Muon trident process at far-forward LHC detectors

TL;DR

This work analyzes the electromagnetic muon trident process in muon-nucleus scattering at far-forward LHC detectors (FASERν and FASERν2). It employs a modified Monte Carlo framework to compute coherent cross sections for $\mu^{\pm}_{i} + A \rightarrow \mu^{\pm}_{f} + A + l^{+} + l^{-}$, including Bethe-Heitler and muon-bremsstrahlung diagrams, and uses the equivalent photon approximation to assess production of QED bound states such as true muonium $(\mu^{+} \mu^{-})_S$ and positronium $(e^{+} e^{-})_S$, with realistic muon fluxes from FLUKA. The results show large event yields for $e^+e^-$ and $\mu^+\mu^-$ pairs across Run 3 and HL-LHC, non-negligible $\tau^+\tau^-$ rates at FASERν, and rare but potentially observable true muonium at FASERν2 alongside abundant positronium, highlighting the potential of forward detectors to probe electromagnetic lepton-pair production and search for bound states. These findings motivate detailed experimental analyses to separate final-state configurations and refine nuclear-form-factor modeling in forward-scattering processes.

Abstract

The electromagnetic production of a dilepton pair in the muon - ion scattering, usually denoted muon trident process, is investigated considering the feasibility of studying processes induced by muons at LHC using its far-forward detectors. The total and differential cross - sections are estimated taking into account of the Bethe-Heitler and bremsstrahlung channels, and predictions for the event rates expected for muon - tungsten ($μW$) collisions at the LHC energies are presented. Our results indicate that the observation of the muon trident process is feasible at FASER$ν$. In particular, our results indicate that the electromagnetic production of $τ^+ τ^-$ pairs in the $μW$ scattering can, in principle, be observed for the first time ever at the LHC. In addition, we present our predictions for the production of QED bound states. Finally, results for the trident process at the FASER$ν$2 detector are also presented.

Figures (5)

• Figure 1: Leading order diagrams for the electromagnetic production of a dilepton pair in a muon - ion scattering. The dilepton pair can be produced by the Bethe-Heitler process (upper diagrams) and by the muon bremsstrahlung channel (lower diagrams).
• Figure 2: Total cross-section as a function of incident muon energy for distinct final states: pairs of electrons (solid back line), muons (dashed red line) and taus (dash-dotted blue line).
• Figure 3: Predictions for the expected number of events associated with the production of a pair of electrons (upper panel), muons (central panel) and taus (lower panel) by an incident muon, binned in the energy of each lepton. The results are for FASER$\nu$ during the run 3 of the LHC, considering an integrated luminosity of 250 fb$^{-1}$.
• Figure 4: Predictions for the number of events associated with the electromagnetic $\mu^+\mu^-$ production by an incident muon, binned in the invariant mass (left panel) and angular opening (right panel) of two muons in the final state. The results are for FASER$\nu$ during the run 3 of the LHC considering an integrated luminosity of 250 fb$^{-1}$.
• Figure 5: Total cross-sections for true muonium and open muon pair production in $\mu W$ interactions as a function of incident muon energy.