A step towards estimation of the neutral-hadron size: the gravitational mass radius of pi0 meson in a relativistic theory of composite particles

We extend our nonperturbative essentially relativistic approach, elaborated previously, to perform an approximate estimation of the size of pi0 meson. We present detailed argumentation for choosing the mass mean square radius (MSR) for this purpose. Its value calculated in our approach, using three model quark-antiquark wave functions in pion, is (0.5-0.53) fm. The neutral-pion electromagnetic form factor is zero in accordance with the charge-conjugation symmetry. We demonstrate ambiguities encountered when using standard definitions of mechanical MSR for the size. We discuss MSRs obtained in various approaches and their comparison with each other and with our results.