Physics opportunities with the fixed-target program of the LHCb experiment using an unpolarized gas target

The LHCb experiment pioneered fixed-target physics with LHC beams, thanks to the SMOG internal gas target. Collisions of proton and heavy-ion beams on targets with different nuclear size can be recorded at a centre-of-mass energy of $\sqrt {s_{NN}}$ ∼ 100 GeV. This note summarizes the physics opportunities offered by the current fixed-target setup and its upgrade envisaged for the LHC Run 3. Unique measurements are being performed with Run 2 data, covering in particular heavy flavour production in nuclear collisions over a wide Feynman-$x$ range and light particle production of particular interest to cosmic ray physics. The increase in luminosity and extension of the choice of target material, which are being pursued for Run 3, open many new possibilities which are reviewed in this document.