First Measurement of the $\hat{I}{½}_e$ and $\hat{I}{½}_{\hat{I}}¼$ Interaction Cross Sections at the LHC with FASER's Emulsion Detector

Abstract

This paper presents the first results of the study of high-energy electron and muon neutrino charged-current interactions in the FASER$\hat{I}½$ emulsion/tungsten detector of the FASER experiment at the LHC. A subset of the FASER$\hat{I}½$ 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 $0.025_{-0.010}^{+0.015}$, leading to a statistical significance of 5.2$\ddot{I}ƒ$. 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 $0.22_{-0.07}^{+0.09}$, leading to a statistical significance of 5.7$\ddot{I}ƒ$. 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 $\hat{I}{½}_e$ ($\hat{I}{½}_{\hat{I}}¼$) to be $(1.2_{-0.7}^{+0.8})\times 10^{-38}{\mathrm{cm}}^2{\mathrm{GeV}}^{-1}$ ($(0.5\pm 0.2)\times 10^{-38}{\mathrm{cm}}^2{\mathrm{GeV}}^{-1}$), consistent with Standard Model predictions. These are the first measurements of neutrino interaction cross sections in those energy ranges.