FENICE results and a proposal for a new measurements of nucleon time-like form factors

Abstract

The nucleon form factors have been extensively studied in the space-like region by several experiments. In the time-like region, only recently some precise measurements of the proton form factor became available and only one measurement of the neutron one has been performed by FENICE. In this paper, the main FENICE results are described, including the measurement of neutron and proton form factors and of the total e + e − → hadrons cross section close to the e +e − → N N threshold. The general behaviour of the neutron time-like form factors is rather unexpected. In the unphysical region, nucleon form factors have been studied by means of dispersion relations, using a model independent regularization method. No φ signal has been found, in contrast with some expectation about the strange quark content in the nucleon. However, large errors are present in these results, due to large errors in present data. In general, more data are demanded in order to have a complete knowledge of the time-like form factors of the neutron, experically close to the threshold, where there are some indications for a steep behaviour. A new experiment is proposed at SLAC in order to measure the nucleon form factors in the time-like region with the same accuracy than we have in the space-like one, disentangling the electric and magnetic contribution to the cross section.