  FASER is a proposed small and inexpensive experiment designed to search for
light, weakly-interacting particles at the LHC. Such particles are dominantly
produced along the beam collision axis and may be long-lived, traveling
hundreds of meters before decaying. To exploit both of these properties, FASER
is to be located along the beam collision axis, 480 m downstream from the ATLAS
interaction point, in the unused service tunnel TI18. We propose that FASER be
installed in TI18 in Long Shutdown 2 in time to collect data from 2021-23
during Run 3 of the 14 TeV LHC. FASER will detect new particles that decay
within a cylindrical volume with radius R= 10 cm and length L = 1.5 m. With
these small dimensions, FASER will complement the LHC's existing physics
program, extending its discovery potential to a host of new particles,
including dark photons, axion-like particles, and other CP-odd scalars. A FLUKA
simulation and analytical estimates have confirmed that numerous potential
backgrounds are highly suppressed at the FASER location, and the first in situ
measurements are currently underway. We describe FASER's location and discovery
potential, its target signals and backgrounds, the detector's layout and
components, and the experiment's preliminary cost estimate, funding, and
timeline.
