FENICE results and a first proposal for a new measurement of the nucleon form factors

Abstract

The FENICE experiment was built in 1990 at the e + e − collider ADONE in Frascati to perform the first measurement of the electromagnetic form factor of the neutron in the time like-region, measuring the cross section of the reaction: e + e − → nn . The non-magnetic apparatus allowed the detection of nn and pp as well as multihadronic events. Results are presented on the first measurement of the neutron form factor from q 2 = 3.69 to q 2 = 5.95 for a total integrated luminosity of about 500 nb −1 , together with new results on the proton form factor in the same q 2 region and a new measurement of the total e + e − → hadrons cross section, close to the nucleon-antinucleon threshold. The neutron form factor turns out to be greater or equal than the proton one, and there are indications of a narrow vector resonance close to the nucleon-antinucleon threshold. The angular distribution of antiprotons and antineutrons has allowed to check the presence of the electric form factor contributions. The general behaviour of the neutron time-like form factor is rather unexpected. A proposal to perform another measurements using of an asymmetric e + e − collider is briefly discussed.