title: Proton size anomaly.
abstract: 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.
authors.affiliation: Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Vernon
authors.initials: V
authors.lastname: Barger
authors.forename: Cheng-Wei
authors.initials: CW
authors.lastname: Chiang
authors.forename: Wai-Yee
authors.initials: WY
authors.lastname: Keung
authors.forename: Danny
authors.initials: D
authors.lastname: Marfatia