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Proton size anomaly
Vernon Barger1, Cheng-Wei Chiang, Wai-Yee Keung
1Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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.
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