Sprecher
Beschreibung
Tetraquark systems are composed of two quarks and two antiquarks which could be understood
as diquark-antidiquark bound states or meson-meson molecules. In the latter case, the
antisymmetrisation of the molecular wave function against quark exchange becomes necessary.
We discuss the resulting Pauli-blocking effect for the all-charm tetraquark Tcc(6900) as a J/ψ-J/ψ
molecule [1] and the light tetraquark a0(500) as a two-pion molecule of the ππ interaction [2,3].
In the case of the heavy-light tetraquark χc1(3872) (formerly X(3872)), we demonstrate that
double quark exchange in a J/ψ-ρ or J/ψ-ω molecule leads to an energy-dependent kernel of the
Bethe-Salpeter equation for the T-matrix of the two-meson molecule which has a pole at the D-
D threshold that leads to a bound state with the mass of X(3872) [4]. In this way we can explain
by a dynamical quark exchange kernel that the X(3872) is a well-defined long-lived resonance
close to the D-D threshold, because it is a quasi-unitary bound state with a very small binding
energy. It follows also in accordance with observations that it dominantly decays into a J/ψ and
two or three pions under strong CP violation. We speculate about modifications of the X(3872)
when it is created in the hot and dense medium of a heavy-ion collision.
