The nature of the newly observed four-charm states is yet to be determined even though a compact tetraquark interpretation is preferred. Rescattering of known hadrons through the strong interaction is also possible to create structures that look like a hadron state. The cc-diquark model successfully predicted the mass of the Ξcc++ baryon observed by LHCb in 2017. As the J/ψ meson contains a charm (c) and an anticharm quark (bar-antidiquark attracting each other. The narrower structure is described as a hadron state of mass about 6900 MeV/c 2, denoted as X(6900). Recently, by studying the invariant mass distribution of two J/ψ mesons produced in proton-proton collisions at center-of-mass energies up to 13 TeV, the LHCb collaboration observed two structures. All these known states contain at most two heavy quarks-the beauty or charm quark. Identification of pentaquark states is even more difficult, and the first candidates were observed by the LHCb experiment in 2015. The explanation of their properties requires the existence of four constituent quarks. In the following years, a few more exotic states were discovered. However, no unambiguous signal of exotic hadrons was observed until 2003, when the X(3872) state was discovered by the Belle experiment. A rich spectrum of exotic hadrons is expected just as for the conventional ones.
The quark model also allows for the existence of so-called exotic hadrons, composed by four (tetraquarks), five (pentaquarks) or more quarks. 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 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. The Large Hadron Collider is the world's largest and most powerful particle accelerator.
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