In 1974 the theory that describes the strong interaction, Quantum Chromodynamics (QCD), was proposed. Its great success is nevertheless restricted to the situations where the coupling constant that determines the interactions between partons, quarks and gluons, is small and perturbative techniques are applicable. Our knowledge on other situations is far less satisfactory. When the coupling is large, partons are confined inside hadrons and the symmetries of QCD are broken. This confinement regime can, at present, only be addressed either by numerical methods or by models. Other situation is when the coupling can be considered small but the number of partons is so large that the field strength is also very large and non-linear phenomena appear. In this region standard perturbative techniques fail. A new regime of QCD where parton densities in hadrons and nuclei are saturated - tamed by the non-linear behaviour of QCD - is expected to dominate the dynamics of high energy collisions involving hadrons and to determine the parton content of hadrons and nuclei when we probe partons that carry a small momentum fraction of the total hadron momentum. This regime shows features that are expected to happen in the confinement region, so its understanding will shed light on the transition between the perturbative and non perturbative regimes of QCD. The present project tries to increase our understanding on this saturation domain by promoting previous calculations to higher precision as required for a meaningful comparison with experimental data, to clearly settle whether this new regime of QCD has been achieved in the existing accelerators or experiments, and for proposing new measurements to be performed in existing or future experiments.
At the Large Hadron Collider (LHC) at CERN, several experiments are analysing proton-proton, proton-nucleus and nucleus-nucleus collisions. All the experiments are either focused on QCD research (e.g. the ALICE experiment) or require knowledge on QCD, QCD backgrounds producing the largestf uncertainty in measurements of characteristics of or deviations from the Standard Model of Particle Physics. The knowledge pursued in these experiments, and therefore in this project, is of fundamental nature. Thus, no immediate application to improve daily life is expected. But large projects as those developed at CERN, and future projects like the Electron Ion Collider (EIC) to be built in the USA for the early 2030’s or others (LHeC, FCC-he) in a more preliminary stage, produce results that become of use for society as it has been in the past: the WWW, accelerator and detector technology for health, new computing techniques,…
The overall objectives of the project aim to increase our precision for understanding the non-linear, high energy regime of QCD: understanding the quantum effects that produce particle correlations observed in data, developing an improved framework for calculations of high order in the saturation domain, a phenomenological description of observables sensitive to saturation in proton and nuclear collisions, and the application of such knowledge to future projects like the EIC, LHeC or FCC-he.
