Opening angle and dineutron correlations in knockout reactions with Borromean two-neutron halo nuclei

Abstract

Background: Knockout reactions with proton targets provide an invaluable tool to access the properties of two-neutron halo nuclei. Recently, experimental results for the average opening angle as a function of the intrinsic neutron momentum in $^{11}\mathrm{Li}$ have shown a localization of dineutron correlations on the nucleus surface.Purpose: Study the model dependence and the effect of distortion and absorption on the opening angle distributions to assess the reliability of this observable to extract properties of Borromean two-neutron halo nuclei.Method: A quasifree sudden model is used to describe the knockout process, where absorption effects are modeled by the eikonal $S$ matrix between the proton target and the core of the Borromean nucleus. Final states in momentum space are built within a three-body model for the projectile, which enables the description of momenta and opening angle distributions.Results: A strong dependence on absorption effects is found for the opening angle at large intrinsic momenta, while the region of lower momenta is mostly insensitive to them. Reasonable agreement with the available data is obtained for $^{11}\mathrm{Li}$ at low momenta with weights for $s$ and $p$ waves different from those previously reported, showing a model dependence in their extraction. For $^{19}\mathrm{B}$, test calculations show marked sensitivity to small $p$-wave components.Conclusions: The opening angle for $(p,pn)$ knockout reactions on Borromean nuclei at small intrinsic momenta is a reliable observable mostly sensitive to the structure of the Borromean nucleus. For larger momenta, the reaction mechanism leads to a larger distortion of the distribution. In the case of nuclei with small components of opposite parity to the dominant ones, this observable can be used to explore them. The relation between dineutron in coordinate space and opening angle in momentum space is found to be model dependent.