The Forward Physics Facility at the HL-LHC and its Synergies with Astroparticle Physics

High-energy collisions at the high-luminosity Large Hadron Collider (HL-LHC) will generate a vast flux of particles along the beam collision axis, a region not accessible by current LHC experiments. The study of multi-particle production in the far-forward region is especially important for astroparticle physics. High-energy cosmic rays create extensive air showers (EAS) in the atmosphere, driven by hadron-ion collisions in the non-perturbative QCD regime. Therefore, understanding high-energy hadronic interactions in the forward region is crucial for interpreting EAS data and estimating backgrounds for searches of astrophysical neutrinos, among other applications. The Forward Physics Facility (FPF) is a proposal to construct a new underground cavern at the HL-LHC, hosting various far-forward experiments designed to detect particles outside the current LHC acceptance. We will outline the current plans for the FPF and highlight its synergies with astroparticle physics. Specifically, we will discuss how FPF measurements will enhance the modeling of high-energy interactions in the atmosphere, helping to reduce the associated uncertainties in multi-messenger astrophysics.