How the X(5568) challenges our understanding of QCD
  We discuss the X(5568) particle recently announced by the D0 Collaboration. Several types of models were proposed to explain this structure in the literature. As pointed out by Burns and Swanson (arXiv:1603.04366), none of them provides a satisfactory description of the observation. We provide additional arguments using general properties of QCD, and conclude that the observation of the X(5568), if confirmed, poses serious challenges to our understanding of nonperturbative QCD. 
Feng-Kun Guo, Ulf-G. Mei{\ss}ner, Bing-Song Zou
hep-ph hep-ex nucl-th