PRECISE DETERMINATION OF THE ELECTROMAGNETIC FORM-FACTOR OF THE PROTON IN THE TIME-LIKE REGION UP TO S=4.2 GEV2

The s dependence of the proton form factor in the time-like region has been determined up to s=4.2 GeV2, assuming the validity of the |Ge| = |Gm| = |G| hypothesis. Data were taken in a dedicated experiment performed at the LEAR antiproton ring at CERN, increasing by one order of magnitude the available statistics on the proton form factor near threshold in the time-like region. Our result consist of cross section measurements of anti-p p to e+e- reaction for different beam momenta in the kinematical r 3.6⩽s⩽4.2 GeV2. The observed s dependence of the form factor close to threshold differs appreciably from the one suggested by previous experiments.