Speaker
Description
Charged pions obviously carry no baryon number. Nevertheless, they possess a nontrivial baryonic structure stemming from isospin breaking by the $u$ and $d$ quark mass difference and from the EM effects. We obtain estimates for the pion baryonic form factor in two very different ways ways: from simple constituent quark models, and from vector meson dominance fits to the experimental $e^+ e^- \to \pi^+ \pi^-$ data. All our estimates consistently yield a positive baryon mean squared radius of pi+, (0.03−0.04 fm)$^2$. Hence, an intuitive picture emerges where the outer spatial region of $\pi^+$ has a net baryon density ($u$ excess), while the inner region a net anti-baryon density ($d$-bar excess), both compensating each other such that the total baryon number is zero. For $\pi^-$ the effect is equal and opposite. We also discuss the challenging lattice prospects of measuring this fundamental property.
Based on Phys. Lett. B 822 (2021) 136680 (2103.09131 [hep-ph])
