QCD at finite T

Quantum Chromodynamics (QCD), the theory which describes the interactions between quarks and gluons, has a very particular feature called confinement. This property states that quarks or gluons can never be isolated and observed individually (at least at low energies), they are always confined inside bound states called hadrons. However, at sufficiently high temperatures or densities confinement is broken and quarks and gluons appear outside of hadrons forming a new state of matter, the so-called quark-gluon-plasma (QGP).

This QGP poses many interesting questions. What is its Equation of State - what are the pressure, energy and entropy densities of the thermal medium? How do these collective properties depend on the temperature? How does the thermal medium affect properties of particles in the medium?

The tools we use to investigate these issues are effective field theories (EFTs) and lattice QCD. In a system of a few heavy quarks inside a plasma of light quarks and gluons there are several scales present: the heavy quark mass, the average distance between the heavy quarks, the temperature, thermal gluon masses, etc. For a weakly coupled plasma, small distances or small/high temperatures several of these are well separated and therefore suited for an EFT description. In the case of highly energetic particles traversing the plasma, different momentum components, (anti-)collinear or transverse, may also be well separated. The non-perturbative calculations provided by lattice QCD often complement the information from EFTs or can probe regimes where EFTs that rely on a perturbative expansion are no longer applicable.

In particular we study the following phenomena:

Quarkonium suppression in heavy-ion collisions is concluded from observations that rates of heavy quarkonia are modified in collisions of heavy ions. Knowledge of the heavy quark interaction in a medium is necessary to describe such phenomena. Using an EFT at finite temperature, we have obtained a description of the heavy quark interaction in the medium which features a large imaginary part responsible for the quarkonium dissolution in the medium. Applications to the calculation of the R_AA factor measured at the ALICE experiment at LHC-CERN is under investigation as well as complementary lattice calculations. However, as the thermal medium is not the only cause for modified properties of heavy quarkonia, the quarkonium suppression is the topic of a world-wide research effort.

Jet quenching refers to the process by which highly energetic parton jets, which are created in the early stages of a collision before the plasma is formed, lose energy through interaction with the plasma. This may be facilitated by gluon bremsstrahlung, pair production, etc. A related medium effect is transverse momentum broadening, by which interactions of the jet partons with the plasma lead to changes in the parton momentum components that are transverse to the initial jet direction without changing the jet's energy. Studying these jet properties can provide information about the properties of the QGP.

Static quark correlators provide a first approximation to the behaviour of heavy quarks in a plasma of light quarks and gluons. In an expansion in the heavy quark mass the leading term treats the heavy quark as static. In that case the heavy quark propagator can be replaced by a purely gluonic operator, a so-called Wilson line or Polyakov loop. The free energy of one or more heavy quarks in a QGP can then be calculated through the thermal average of a Polyakov loop or correlators thereof. In this context there is also a lot of interest in the Polyakov loop as an order parameter of deconfinement. In a confining medium a single heavy quark should not be able to exist, which corresponds to an infinite free energy or a zero-value Polyakov loop. A non-zero Polyakov loop is therefore a sign of deconfinement. The crossover from zero to non-zero values for the Polyakov loop is a purely non-perturbative effect and can only be seen in lattice calculations. In the perturbative calculation of the Polyakov loop and related correlators in the deconfinement region there appear particular divergences that need to be renormalized in addition to regular charge renormalization and which can be categorized into loop mass, cusp and intersection divergences. Since these divergences appear in both perturbative and non-perturbative calculations, a comparison of both methods can shed light on the understanding of each.

Written by M. Berwein and J. Weber.


Nora Brambilla, Antonio Vairo, Peter Petreczky (Brookhaven), Alexei Bazavov (Iowa), Michael Benzke (Hamburg), Miguel Escobedo (Saclay), Johannes Weber, Matthias Berwein, Simone Biondini

Some related publications:

  • A. Bazavov, N. Brambilla, X. Garcia i Tormo, P. Petreczky, J. Soto and A. Vairo, Determination of αs from the QCD static energy: An update Phys. Rev. D 90 (2014), 074038, arXiv:1407.8437, Inspire

  • M. Benzke, N. Brambilla, M.A. Escobedo and A. Vairo, Gauge invariant definition of the jet quenching parameter JHEP 02 (2013) 129, arXiv:1208.4253, Inspire

  • M. Berwein, N. Brambilla, J. Ghiglieri and A. Vairo, Renormalization of the Cyclic Wilson Loop JHEP 03 (2013) 069, arXiv:1212.4413, Inspire