First Measurement of the ν_e and ν_μ Interaction Cross Sections at the LHC with FASER's Emulsion Detector

This paper presents the first results of the study of high-energy electron and muon neutrino charged-current interactions in the FASERν emulsion/tungsten detector of the FASER experiment at the LHC. A subset of the FASERν volume, which corresponds to a target mass of 128.6 kg, was exposed to neutrinos from the LHC pp collisions with a centre-of-mass energy of 13.6 TeV and an integrated luminosity of 9.5 fb^-1. Applying stringent selections requiring electrons with reconstructed energy above 200 GeV, four electron neutrino interaction candidate events are observed with an expected background of 0.025^+0.015_-0.010, leading to a statistical significance of 5.2σ. This is the first direct observation of electron neutrino interactions at a particle collider. Eight muon neutrino interaction candidate events are also detected, with an expected background of 0.22^+0.09_-0.07, leading to a statistical significance of 5.7σ. The signal events include neutrinos with energies in the TeV range, the highest-energy electron and muon neutrinos ever detected from an artificial source. The energy-independent part of the interaction cross section per nucleon is measured over an energy range of 560–1740 GeV (520–1760 GeV) for ν_e (ν_μ) to be (1.2_-0.7^+0.8) × 10^-38 cm^2 GeV^-1 ((0.5±0.2) × 10^-38 cm^2 GeV^-1), consistent with Standard Model predictions. These are the first measurements of neutrino interaction cross sections in those energy ranges.