Linhas de Pesquisa

Research line 

       Nuclear reactions are a good example of a many-body mechanism: the nucleus contains up to hundreds of nucleons and during the reaction other particles are generated. At high energies, even proton-proton collisions generate hundreds of particles. If, on the one hand, the large number of particles in the interacting system causes the natural diculties of many-body systems, on the other hand, they offer an excellent opportunity to deal with the statistical and thermodynamic aspects of strongly interacting quantum systems. The statistical aspects of nuclear reactions have some important characteristics that make them a unique set among all those found in nature: 

  

1. Constituents interact strongly, and this QCD-governed interaction has not yet been fully tested. While in Particle Physics the so-called perturbative QCD (pQCD) is a useful theoretical tool, due to the property of asymptotic freedom characteristic of QCD, in Nuclear Physics the interactions between the constituents are strong enough so that the technique of expansion in perturbative series is not applicable. 

  

  

2. Being formed by bosons and fermions, the system offers all the complexity (in terms of calculation) involved in the symmetrization or anti-symmetrization of the wave function. The diculty in dealing with fermions is notorious, for example in the area of network QCD (LQCD). Therefore, there are evident theoretical data in this area. These characteristics cause restrictions and facilitations to the theoretical treatment that are different when the energy of the nuclear reaction is varied, hence the need to classify the reactions as a function of energy: low, intermediate, high and relativistic. Below, the criteria for this classification, always arbitrary, will be clarified: 

  

i) Low Energy 

  

We consider as being of low energy the reactions in which: 

  

a) The antisymmetrization of the wave function causes fundamental effects for the understanding of nuclear processes, so the nuclear structure is a fundamental ingredient. 

  

b) The nucleonic degrees of freedom are constrained due to the blocking of the end states according to the Pauli Exclusion Principle (Pauli blocking). 

  

c) In this case the core is considered fluid and hydrodynamic models are generally used, and collective degrees of freedom dominate the process. 

  

d) The excitation energy is generally less than 50MeV. 

  

ii) Intermediate Energies 

  

The. In this energy range, the Pauli block loses its effectiveness. The nucleonic degrees of freedom start to gain importance and the transition of these particles between the nuclear levels of independent particles becomes fundamental. 

  

B. While at low energies the antisymmetrization generated the facility to reduce the effects of the nucleonic degrees of freedom, allowing the domain of the collective degrees of freedom, here the system is in a complicated transition phase between the two regimes, being independent particle degrees of freedom fundamental to understanding the reaction while the nuclear structure still strongly determines the characteristics of the system. 

  

ç. The accurate calculation of the Pauli block becomes really important in the description of nuclear processes, the accurate calculation of the effects of the wave function antisymmetry and determinant in the theoretical result. In fact, if at lower energies all processes below the Fermi level were blocked, simplifying the calculations, here a significant but non-dominant fraction of the processes below the Fermi level are allowed. 

  

d) This range of energies goes from 50MeV to 1GeV. 

  

iii) High Energy 

  

a) At high energies the relativistic aspects become important. The number of particles is no longer constant and the production of secondary mesons mainly, but bosons and baryons at higher energies) becomes fundamental. 

  

b) The collective degrees of freedom are negligible and the process is dominated by nucleonic and subnucleonic degrees of freedom, these determinants in the description of the production of baryonic resonances or in the production of secondary particles in general. Models that describe the nucleon and its structure become important. 

  

c) As the energy increases, the Pauli block becomes less important and the treatment of the system as a quantum gas becomes easier because the antisymmetrization effects are no longer fundamental in the description of nuclear processes. 

  

d) We consider high energies the range between 1GeV and 10GeV. 

  

iv) Relativistic Energies. In this range the nuclear structure is totally ignored. Nucleonic degrees of freedom are unimportant because the binding energy is much less than the collision energy. The nucleus is often seen as a set of practically disconnected particles. 

  

a) The nucleonic structure becomes relevant according to the dominance of subnuclear degrees of freedom. 

  

b) In the lowest energy range, antisymmetrization is implemented again for the description of the nucleonic structure. 

  

c) At higher energies a system in thermodynamic equilibrium is formed. This system can be understood as a hadronic resonance gas and the antisymmetrization effects are negligible due to the large available phase space. 

  

d) At even higher energies, baryons and mesons cease to be important (with degrees of freedom) and the description of the system becomes in terms of its constituents quarks and mesons. We have the plasma of quarks and gluons. 

  

e) This range corresponds to energies above 10GeV. 

  

It should be noted that the above division is merely schematic and that the transition from one band to another presents superpositions of phenomena, which usually brings additional diculties to the description of the nuclear processes that occur during the reaction. It is also important to note that in many cases reactions in an energy range also allow the study of lower energy ranges. A striking example is the photon-induced intermediate-energy reactions: one can study the formation of baryonic resonances, the production of scalar and vector mesons, the emission of fast neutrons and protons, and the formation of a residual nucleus with a given energy of excitation that is sufficiently low for this nucleus to continue to decay by processes such as spallation. In higher energy cases, multifragmentation can also be an important mechanism. The theoretical treatment of each of the described categories tends to be different. In the present project we have two theoretical tools that allow us to deal with at least some aspects of reactions in all categories: 

  

a) The CRISP model (acronym for Rio-Ilhéus-São Paulo Collaboration), which uses the Monte Carlo method to address the intranuclear cascade processes that occur at intermediate and high energies and the evaporation processes and what occur at low energies and intermediate. 

  

b) The Self-Consistent Non-Extensive Theory (NACT) that introduces the non-extensive statistics proposed by Tsallis in self-consistent thermodynamics, and with that it manages to describe in a simple way many experimental results of relativistic collisions.