The D0 collaboration recently claimed evidence for the existence of a tetraquark state with very curious properties. Called, the X(5568), it was seen as a "bump" in the mass of combinations of Bs mesons and charged pions. Since these particles contain different quark flavours (a Bs mesons contains beauty and strange; a pion contains up and down flavours), if confirmed this would be the first known particle with four different flavours. Such a state is hard to explain in most models of quantum chromodynamics -- the theory that explains how quarks are bound together. The LHCb collaboration has searched for the claimed state in a large data sample. Finding no signal, limits are set which are apparently at odds with the D0 claim.

• Publication: PRL 117 (2016) 152003
• DOI: https://doi.org/10.1103/PhysRevLett.117.152003