How to determine a dineutron correlation in Borromean nuclei via a quasi-free knockout ($p,pn$) reaction?
The quasi-free neutron knockout reaction on $^6$He is investigated to discuss the dineutron correlation in the ground state. In the present work, the momentum distributions of the two emitted neutrons are calculated with the $\alpha$~+~$n$~+~$n$ three-body model and the simple reaction model, and the effects of the knockout process via the $^5$He resonance and the target dependence in the momentum distributions are discussed. From the calculation, it is found that a clear signature of the dineutron correlation can be obtained by choosing the kinematical condition to exclude the process via the $^5$He resonance, while the inclusion of the $^5$He resonance drastically changes the momentum distributions. It is also shown to be important to use the proton target in the quantitative discussion on the dineutron correlation by the knockout reaction. In addition to theoretical arguments, a possible experimental setup to determine the dineutron correlation via the quasi-free knockout reaction is considered.