exotic hadrons (tetraquarks and pentaquarks)
The simple quark model offers two recipes for a hadron: three quarks, or one quark with one antiquark. For decades those seemed to be the only options nature used. But the underlying theory of the strong force never forbade other combinations — it only said that the total color charge must cancel out. Particles built from more than the usual number of quarks are called exotic hadrons.
Two kinds have been firmly established. A tetraquark is a hadron made of two quarks and two antiquarks — four particles instead of the usual two or three. A pentaquark is made of four quarks and one antiquark — five particles. Both still satisfy the color-neutrality rule, just in a more elaborate way. An open question is how the pieces are arranged: are they all bound tightly together as one blob, or are they really a meson and a baryon (or two mesons) loosely orbiting each other like a molecule? For several exotic states the molecular picture fits the data well.
These particles were long predicted but hard to pin down, because they are easily faked by ordinary processes. Convincing tetraquark and pentaquark signals only emerged in the 2000s and 2010s, largely from the LHCb experiment at CERN, which sifts enormous numbers of collisions for the telltale bumps. Exotic hadrons matter because they are a fresh testing ground for the theory of the strong force in regimes the simple quark model never reached, probing how quarks can cluster in ways beyond the textbook two recipes.
In 2015 the LHCb experiment reported pentaquark states containing a charm quark and its antiquark plus three light quarks; tetraquark candidates like the X(3872) had been seen earlier.
Exotic hadrons go beyond the two standard recipes, with four or five quarks held together while staying color-neutral.
Exotic does not mean beyond the Standard Model. Tetraquarks and pentaquarks are still ordinary quarks bound by the known strong force; they are exotic only relative to the textbook two recipes, not signs of new physics.