High Energy QCD

Abstract

The project significantly advanced our understanding of the theory of strong interactions known as quantum chromodynamics (QCD) in high energy collisions of elementary particles and nuclei. QCD is one of the four fundamental forces of nature, but is understood quite poorly due to the complexity of strong interactions. This project advanced our understanding of QCD in the very high energy collisions of protons and nuclei, where densities of quarks and gluons inside the colliding particles are so high (due to high energy) that complicated nonlinear interactions between quarks and gluons become important. This regime is known as gluon (or parton) saturation. The result of the project is a significant improvement of our understanding of the physics of gluon saturation: important (running coupling) corrections to the existing description of the process (the so-called Balitsky-Kovchegov equation) have been calculated, placing the BK equation in good agreement with the experimental data on deep inelastic scattering (DIS), a process where an electron is collided with the proton to probe the proton's internal structure. Corresponding cross section for quark and gluon production in DIS and nuclear collisions have been calculated and corrected correspondingly, resulting in new and interesting predictions for the physics to be probed in heavy ion collisions at the Large Hadron Collider (LHC). The dense gluon systems play an important role in collisions of ultrarelativistic large nuclei, which are performed in order to create a plasma of quarks and gluons (QGP). An important question in the field is how exactly this QGP is produced in a collision of two heavy ions. The conclusion of this project is that QGP production happens due to strong coupling effects between quarks and gluons. This made theoretical description of QGP production only possible using the methods emerging from string theory. Using these methods QGP production was well understood as a result of this project. The project was mainly accomplished by analytic calculations, with occasional use of personal computers and a supercomputer to perform numerical calculations. As such it is very economic and technically effective. The benefit to public is by broadening our understanding of fundamental physics. Strong interactions are responsible for about 98% of the visible mass in the Universe, and their understanding is vital both for understanding how the Universe works and for which physics lies beyond the known interactions of Standard Model of particle physics.