Extreme QCD: Quantifying the QCD Phase Diagram ’22

Extreme QCD: Quantifying the QCD Phase Diagram ’22

Project: dp006

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

New York).

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