First measurement of the absorption of $^{3}\overline{\rm He}$ nuclei in
  matter and impact on their propagation in the galaxy
  Antimatter particles such as positrons and antiprotons abound in the cosmos.
Much less common are light antinuclei, composed of antiprotons and
antineutrons, which can be produced in our galaxy via high-energy cosmic-ray
collisions with the interstellar medium or could also originate from the
annihilation of the still undiscovered dark-matter particles. On Earth, the
only way to produce and study antinuclei with high precision is to create them
at high-energy particle accelerators like the Large Hadron Collider (LHC).
Though the properties of elementary antiparticles have been studied in detail,
knowledge of the interaction of light antinuclei with matter is rather limited.
This work focuses on the determination of the disappearance probability of
\ahe\ when it encounters matter particles and annihilates or disintegrates. The
material of the ALICE detector at the LHC serves as a target to extract the
inelastic cross section for \ahe\ in the momentum range of $1.17 \leq p < 10$
GeV/$c$. This inelastic cross section is measured for the first time and is
used as an essential input to calculations of the transparency of our galaxy to
the propagation of $^{3}\overline{\rm He}$ stemming from dark-matter decays and
cosmic-ray interactions within the interstellar medium. A transparency of about
50% is estimated using the GALPROP program for a specific dark-matter profile
and a standard set of propagation parameters. For cosmic-ray sources, the
obtained transparency with the same propagation scheme varies with increasing
$^{3}\overline{\rm He}$ momentum from 25% to 90%. The absolute uncertainties
associated to the $^{3}\overline{\rm He}$ inelastic cross section measurements
are of the order of 10%$-$15%. The reported results indicate that
$^{3}\overline{\rm He}$ nuclei can travel long distances in the galaxy, and can
be used to study cosmic-ray interactions and dark-matter decays.
