Research in the Particle Theory group

(Gert Aarts, Chris Allton, Simon Hands)

When quarks and gluons are put under the extreme conditions of non-zero temperature and/or density, new phases of strongly interacting matter appear. The best known example is the quark-gluon plasma (QGP), currently probed at the Large Hadron Collider in CERN. In order to gain insight into the nature of the strong interactions under these conditions, we have studied a number of questions, using a combination of lattice QCD and analytical techniques.

Quarkonia, bound states of a heavy quark and anti-quark, were first proposed as hard probes of the QGP in 1986. The initial focus has been on charmonium, bound states of charm quarks. With the advent of the LHC, running at much higher energies, attention has shifted to bottomonium. In 2011 CMS published first results on the survival of the Υ(1S) ground state, but a suppression of excited Υ(2S, 3S) states in the QGP. We have studied the fate of bottomonium in the QGP using lattice QCD, finding agreement with the experimental results.We have also studied the force between charm quarks, when they are immersed in the quark-gluon plasma.

Transport plays an important role in the hydrodynamical description of heavy-ion collisions. In our work we focus on the electrical conductivity and the diffusion constant, using lattice QCD simulations at finite temperature. Our results are the first with 2+1 flavours of dynamical quarks, covering a wide temperature range, and we uncovered a nontrivial temperature dependence when moving from the confined to the deconfined phase.