Heavy ion collisions: An example of indirect observations of physical phenomena

Since Galileo and Newton, Physics has consolidated itself as a field of knowledge dedicated to the observation of nature and the creation of models that explain the phenomena. With thermodynamics combined with the mechanics of fluids we can have an idea of the behavior of the climate, for example. Over time, knowledge about nature has been improving and new questions about the constitution of matter and our own origin were becoming more complex. In this scenario, it appears as we know it today, the Physics of elementary particles, Quantum Field Theory and Chromodynamics quantum theory, which deal with what we understand today to be “the fundamental blocks” of constitution of all the universe, namely leptons (with their neutrinos) and quarks. According to quantum chromodynamics, in particular the properties of confinement and asymptotic freedom, it is impossible to detect an isolated quark, that is, they are always in states connected in pairs, forming mesons, trios, forming baryons and in more complex states, such as tetraquark and pentaquark. However, under extreme conditions of temperature and pressure, it has been hypothesized that quarks and gluons (particles that mediate the interaction between quarks) can be “free” in a given volume. This hypothetical phase of matter is called a quark-gluon plasma, or QGP. It is speculated to have existed in the first instants after the Big Bang and that exists inside neutron stars due to the enormous energy density at these locations. These conditions of very high temperature and energy density can be reproduced in the laboratory with the collision of heavy ions in an ultrarelativistic regime in accelerators such as the RHIC and the LHC. Due to the extremely short duration of the QGP phase after the collision, we were unable to directly observe the plasma, only the so-called final observables, such as the particles generated by this set of quarks, gluons and energy and the momentum distribution of these particles. In this text, we will discuss the fundamental elements of this theory and its main advances.

Keywords:
QGP; hadrons; plasma