Deep-Inelastic Scattering at TeV Energies with LHC Muons

The LHC far-forward experiments FASER and SND@LHC have pioneered the detection of TeV-energy neutrinos produced in hard-scattering proton-proton collisions at the LHC. In addition to neutrinos, an intense flux of TeV-energy muons reaches these detectors, representing a dominant background for both neutrino studies and beyond the Standard Model searches. Here we demonstrate that this forward muon flux enables a comprehensive neutral-current deep-inelastic scattering (DIS) program at FASER with a strong kinematical overlap with the Electron Ion Collider. For the Run 3 luminosity of $\mathcal{L}_{\rm pp}=250$~fb$^{-1}$, more than $10^5$ inclusive muon DIS events, of which up to $10^4$ from charm production, are expected at FASER$\nu$. As a representative application, we demonstrate the sensitivity of muon DIS at FASER$\nu$ to probe the (intrinsic) charm content of the proton at large-$x$. We also provide predictions for event yields of muon DIS for future FASER runs and for the proposed Forward Physics Facility.