Electron Scattering From a High Momentum Neutron in Deuterium

The deuterium nucleus is a system of two nucleons (proton and neutron) bound together. The configuration of the system is described by a quantum-mechanical wave function and the state of the nucleons at a given time is not known a priori. However, by detecting a backward going proton of moderate momentum in coincidence with a reaction taking place on the neutron in deuterium, the initial state of that neutron can be inferred if we assume that the proton was a spectator to the reaction. This method, known as spectator tagging, was used to study the electron scattering from high-momentum neutrons in deuterium. The data were taken with a 5.765 GeV polarized electron beam on a deuterium target in Jefferson Laboratory\u27s Hall B, using the CLAS detector. The accumulated data cover a wide kinematic range, reaching values of the invariant mass of the unobserved final state W* up to 3 GeV. A data sample of approximately 5 · 105 events, with protons detected at large scattering angles (as high as 136°) in coincidence with the forward electrons, was selected. The product of the neutron structure function with the initial nucleon momentum distribution F2n · S was extracted for different values of W*, backward proton momenta ps and momentum transfer Q2. The data were compared to a calculation based on the spectator approximation and using the free nucleon form factors and structure functions. A strong enhancement in the data, not reproduced by the model, was observed at cos (θpq ) \u3e −0.3 (where θpq is the proton scattering angle relative to the direction of the momentum transfer) and can be associated with the contribution of final state interactions (FSI) that were not incorporated into the model. The bound nucleon structure function F2n was studied in the region cos(θpq) \u3c −0.3 as a function of W* and scaling variable x*. At high spectator proton momenta the struck neutron is far off its mass shell. At ps \u3e 400 MeV/c the model overestimates the value of F2n in the region of x* between 0.25 and 0.6. A modification of the bound neutron structure is one of possible effects that can cause the observed deviation