Multiple-Parton Interactions and the Exotic Hadrons

The particle colliders are the main method for physicists to study the structures of particles and their interactions. A typical kind of collision in use is the pp-collision (Fig.??). Protons consist of more fundamental particles called partons (e.g. quarks and gluons), which means we must study the collisions at the parton level. Most of the interactions in a pp-collision are soft, with low transverse momentum exchanges, in which the Multiple-Parton Interactions (MPIs) take an important part.
The simplest and most common hadrons detected in the colliders are baryons and mesons. They consist of three quarks (or anti-quarks) and quark-antiquark pairs, respectively. However, it has been proven that there also exist hadrons of more complex structures, such as tetraquarks and pentaquarks. They can be composite objects of more quarks and gluons. These hadrons are called the exotic hadrons.