New Physics and the Proton Radius Problem

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.Comment: 6 pages, 6 figures, v2 contains revised comment on competing model of Lamb Shift discrepanc

Resonance Region Structure Functions and Parity Violating Deep Inelastic Scattering

The primary motive of parity violating deep inelastic scattering experiments has been to test the standard model, particularly the axial couplings to the quarks, in the scaling region. The measurements can also test for the validity of models for the off-diagonal structure functions $F_{1,2,3}^{\gamma Z}(x,Q^2)$ in the resonance region. The off-diagonal structure functions are important for the accurate calculation of the $\gamma Z$-box correction to the weak charge of the proton. Currently, with no data to determine $F_{1,2,3}^{\gamma Z}(x,Q^2)$ directly, models are constructed by modifying existing fits to electromagnetic data. We present the asymmetry value for deuteron and proton target predicted by several different $F_{1,2,3}^{\gamma Z}(x,Q^2)$ models, and demonstrate that there are notable disagreements.Comment: 6 pages, 3 figures. New version contains additional descriptions of competing structure function model

Gamma-Z box contributions to parity violating elastic e-p scattering

Parity-violating (PV) elastic electron-proton scattering measures Q-weak for the proton, $Q_W^p$. To extract $Q_W^p$ from data, all radiative corrections must be well-known. Recently, disagreement on the gamma-Z box contribution to $Q_W^p$ has prompted the need for further analysis of this term. Here, we support one choice of a debated factor, go beyond the previously assumed equality of electromagnetic and gamma-Z structure functions, and find an analytic result for one of the gamma-Z box integrals. Our numerical evaluation of the gamma-Z box is in agreement within errors with previous reports, albeit somewhat larger in central value, and is within the uncertainty requirements of current experiments.Comment: 4 pages, 4 figures, v2: reference added, typo fixe

Modification of electromagnetic structure functions for the gamma Z-box diagram

The gamma Z-box diagram for parity violating elastic e-p scattering has recently undergone a thorough analysis by several research groups. Though all now agree on the analytic form of the diagram, the numerical results differ due to the treatment of the structure functions, F-1,2,3(gamma Z) (x, Q(2)). Currently, F-1,2,3(gamma Z)(x, Q(2)) at low Q(2) and W-2 must be approximated through the modification of existing fits to electromagnetic structure function data. We motivate and describe the modification used to obtain F-1,2(gamma Z)(x, Q(2)) in our previous work. We also describe an alternative modification and compare the result to our original calculation. Finally, we present a new modification procedure to acquire F-3(gamma Z) (x, Q(2)) in the resonance region and calculate the axial contribution to the gamma Z-box diagram. Details of these modifications will illuminate where discrepancies between the groups arise and where future improvements can be made