Momentum Measurement of Charged Particles in FASER's Emulsion Detector at the LHC

Abstract

We present a momentum measurement method based on multiple Coulomb scattering (MCS) in the FASER$\hat{I}½$ emulsion detector. The measurement of charged-particle momenta is essential for studying neutrino interactions in the TeV energy range at the FASER experiment. This method exploits the sub-micron spatial resolution and long tracking length of the FASER$\hat{I}½$ detector, enabling momentum determination from a few GeV up to a few TeV. The performance was evaluated using Geant4-based Monte Carlo simulations and validated with muon test beam data in the momentum range 100-300 GeV. As a first probe of the method for higher momentum muons, background muons recorded by the FASER$\hat{I}½$ detector were examined, showing reconstructed momenta consistent with expectations from their angular spread.