  Background: Information about the structure of halo nuclei are often inferred
from one-neutron knockout reactions. Typically the parallel-momentum
distribution of the remaining core is measured after a high-energy collision of
the exotic projectile with a light target.
  Purpose: We study how the structure of halo nuclei affects knockout
observables considering an eikonal model of reaction.
  Method: To evaluate the sensitivity of both the diffractive and stripping
parallel-momentum distributions to the structure of halo nuclei, we consider
several descriptions of the projectile within a halo effective field theory. We
consider the case of 11Be, the archetypical one-neutron halo nucleus, impinging
on 12C at 68 MeV/nucleon, which are usual experimental conditions for such
measurements. The low-energy constants of the description of 11Be are fitted to
experimental data as well as to predictions of an ab initio nuclear-structure
model.
  Results: One-neutron knockout reaction is confirmed to be purely peripheral,
the parallel-momentum distribution of the remaining core is only sensitive to
the asymptotics of the ground-state wavefunction and not to its norm. The
presence of an excited state in the projectile spectrum reduces the amplitude
of the breakup cross section; the corresponding probability flux is transferred
to the inelastic-scattering channel. Although the presence of a resonance in
the core-neutron continuum significantly affects the energy distribution, it
has no impact on the parallel-momentum distribution.
  Conclusions: One-neutron knockout cross section can be used to infer
information about the tail of the ground-state wavefunction, viz. its
asymptotic normalization coefficient (ANC). The independence of the
parallel-momentum distribution on the continuum description makes the
extraction of the ANC from this observable very reliable.
