Background: The study of exotic nuclei far from the $\beta$ stability line is stimulated by the development of radioactive ion beam facilities worldwide and brings opportunities and challenges to existing nuclear theories. Including self-consistently the nuclear superfluidity, deformation, and continuum effects, the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) has turned out to be successful in describing both stable and exotic nuclei. Due to several challenges, however, the DRHBc theory has only been applied to study light nuclei so far.

A kinematically complete quasifree $(p,pn)$ experiment in inverse kinematics was performed to study the structure of the Borromean nucleus ${}^{17}\mathrm{B}$, which had long been considered to have a neutron halo. By analyzing the momentum distributions and exclusive cross sections, we obtained the spectroscopic factors for $1s_{1/2}$ and $0d_{5/2}$ orbitals, and a surprisingly small percentage of 9(2)% was determined for $1s_{1/2}$. Our finding of such a small $1s_{1/2}$ component and the halo features reported in prior experiments can be explained by the deformed relativistic Hartree-Bogoliubov theory in continuum, revealing a definite but not dominant neutron halo in ${}^{17}\mathrm{B}$. The present work gives the smallest $s$- or $p$-orbital component among known nuclei exhibiting halo features and implies that the dominant occupation of $s$ or $p$ orbitals is not a prerequisite for the occurrence of a neutron halo.