Extreme QCD: Quantifying the QCD Phase Diagram ’19

Extreme QCD: Quantifying the QCD Phase Diagram ’19

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.

Figure 1.

Our most recent results are summarized in the plot where we show the proton mass as a function of temperature up to around Tc in the top left panel. The other panels show results for hadrons containing various numbers of strange quarks. As can be seen, the mass of positive parity states have fairly constant masses, whereas their negative parity partners’ masses decrease substantially as the temperature increases until they become nearly degenerate with their positive parity partners at around Tc, as dictated by the global chiral symmetry of the QCD Lagrangian. Beyond Tc masses are difficult to determine which is consistent with the QCD force becoming so weak that the hadrons break apart. This calculation supplements our theoretical understanding of QCD with quantitative detail..