The three-dimensional structure of nucleons (protons and neutrons) is
embedded in so-called generalized parton distributions, which are accessible
from deeply virtual Compton scattering. In this process, a high energy electron
is scattered off a nucleon by exchanging a virtual photon. Then, a
highly-energetic real photon is emitted from one of the quarks inside the
nucleon, which carries information on the quark's transverse position and
longitudinal momentum. By measuring the cross-section of deeply virtual Compton
scattering, Compton form factors related to the generalized parton
distributions can be extracted. Here, we report the observation of unpolarized
deeply virtual Compton scattering off a deuterium target. From the measured
photon-electroproduction cross-sections, we have extracted the cross-section of
a quasi-free neutron and a coherent deuteron. Due to the approximate isospin
symmetry of quantum chromodynamics, we can determine the contributions from the
different quark flavours to the helicity-conserved Compton form factors by
combining our measurements with previous ones probing the proton's internal
structure. These results advance our understanding of the description of the
nucleon structure, which is important to solve the proton spin puzzle.
