Proton polarization shifts in electronic and muonic hydrogen

The contribution of virtual excitations to the energy levels of electronic and muonic hydrogen is investigated combining a model-independent approach for the main part with quark model predictions for the remaining correction s. Precise values for the polarization shifts are obtained in the long-wave length dipole approximation by numerically integrating over measured total photoabsorption cross sections. These unretarded results are considerably r educed by including retardation effects in an approximate way since the ave rage momentum transfer (together with the mean excitation energy) turns out to be larger than usually assumed. Transverse and seagull contributions ar e estimated in a simple harmonic oscillator quark model and found to be non -negligible. Possible uncertainties and improvements of the final results a re discussed. (C) 1999 Elsevier Science B.V. All rights reserved.