Science Highlights 2022
The FASTSUM collaboration uses DiRAC supercomputers to simulate the interaction of quarks, the
fundamental particles which make up protons, neutrons and other hadrons. The force which holds
quarks together inside these hadrons is Quantum ChromoDynamics, “QCD”. We are particularly
interested in the behavior of QCD as the temperature increases to billions, and even trillions of Kelvin.
These conditions existed in the first moments after the Big Bang, and are recreated on a much smaller
scale in heavy ion collision experiments in CERN (near Geneva) and the Brookhaven laboratory (near
The intriguing thing about QCD at these temperatures is that it undergoes a substantial change in
nature. At low temperatures, QCD is an extremely strong, attractive force and so it’s effectively
impossible to pull quarks apart, whereas at temperatures above the “confining” temperature Tc, it is
much weaker and the quarks are virtually free and the hadrons they once formed “melt”.
We study this effect by calculating the masses of protons and other hadrons and their “parity partners”,
which are like their mirror-image siblings. Understanding how these masses change with temperature
can give deep insight into the thermal nature of QCD and its symmetry structure.
Our most recent results are summarized in the plots below. On the left we show the temperature
variation of the masses of the D and D* mesons (which are made up of a charm and a light quark).
This shows that they become nearly degenerate at the deconfining temperature indicated by the vertical
red line. On the right we show the R parameter which measures the degeneracy of the positive and
negative parity states of particular baryons. Results are plotted for the N (nucleon, i.e. proton/neutron)
as well as three other baryons which contain strange quark(s). This shows that the two parity states
become near degenerate (corresponding to R→ 0) in the high temperature regime above the vertical