FASER experiment: An introduction and research progress

The paper introduces the FASER experiment at CERN and its research progress. FASER, the ForwArd Search ExpeRiment, is an experiment dedicated to searching for light, forward direction particles at Large Hadron Collider (LHC). Such particles often fly along the LHC beam and escape the coverage of traditional particle detectors. The detection of these forward particles is a good complement to other experiments at LHC. The FASER detector is located 480 m downstream of the ATLAS interaction point, aligned with the beam collisions axis. It consists of multiple modules, including an emulsion and tungsten detector, three permanent magnets, four tracking spectrometer stations, four scintillator detectors, and an electromagnetic calorimeter. The emulsion and tungsten detector, FASER nu, is designed to detect high energy neutrinos produced at LHC and to study their properties. The physics motivation of FASER is mainly to search for potential dark matter candidates and study neutrinos produced by the collider. Back to 2018, a pilot emulsion detector was installed in the far-forward region from the ATLAS interaction point and observed the first candidates of the neutrino interactions at the LHC. Recently, using the proton-proton collision data collected at center of mass energy root s = 13.6 TeV in 2022 in LHC Run-3, first physics results from FASER have been released. In March 2023, FASER reported the first direct observation of collider neutrinos at LHC. Using the active electronic components of FASER, approximately 153 neutrino interactions in the tungsten-emulsion detector were observed, with a signal significance of 16 standard deviations. Five months later, FASER reported the direct observation of electron neutrino interactions in the FASER. sub-detector, with a statistical significance of 5 standard deviations. The signal events include neutrinos with TeV energies, which are the highest energy electron neutrinos detected from an artificial source, the particle collider. In August 2023, FASER also presented its first dark photon search result. Although no signal events were found, it provided world-leading constraints on dark photons with mass within the range of 17 to 70 MeV and couplings epsilon within the range of 2x10(-5) to 1x10(-4). As an important milestone in forward physics, FASER will continue data-taking during the full LHC Run-3. In December 2023, FASER has been officially approved to run in the future HL-LHC era. In the meantime, the recently proposed Forward Physical Facility (FPF) might become a new frontier for forward physics research for HL-LHC. FPF will house several forward experiments, such as FASER2, FASER nu 2, FLArE and FORMOSA, with physics potential not only on collider neutrinos and dark photons, but also on other beyond the standard model (BSM) particles, such as millicharged particles. In this paper, we present an overview of the FASER experiment, encompassing its instrumentation and physics objectives, and briefly report the preliminary experimental results of the FASER.