Measurements of the Electric and Magnetic Form Factors of the Neutron for Timelike Momentum Transfer

We present the first measurements of the electric and magnetic form factors of the neutron in the timelike (positive q2) region as function of four-momentum transfer. We explored the differential cross sections of the reaction e+e- -> n over bar n with data collected with the BESIII detector at the BEPCII accelerator, corresponding to an integrated luminosity of 354.6 pb-1 in total at twelve center-of-mass energies ffiffibetweenps = 2.0-2.95 GeV. A relative ffiffiuncertainty of 18% and 12% for the electric and magnetic form factors, respectively, is achieved at ps= 2.3935 GeV. Our results are comparable in accuracy to those from electron scattering in the comparable spacelike region of four-momentum transfer. The electromag-netic form factor ratio Rem equivalent to IGEI=IGMI is within the uncertainties close to unity. We compare our result on IGEI and IGMI to recent model predictions, and the measurements in the spacelike region to test the analyticity of electromagnetic form factors.