The question of X(3872)

X(3872) is a resonance observed a few years ago at Belle and Tevatron and what hit immediately physicists imagination was that it has roughly two times the mass of meson. This would imply that it could be a neat example of hadron molecule being a combination of two couples of quarks. As you may know, there is a lot of activity in QCD to understand if tetraquarks exist or not and notable physicists are involved in this quest. Several proposals emerged showing how tetraquarks can be the answer to the spectrum of light unflavored mesons. X(3872) could be a particular tetraquark state with diquarks combining with a very low binding energy (about 0.25 MeV) forming a molecule. Whatever its nature, this resonance appears quite exotic indeed. But a recent paper (see here) sheds some light about what this particle cannot be. The authors derive some bounds on the production cross section of it showing that is not plausible to consider this particle as a diquark state. They carry on simulations of production of the resonance proving that is unlikely the formation in S-wave of a molecular state. The paper appeared in this days in Physical Review Letters (see here).

The interest arisen on this particle at the time makes important this article giving a significant clarification about the direction to take to have an understanding of its very nature.