Department of Physics

The exotic X(3872) hadron, discovered by the Belle collaboration in 2003, has a natural width of about 1 MeV, which is unexpectedly narrow for a state with a mass very close to the D0-D0* threshold. The internal structure of the exotic meson X(3872) is still under debate. The similarity of the X(3872) mass and the D0-D* mass threshold inspired the interpretation of X(3872) as a D0-D0* mesonic molecule with small binding energy. Another explanation is that this meson is a tetraquark composed of four valence quarks.

Relativistic heavy-ion collisions produce an extremely hot and strongly interacting medium, the Quark-Gluon Plasma, which once filled the early universe at a time scale 10 microseconds after the Big-Bang. Heavy-ion collisions provide a new environment to study the nature of multi-quark states. The radii of a diluted D0-D0* mesonic molecule and a compact tetraquark are very different. The production rates in the two proposed scenarios are different in the presence of the Quark-gluon plasma. Tetraquarks and mesonic molecules interact differently with the co-moving particles. It is exciting to study X(3872) in heavy-ion collisions, which would reveal its internal structure. Studies along this line will broaden our view of the kinds of particles produced abundantly in the early universe.