The X is an anomalous particle with an energy of 3871.2 MeV which does not fit into the quark model

Quark model

In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....

 because of its quantum numbers. It was discovered by the Belle experiment

Belle experiment

The Belle experiment is a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers investigating CP-violation effects at the High Energy Accelerator Research Organisation in Tsukuba, Ibaraki Prefecture, Japan.The...

 in Japan and later confirmed by several other experimental collaborations. Several theories have been proposed for its nature, such as a mesonic molecule

Mesonic molecule

A mesonic molecule is a set of two or more mesons bound together by the strong force. Unlike baryonic molecules, which form the nuclei of all elements in nature save hydrogen-1, a mesonic molecule has yet to be definitively observed. The X discovered in 2003 and the Z discovered in 2007 by the...

 or a diquark

Diquark

In quark–diquark models, a diquark, or diquark correlation/clustering, is the hypothetical state of two quarks grouped inside a baryon . The diquark is often treated as a single particle with which the third quark interacts via the strong interaction...

-antidiquark pair.