An introduction to particle physics with emphasis in exotics hadrons

Abstract

The study of exotic hadrons, i.e., those with a nature different than that expected in the traditional quark model, and their properties has recently experienced a colossal boom. The development in this area is so tremendous that studying exotic hadrons has become one of the main, and more prolific, research lines at experimental facilities like BES (China), LHC (Switzerland), and soon it will be at FAIR (Germany), the new large scale accelerator facility in Europe. For instance, the observation of the Pc baryons with hidden charm (minimum quark content being five quarks) first in 2015, and latter on in 2019 with better statistics; the discovery of exotic mesons with two open flavors (minimum quark content being four quarks) during the fall of 2020, and the ob- servation of exotic narrow doubly charmed mesons (minimum quark content being four quarks too) at the end of 2021 is a proof of the affirmation made above. Quantum chromodynamics (QCD) describes the strong interactions between quarks, but the great challenge of studying the nature and properties of these exotic hadrons lies in the fact that having masses in the range of 3-4 GeV they belong to the non-perturbative region of QCD. The use of effective Lagrangians to study hadron systems at such energies is a very powerful tool nowadays. The strategy consists in using these Lagrangians, which implement the relevant symmetries for the system, to calculate the corresponding T matrix in a coupled channel approach. Once the T matrix is obtained, observables as cross sections, scattering length, decay widths, invariant mass distributions, etc., can be determined and compared with experimental results.