Deep-Inelastic Scattering at TeV Energies with LHC Muons

TL;DR

The study demonstrates that TeV-energy forward muons at the LHC can perform a rich neutral-current DIS program on tungsten, yielding large inclusive and charm-tagged samples that rival neutrino DIS in constraining proton structure at large $x$. By combining a detailed muon-flux model with NLO DIS generation and realistic detector cuts, the work shows robust Run 3 yields and compelling HL-LHC projections, including the potential to distinguish between intrinsic and perturbative charm via large-$x$ measurements and charm asymmetries. It also discusses systematic uncertainties, selection optimizations (e.g., $n_{\rm tr}$ cuts and muon-energy resolution), and how muon-DIS data could be integrated into global (n)PDF fits, potentially matching the physics reach of future facilities like the EIC in a complementary regime. The findings highlight forward muon DIS as a valuable QCD and proton-structure probe, motivating upgrades to FASER and planning for future Forward Physics Facility measurements, with significant implications for PDFs, IC models, and SMEFT studies.

Abstract

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$ν$. As a representative application, we demonstrate the sensitivity of muon DIS at FASER$ν$ 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.

Figures (18)

• Figure 2.1: The (anti)muon fluxes for the rectangular geometry of FASER$\nu$ (left) and the circular geometry of the FASER spectrometer (right panel). An integrated luminosity of $\mathcal{L}_{\rm pp}=250$ fb$^{-1}$ is assumed. The dashed line indicates the interpolation provided by LHAPDF and which is used as input to the POWHEG simulations of muon DIS presented in this work. These fluxes are normalised according to the definition of Eq. (\ref{['eq:muon_fluxes']}).
• Figure 3.1: The number of muon DIS neutral-current inclusive events predicted for FASER$\nu$ as a function of $Q^{2}$ and $x$ satisfying the acceptance and selection cuts described in Sect. \ref{['sec:muonDIS']} for an integrated luminosity of $\mathcal{L}_{\rm pp}=250$ fb$^{-1}$. We display results for the NNPDF4.0 (left) and CT18 (right) baseline sets, see also Table \ref{['table:Nevents']} for the corresponding integrated yields. Similar results are obtained for other PDF sets. The curves indicate fixed values of $W$.
• Figure 3.2: The number of inclusive muon DIS events at FASER$\nu$ for $\mathcal{L}_{\rm pp}=250$ fb$^{-1}$ as a function of the incoming muon energy $E_\mu$. The left (right) panel shows the predictions based on NNPDF4.0 (CT18) for both the fitted and perturbative charm variants. The error bars (bands) in each bin indicates the associated statistical (PDF) uncertainty.
• Figure 3.3: Same as Fig. \ref{['fig:DIStotal_cuts_E_distrib']} now with events binned in terms of $x$.
• Figure 3.4: Same as Fig. \ref{['fig:DIStotal_Q_x']} with the selected events binned in $(x,E_\mu)$.