Mesons have a wide variety. Some are quarks plus different anti-quarks (e.g. up + anti-down). Neutral mesons are not just quarks plus their anti-quarks, they ar

Mesons have a wide variety. Some are quarks plus different anti-quarks (e.g. up + anti-down). Neutral mesons are not just quarks plus their anti-quarks, they are usually quantum superpositions of multiple quark/anti-quark pairs. For example, the pi-0 meson is a superposition of up + anti-up minus down + anti-down, all divided by the square root of 2 (quantum mechanics is weird). However, it gets even more complicated when spin comes into play, an up + anti-down meson could be a charged pion with 0 spin and a mass of 139 MeV or it could be a rho meson with a spin of 1 and a mass of 775 MeV.