  The LHCb experiment has the unique possibility, among the LHC experiments, to
be operated in fixed target mode, using its internal gas target. The energy
scale achievable at the LHC, combined with the LHCb forward geometry and
detector capabilities, allow to explore particle production in a wide
Bjorken-$x$ range at the $\sqrt{s_{\scriptscriptstyle\rm NN}} \sim 100$ GeV
energy scale, providing novel inputs to nuclear and cosmic ray physics. The
first measurement of antiproton production in collisions of LHC protons on
helium nuclei at rest is presented. The knowledge of this cross-section is of
great importance for the study of the cosmic antiproton flux, and the LHCb
results are expected to improve the interpretation of the recent high-precision
measurements of cosmic antiprotons performed by the space-borne PAMELA and
AMS-02 experiments.
