Extreme QCD: Quantifying the QCD Phase Diagram

Extreme QCD: Quantifying the QCD Phase Diagram

Our collaboration uses the 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 is Quantum ChromoDynamics, “QCD”. We are particularly interested in the behavior of QCD as the temperature increases to billions and even trillions of Celsius. 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 (in New York State).

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 prise 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. At low temperatures, hadrons have lower masses than their partners. Above Tc, due to the fact that QCD acquires more symmetry, hadrons and their partners may become equal in mass.

 Figure 1: Plot of the “R”-value as function of temperature.

Our results are summarized in the plot where we show the “R” value which measures how close the masses of the partners are: R=1 corresponds to very different masses, and R=0 means that they have identical masses. For a variety of hadrons including the proton (N), delta (Δ) and omega (Ω) as the temperature increases above Tc, we see that R is close to zero implying that the parity partners have near equal masses. This confirms the long-held, but unconfirmed suspicions regarding the nature of QCD. Drilling deeper into this data, it’s clear from the figure, that the Ω hadron partners are the least close in mass. This can be understood because the constituent quarks in Ω are the “strange” quarks which are significantly heavier than the virtually massless “up” and “down” quarks inside the N and Δ. This extra mass means that the symmetry mechanism mentioned does not work as well.