Objects made of two quarks and two anti-quarks are called tetraquarks. There are two ways to build a tetraquark state: 1) a “genuine tetraquark” – an object where all 4 quarks are bound into compact multuiquark object by gluon exchange. 2) a “meson molecule” – an object where each of pair form a meson - with the two mesons bound by a long-range meson exchange. Meson molecules can be bound or quasi-bound. In the latter case they are often referred as dynamically generated resonances. Objects with the same quantum numbers are allowed to mix. Therefore some tetraquarks can spend part of their life in the form of multiquark and part as a meson molecule or even a meson.


Meson-meson molecule