Quantum Chromodynamics (QCD) encodes a large richness of physical phenomena, and is intensively studied theoretically and experimentally. However, in spite of its success, some of its aspects are not yet fully understood; there remain open questions that demand answers. Most of these questions have important implications in cosmology and astroparticle physics.
The QCD Lagrangian contains ingredients that can clarify key questions concerning cosmology. The term, which breaks conformal symmetry, even in the massless case, is related to the axion field and its search concerns the nature of dark matter and also could contribute to the cosmological constant. Other possibilities of dark matter have been speculated, such as the existence of exotic hadrons made of color-octet complexes. The mentioned term, which is not CP invariant, plays an important role in the equation of the state of the deconfinement transition from hadronic matter to quark gluon matter, such as what happened in the first moments of the Universe after the Big Bang. This term is proportional to the trace anomaly, which measures the departure from a free quark-gluon gas of the obtained strongly-coupled quark gluon matter, and is also related to vacuum structure.
The experiments of the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have created strongly-coupled quark gluon matter in nucleus–nucleus collisions. Most of the observed collective effects have also been seen in pp collisions. In this case, it is not clear how hydrodynamic models can be applied. There is not a unified picture of the transverse momentum distribution of pp data, as well as its azimuthal distribution. The interplay between soft and hard collisions can show interesting relationships between parton entanglement and thermalization. On the other hand, the forward LHC detectors provide important information on elastic and diffractive scattering, which play important roles in determining the hadronic cascade produced in ultrarelativistic cosmic rays. Usual hadronic models, previously-matched to LHC data, are not able to describe some of the cosmic ray data at higher energies, such as the excess of muons and the energy dependence of the distribution of the length of maximum depth. Phenomena like gluon saturation, color reconnection, string interactions, percolation, and string junction working at LHC energies could have implications in the hadronic cascade.
The QCD conformal breaking term, the axion field and the relation to dark matter and the cosmological constant, the strong CP problem, the dependence on the temperature of the trace anomaly, the equation of state close to the deconfinement phase transition, the collective effects produced in colliding small systems and its thermalization, the transverse momentum distributions, including azimuthal distributions and the interplay between soft and hard interactions, the elastic and diffractive scatterings and in general forward physics at LHC and ultrahigh cosmic ray energies, models of hadronic cascade are the subject of special interest at an interplay of QCD with two related fields: Cosmology and Astroparticle Physics.
Prof. Dr. Carlos Pajares