Unravelling the richness of dark sector by FASER nu

FASER nu is a newly proposed experiment which will take data in run III of the LHC during 2021-2023. It will be located in front of the FASER detector, 480 m away from the ATLAS interaction point in the forward direction. Its main goal is to detect neutrinos of all flavors produced at the interaction point with superb precision in reconstructing charged tracks. This capability makes FASER nu an ideal setup for uncovering the pattern and properties of a light dark sector. We demonstrate this capability for a well-motivated class of models with a dark matter candidate of mass around a few GeV. Dark matter annihilates to a pair of intermediate neutral particles that subsequently decay into the standard model charged fermions. We show how FASER nu can shed light on the structure of the dark sector by unravelling the decay chain within such models.