Measurement of the structure function of the nearly free neutron using spectator tagging in inelastic ²H(e,e′p_s)X scattering with CLAS

Background: Much less is known about neutron structure than that of the proton due to the absence of free neutron targets. Neutron information is usually extracted from data on nuclear targets such as deuterium, requiring corrections for nuclear binding and nucleon off-shell effects. These corrections are model dependent and have significant uncertainties, especially for large values of the Bjorken scaling variable <i>x</i>. As a consequence, the same data can lead to different conclusions, for example, about the behavior of the d quark distribution in the proton at large<i> </i>x. Purpose: The Barely Off-shell Nucleon Structure experiment at Jefferson Lab measured the inelastic electron-deuteron scattering cross section, tagging spectator protons in coincidence with the scattered electrons. This method reduces nuclear binding uncertainties significantly and has allowed for the first time a (nearly) model-independent extraction of the neutron structure function F<sub>2</sub>(x,Q<sup>2</sup>) in the resonance and deep-inelastic regions. Method: A novel compact radial time projection chamber was built to detect protons with momentum between 70 and 150 MeV/c and over a nearly 4π angular range. For the extraction of the free-neutron structure function F<sup>n2</sup>, spectator protons at backward angles (>100∘ relative to the momentum transfer) and with momenta below 100 MeV/c were selected, ensuring that the scattering took place on a nearly free neutron. The scattered electrons were detected with Jefferson Lab's CLAS spectrometer, with data taken at beam energies near 2, 4, and 5 GeV. Results: The extracted neutron structure function F<sup>n2</sup> and its ratio to the inclusive deuteron structure function F<sup>d2</sup> are presented in both the resonance and the deep-inelastic regions for momentum transfer squared Q<sup>2</sup> between 0.7 and 5 GeV<sup>2</sup>/c<sup>2</sup>, invariant mass W between 1 and 2.7 GeV/c<sup>2</sup>, and Bjorken x between 0.25 and 0.6 (in the deep-inelastic scattering region). The dependence of the semi-inclusive cross section on the spectator proton momentum and angle is investigated, and tests of the spectator mechanism for different kinematics are performed. Conclusions: Our data set on the structure function ratio F<sup>n2</sup>/Fd2 can be used to study neutron resonance excitations, test quark-hadron duality in the neutron, develop more precise parametrizations of structure functions, and investigate binding effects (including possible mechanisms for the nuclear EMC effect) and provide a first glimpse of the asymptotic behavior of d/u at x→1