  A measurement of the Lamb shift in muonic hydrogen yields a charge radius of
the proton that is smaller than the CODATA value by about 5 standard
deviations. We explore the possibility that new scalar, pseudoscalar, vector,
and tensor flavor-conserving nonuniversal interactions may be responsible for
the discrepancy. We consider exotic particles that among leptons, couple
preferentially to muons, and mediate an attractive nucleon-muon interaction. We
find that the many constraints from low energy data disfavor new spin-0, spin-1
and spin-2 particles as an explanation.
