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Form Factors

The study of nucleon form factors, which provide the most detailed information on the spatial distribution of quarks and currents in the nucleon, has entered a new era with the availability of high duty factor beams of polarised electrons. In particular this enables study of the neutron form factors, which have been notoriously difficult to measure. The programme includes (e,e'n) coincidence experiments and $\vec{A}(\vec{e},e^{\prime}n)$ experiments (where the nucleus A contains a polarised neutron) with an order of magnitude improvement in precision. The first precise results using an intense beam of polarised electrons, a polarised target and a neutron polarimeter have been obtained at MAMI. In the time-like region, the FENICE experiment at Frascati measured the $ e^{+}e^{-}\rightarrow \bar{n}n$ cross section near threshold. The analysis has shown quite unexpected and interesting results: While PQCD expects that the ratio between neutron and proton magnetic form factors should be $\sim \left\vert q_{d}/q_{u}=\frac{1}{2}\right\vert $, the neutron magnetic form is found to be larger than the proton one. The data suggest that $ \vert G_{E}^{n}\vert=\vert G_{M}^{n}\vert\simeq 0$ at threshold and that |GMp| has a very steep rise. The differing steep Q2 behaviours of the neutron and proton form factors near threshold hint at an interference with a narrow structure below threshold. QCD implies that there is non-trivial mixing of D-states, quark-gluon and SU(6) representations in the nucleon. These poorly known effects give characteristic behaviour to both elastic scattering and also N* resonance excitation. The use of polarised GeV energy electron beams and polarised targets in particular will be of special interest to disentangle the structure of the nucleon (transition) form factors in the resonance region (cf. N* programme at CEBAF). Experiments on parity violation will provide important additional knowledge on questions such as the contribution of strange quarks to the spin of the nucleon. A possible strange component in the proton ground state would induce new terms in the electromagnetic current which violate parity conservation. The determination of the corresponding form factors is ongoing at BATES and will be carried out at MAMI and CEBAF (Jefferson Lab) during the next years. Finally, the determination of the Q2 evolution of the spin response function of the nucleon, as well as of the Gerasimov Drell Hearn sum rule, will provide a link between the low energy sector and the asymptotic energy regime. An extensive research programme on the Gerasimov Drell Hearn sum rule, is planned at ELSA, GRAAL, MAMI, in Europe and at Jefferson Lab and LEGS (Brookhaven) in the United States. 
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Next: Quark and Gluon Densities Up: Hadron Structure Previous: Glueballs 

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