A New Window into Nucleon Structure: Flavor-Decomposed Form Factors
The u- and d-quark contributions to the elastic nucleon form factors can be determined using form-factor data from both the proton and the neutron together with the assumption of isospin symmetry. Accurate data on the ratio of the electric and magnetic form factors of the proton, GpE/GpM, have been available for over a decade, and brought with them considerable new insight into nucleon structure. Data on the ratio of the electric and magnetic form factors of the neutron, GnE/GnM, up to Q2 = 3.4 GeV2, have only been available more recently. Taken together, the neutron and proton data have made it possible to extract the flavor-decomposed form factors in the Q2 regime in which the data on GpE/GpM originally caused considerable excitement. These flavor-decomposed form factors exhibit several surprising behaviors. For example, The Dirac form-factor contribution from the down quark Fd1 appears to scale like 1/Q4 at the surprisingly low Q2 of about 1 GeV2. In sharp contrast, the up-quark contribution Fu1 has a very different behavior, and if anything, scales more like 1/Q2. Several theoretical arguments, including calculations based on QCD's Dyson-Schwinger equations, suggest that the behavior of Fd1 and Fu1 can be interpreted as providing evidence for the importance of diquark degrees of freedom. This interpretation, if it proves correct, could alter our qualitative notions of nucleon structure. The existence of diquark-like structures within the nucleon, while an old and important idea, has seen little supporting experimental evidence since the observation of missing states in the N* spectrum. Looking forward, measurements planned at the upgraded CEBAF will determine the flavor-separated form factors up to Q2 = 10 GeV2.
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