Mass number dependence of the difference (rel−rmu), a dispersion effect in electron scattering

A systematic analysis of rms charge radii determined by elastic electron scattering (r_el) and muonic atom X-rays (r_mu), respectively, shows that — on the average — the former is less then the latter. This difference is probably caused by virtual excitation and deexcitation of the nucleus: a dispersion effect. The mass number dependence of the difference is also investigated. Its absolute value decreases with increasing mass number as (r_el?r_mu)≈ ?12+0.03 A (am). ThisA-dependence is compared with the available theoretical predictions using low-q, low-Z approximations.