  Microlensing can access planet populations that no other method can probe:
cold wide-orbit planets beyond the snow line, planets in both the Galactic
bulge and disk, and free floating planets (FFPs). The demographics of each
population will provide unique constraints on planet formation.
  Over the past 5 years, U.S. microlensing campaigns with Spitzer and UKIRT
have provided a powerful complement to international ground-based microlensing
surveys, with major breakthroughs in parallax measurements and probing new
regions of the Galaxy. The scientific vitality of these projects has also
promoted the development of the U.S. microlensing community.
  In the 2020s, the U.S. can continue to play a major role in ground-based
microlensing by leveraging U.S. assets to complement ongoing ground-based
international surveys. LSST and UKIRT microlensing surveys would probe vast
regions of the Galaxy, where planets form under drastically different
conditions. Moreover, while ground-based surveys will measure the planet
mass-ratio function beyond the snow line, adaptive optics (AO) observations
with ELTs would turn all of these mass ratios into masses and also distinguish
between very wide-orbit planets and genuine FFPs. To the extent possible,
cooperation of U.S. scientists with international surveys should also be
encouraged and supported.
