Measurement of the Neutron F-2 Structure Function via Spectator Tagging with CLAS

We report on the first measurement of the F2 structure function of the neutron from the semi-inclusive scattering of electrons from deuterium, with low-momentum protons detected in the backward hemisphere. Restricting the momentum of the spectator protons to ≲100  MeV/c and their angles to ≳100° relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The F2n data collected cover the nucleon-resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65<Q2<4.52  GeV2, with uncertainties from nuclear corrections estimated to be less than a few percent. These measurements provide the first determination of the neutron to proton structure function ratio F2n/F2p at 0.2≲x≲0.8 with little uncertainty due to nuclear effects.