Exotic hadron

Experimental signatures for such exotic hadrons had been seen by 2003 at the latest,[3][4] but they remain a topic of controversy in particle physics.

As quantum chromodynamics (QCD) developed over the next decade, it became apparent that there was no reason why only three-quark and quark-antiquark combinations could exist.

[6] In addition, it seemed that gluons, the mediator particles of the strong interaction, could also form bound states by themselves (glueballs) and with quarks (hybrid hadrons).

In April 2014, the LHCb collaboration confirmed the existence of the Z(4430)−, discovered by the Belle experiment, and demonstrated that it must have a minimal quark content of ccdu.

[7] In July 2015, LHCb announced the discovery of two particles, named P+c(4380) and P+c(4450), which must have minimal quark content ccuud, making them pentaquarks.

One model of a pentaquark : q is a quark and q an antiquark ; gluons (wavy lines) mediate strong interactions between quarks; red, green, and blue color charges must each be present, while the remaining quark and antiquark must share a color and its anticolor, in this example blue and antiblue (shown as yellow).