  This article describes the physics and nonproliferation goals of WATCHMAN,
the WAter Cherenkov Monitor for ANtineutrinos. The baseline WATCHMAN design is
a kiloton scale gadolinium-doped (Gd) light water Cherenkov detector, placed 13
kilometers from a civil nuclear reactor in the United States. In its first
deployment phase, WATCHMAN will be used to remotely detect a change in the
operational status of the reactor, providing a first- ever demonstration of the
potential of large Gd-doped water detectors for remote reactor monitoring for
future international nuclear nonproliferation applications.
  During its first phase, the detector will provide a critical large-scale test
of the ability to tag neutrons and thus distinguish low energy electron
neutrinos and antineutrinos. This would make WATCHMAN the only detector capable
of providing both direction and flavor identification of supernova neutrinos.
It would also be the third largest supernova detector, and the largest
underground in the western hemisphere. In a follow-on phase incorporating the
IsoDAR neutrino beam, the detector would have world-class sensitivity to
sterile neutrino signatures and to non-standard electroweak interactions (NSI).
WATCHMAN will also be a major, U.S. based integration platform for a host of
technologies relevant for the Long-Baseline Neutrino Facility (LBNF) and other
future large detectors.
  This white paper describes the WATCHMAN conceptual design,and presents the
results of detailed simulations of sensitivity for the project's
nonproliferation and physics goals. It also describes the advanced technologies
to be used in WATCHMAN, including high quantum efficiency photomultipliers,
Water-Based Liquid Scintillator (WbLS), picosecond light sensors such as the
Large Area Picosecond Photo Detector (LAPPD), and advanced pattern recognition
and particle identification methods.
