Nonstandard heavy mesons and baryons: Experimental evidence

Since the inception of the quark model we have known the simplest examples, mesons made of a quark and antiquark and baryons made of three quarks. But it has long been recognized that more complex structures such as quark-quark-antiquark-antiquark or quark-antiquark-gluon mesons were possible. Such exotic states involving only the light up, down, and strange quarks are difficult to recognize experimentally but those containing the heavier charm or bottom quarks are more distinctive. This paper reviews the experimental status and phenomenological characterization of such exotic hadrons containing heavy quarks.