K0 and K+-meson electromagnetic form factors: A nonperturbative relativistic quark model versus experimental, perturbative, and lattice quantum-chromodynamics results

It has been previously shown that a particular nonperturbative constituent-quark model of hadrons describes experimental measurements of electromagnetic form factors of light charged mesons through a small number of common phenomenological parameters, matching at the same time the quantum-chromodynamics (QCD) asymptotics for the π-meson form factor at large momentum transfer. Here we start with the determination of the K0 electromagnetic form factor in this approach. Precise measurement of the K0 charge radius makes it possible to constrain model parameters with high accuracy. Then, with all parameters fixed, we revisit the K+ form factor and find that it matches experimental measurements in the infrared, lattice results at moderate momentum transfer, and the perturbative QCD asymptotics in the ultraviolet. In this way we obtain a narrow constraint on the K+ charge radius, ⟨r2K+⟩ = 0.403+0.007−0.006 fm2, and extend the successful infrared-ultraviolet connection from π to K mesons.