New physics and the proton radius problem

Abstract

Background: The recent disagreement between the proton charge radius extracted from Lamb shift measurements of muonic and electronic hydrogen invites speculation that new physics may be to blame. Several proposals have been made for new particles that account for both the Lamb shift and the muon anomalous moment discrepancies. Purpose: We explore the possibility that new particles' couplings to the muon can be fine-tuned to account for all experimental constraints. Method: We consider two fine-tuned models, the first involving new particles with scalar and pseudoscalar couplings, and the second involving new particles with vector and axial couplings. The couplings are constrained by the Lamb shift and muon magnetic moments measurements while mass constraints are obtained by kaon decay rate data. Results: For the scalar-pseudoscalar model, masses between 100 to 200 MeV are not allowed. For the vector model, masses below about 200 MeV are not allowed. The strength of the couplings for both models approach that of electrodynamics for particle masses of about 2 GeV. Conclusions: New physics with fine tuned couplings may be entertained as a possible explanation for the Lamb shift discrepancy.

Refined determination of the muonium-deuterium 1S-2Sisotope shift through improved frequency calibration of iodine lines

“…One possible theoretical explanation for the size puzzle is unknown quantum-electrodynamics (QED) corrections on the order of 310 μeV causing μH results to be wrongly attributed to the nuclear size effect [5]. Other theories for the new interactions have also been suggested [6,7].…”

Modification of electromagnetic structure functions for theγZ-box diagram

“…For the Roper, the quark model calculates a constant corrective prefactors while the MAID ratio rapidly approaches Q p,LO W as Q 2 increases. The differences in the Roper resonance corrective prefactors were also the primary cause for the different deuteron asymmetry predictions in [9].…”

“…Indeed, an analysis of the integral indicates that the mean Q 2 value is 0.4 GeV 2 . In applications with higher Q 2 , such as the calculation of the deuteron asymmetry in [9], the quark model and photoproduction corrective prefactors give quite different values.…”

“…Bosted and Christy [18] also have a fit for deuteron and neutron electromagnetic data which we used to modify the deuteron structure functions in [9]. The corrective ratios for the deuteron resonances are listed in Table III The corrective prefactors for the Christy-Bosted fit can be modeled using a different technique.…”

“…The PVDIS experiment [8] at Jefferson Lab has several data points for the deuteron's F γZ 1,2,3 in the resonance region. These data will be insufficient to produce a model-independent fit, but provide a first step in testing the validity of the modifications [9]. Rislow and Carlson [5] 5.7 ± 0.9 Gorchtein et al [6] 5.4 ± 2.0 Hall et al [7] 5.60 ± 0.36…”