The strong interaction is one of the fundamental forces of nature, which binds quarks into hadrons such as the proton and the neutron, the building blocks of atoms. According to the quark model, hadrons can be formed by two or three quarks, called mesons and baryons respectively, and collectively referred to as conventional hadrons. The quark model also allows for the existence of so-called exotic hadrons, composed by four (tetraquarks), five (pentaquarks) or more quarks. A rich spectrum of exotic hadrons is expected just as for the conventional ones. However, no unambiguous signal of exotic hadrons was observed until 2003, when the X(3872) state was discovered by the Belle experiment. In the following years, a few more exotic states were discovered. The explanation of their properties requires the existence of four constituent quarks. Identification of pentaquark states is even more difficult, and the first candidates were observed by the LHCb experiment in 2015. All these known states contain at most two heavy quarks—the beauty or charm quark.
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Source: Phys.org