The electromagnetic form factors of pseudoscalar mesons within the light-front quark model

In this paper, we investigate the electromagnetic form factors (EMFFs) and charge radii of pseudoscalar mesons within the light-front quark model (LFQM). Using parameters derived from the confinements of mesonic decay constants, we obtain numerical results, which indicate the following: (1) the EMFFs of charged and neutral mesons exhibit significant differences in their endpoint behaviors but show similar asymptotic behavior in the momentum transfer regions $Q^2>2~\text{GeV}^2$. For the EMFFs of light mesons such as $\pi$ and $K^+$, our results are in excellent agreement with experimental data in the small momentum transfer regions ($Q^2<0.3$ GeV$^2$). For the charge radii of mesons, our results also show rough consistency with predictions from other approaches. (2) For charged mesons, the peak values of $Q^2~F_P(Q^2)$ are approximately proportional to the mass difference $\Delta~m$ between their constituent quarks. Moreover, the mean square radii $\langle~r^2\rangle_P$ of charged mesons decrease with increasing meson mass and decreasing $\Delta~m$. For neutral mesons, their charge radii are primarily determined by the electric charge of the heavy quark. These results indicate that quark mass asymmetry significantly influences the behavior of the EMFFs and charge radii of mesons. Experimental data to test these predictions would thus be of great interest.