Heavy Quarks in the Early Universe's Plasma Move Unlike Einstein Predicted
MIT and UC Santa Barbara theorists find that kicks in a fast quark's momentum are lopsided, hinting that quark-gluon plasma behaves more like a liquid than a gas.
A new calculation suggests that a fast-moving heavy quark crossing the quark-gluon plasma does not jiggle the way Einstein's famous 1905 theory of Brownian motion says it should. The finding, published in Physical Review Letters, gives experimenters a new fingerprint to look for in collider data and hints at how the universe's first fluid is put together.
Quark-gluon plasma filled the cosmos for a few microseconds after the Big Bang. Physicists recreate it by smashing heavy ions together at nearly the speed of light, which produces droplets about the size of an atomic nucleus that live for tens of septillionths of a second. Heavy quarks that pass through the droplet are useful probes, because the kicks they receive reveal the plasma's microscopic structure.
Einstein's 1905 paper tied a particle's motion in a fluid to the random kicks it gets from the surrounding molecules. It was inspired by Robert Brown's 1827 observations of pollen grains jittering in water. For small kicks, the distribution of those kicks follows a bell curve. Physicists have known for roughly 20 years that Einstein's framework breaks down for particles moving at relativistic speeds, which left a gap in how to interpret heavy-quark data.
Jean F. Du Plessis of MIT's Center for Theoretical Physics and Bruno Scheihing-Hitschfeld of the Kavli Institute for Theoretical Physics at UC Santa Barbara took up the problem in a paper titled "Heavy Quark Transport Is Non-Gaussian beyond Leading Logarithm." They modeled a relativistic heavy quark moving through a weakly interacting gas of quarks and gluons, a simplified starting point. They calculated the strong nuclear force acting on the quark and the resulting probability distribution of its momentum changes, then repeated the calculation for plasmas made of different particles.
For small kicks, the distribution is bell-shaped, as in Einstein's picture. But the tails of the distribution, which describe large kicks, are lopsided for a relativistic heavy quark. Large kicks that slow the quark down are more likely than equally large kicks that speed it up. Large kicks of either kind are also more common than the standard bell curve predicts.
The same asymmetric pattern appeared across the different types of plasma the team tested, and it was already present in an earlier calculation for a strongly coupled plasma. That consistency suggests the lopsidedness is a general feature rather than a quirk of one model.
If experiments can measure heavy-quark motion with enough precision, the shape of this distribution could help characterize the plasma's microscopic structure. Du Plessis said the results suggest the early universe's quark-gluon plasma is more strongly interacting than Brownian models imply, making it "more like a liquid and less like a gas."
The work is theoretical, and the weakly interacting gas used here is a deliberate simplification. Testing the prediction against data from facilities such as the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, whose STAR detector studies these collisions, is the obvious next step.
