HIP Theory Project: QCD

Scientific background

Quantum Chromodynamics (QCD) is firmly established as the correct theory of the strong interaction and a cornerstone in the Standard Model of particle physics. It describes the confinement of quarks and gluons into hadrons, and their deconfinement in hot and dense systems. In spite of the apparent simplicity of the Lagrangian that defines the theory, QCD presents a complicated challenge for attempts to calculate physical observables.

The Helsinki Institute of Physics coordinates the Finnish participation at CERN and other large international experiments in particle and nuclear physics. There is a substantial involvement in the ALICE experiment, aimed at studying the properties of hot and dense QCD matter using collisions of relativistic heavy ions. The CMS experiment is, in addition to the electroweak and beyond-the-standard model program, a major facility for studying the strong interaction. HIP also participates in experimental studies of forward hadronic collisions at the TOTEM experiment, which measures the interactions of hadrons in the nonperturbative high energy regime. These activities offer the physics motivation for the research in this theory project.

Studying QCD in various kinematical regimes of the theory requires different approaches. Perturbative calculations can be used for individual parton-parton collisions in dilute systems. They successfully describe rare, hard processes with large virtuality (or transverse momentum). Lattice calculations are performed by numerically evaluating the path integral defining the theory in a discrete spacetime. They are most useful for understanding static, equilibrium properties QCD. Scattering at high energy and dynamical real time processes in strongly interacting matter require the use of effective theories derived from QCD, such as the CGC or HTL resummation. The spacetime evolution of QCD matter produced in a heavy ion collision can be calculated using hydrodynamics, using thermodynamic and transport properties derived from the fundamental theory.

Theoretical research of QCD and strongly interacting gauge theory dynamics in Finland is performed at the universities of Jyväskylä and Helsinki, in research groups that have a long history of collaboration and a strong international visibility. This project aims to continue the domestic and international collaboration in strong interaction physics.