl, the transverse and longitudinal recoil proton polarization in ep --> e'p reaction. In one-photon approximation GEp=GMp is proportional to Pt=Pl; the great strength of this method is the tight control of systematics it affords. The ratio mu GEp=GMp was found to decrease linearly with Q2, down to 0.29 at Q2=5.6 GeV2; revealing a definite difference between the spatial distributions of charge and magnetization at short distances. The combined results of the two JLab experiments were surprising as they appeared to contradict the consensus based on Rosenbluth separation results for (GEp)2 and (GMp)2: the ratio mu pGEp=GMp obtained with the Rosenbluth method is {approx}1 up to 5 GeV2. This unbridgeable difference between cross section and polarization experiments has been reinforced with two recent JLab Rosenbluth experiments; it appears increasingly difficult to explain it away by methodological or systematic errors. We are currently preparing a third experiment at JLab, this time in Hall C, to extend the Q2-range to 9 GeV2.
- Research Organization:
- Thomas Jefferson National Accelerator Facility, Newport News, VA
- Sponsoring Organization:
- USDOE - Office of Energy Research (ER)
- DOE Contract Number:
- AC05-84ER40150
- OSTI ID:
- 885176
- Report Number(s):
- JLAB-PHY-06-501; DOE/ER/40150-3953
- Country of Publication:
- United States
- Language:
- English
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