  Low-energy muon beams are useful for rare decay searches, which provide
access to new physics that cannot be addressed at high-energy colliders.
However, muons are produced within a broad energy spread unmatched to the
low-energy required. In this paper we outline a phase rotation method to
significantly increase the intensity of low-energy muons. The muons are
produced from a short pulsed proton driver, and develop a time-momentum
correlation in a drift space following production. A series of rf cavities is
used to bunch the muons and phase-energy rotate the bunches to a momentum of
around 100 MeV/c. Then another group of rf cavities is used to decelerate the
muon bunches to low-energy. This obtains ~0.1 muon per 8 GeV proton, which is
significantly higher than currently planned Mu2e experiments, and would enable
a next generation of rare decay searches, and other intense muon beam
applications.
