MIT theoretical study predicts asymmetric changes in heavy quarks within primordial plasma

A calculation to study primordial plasma

In the first fraction of a second after the Big Bang, matter existed as a quark-gluon plasma at a trillion degrees. This mixture of particles lasted a few microseconds, before its components joined together to form more stable protons and neutrons. To study this primordial fluid, scientists recreate quark-gluon plasmas in particle accelerators: ion collisions produce droplets roughly the size of an atomic nucleus, which exist for only tens of septillionths of a second. MIT

One strategy is to track heavy particles that are not part of the plasma but pass through it. The way they move can reveal aspects of the fluid’s microscopic structure. However, simply applying the classical description of Brownian motion unchanged to these extreme conditions presents a problem: at speeds close to the speed of light, the expected relationship between drag and fluid fluctuations does not fully capture the interactions.

Physicists at MIT and the University of California, Santa Barbara (UCSB) developed a theory that predicts how a fluid can push and slow a relativistic particle. According to MIT’s news report, the team proposes that the theory could be applied to heavy quarks passing through the quark-gluon plasma. Its predictions could be compared with experiments to characterize the plasma’s microscopic structure; the report does not say that this test has already been carried out. MIT

From Brownian kicks to relativistic changes

In 1905, Albert Einstein described how the irregular motion of particles suspended in a fluid can provide information about the medium’s microscopic forces. In this explanation, the motion results from a balance between drag, which tends to slow the particle, and random pushes from the fluid. The probability of these pushes is represented by a bell-shaped curve: pushes of intermediate intensity are more likely, while the extremes are less likely.

In the new study, Jean Du Plessis, of MIT’s Center for Theoretical Physics, and Bruno Scheihing-Hitschfeld, of UCSB’s Kavli Institute for Theoretical Physics, calculated the forces a heavy quark would experience while moving through a fluid at relativistic speeds. To do so, they performed calculations of the strong nuclear force, which holds quarks and gluons together. They chose a weakly interacting gas of quarks and gluons as their starting point, because weak interactions made the calculations manageable.

From this scenario, they calculated how much the quark’s momentum might change and obtained a probability distribution for those changes. For small kicks, the curve retains a shape similar to a bell. Its tails, however, are unequal: large kicks are more frequent than the Brownian curve would predict, and they are not equally likely in opposite directions. In particular, a large kick that slows the quark more than expected from drag is more likely than an equivalent kick that leaves it moving faster than expected. MIT

The table summarizes the theoretical calculation reported by MIT; it does not represent an experimentally measured distribution. MIT

A structure that could offer new clues

The team repeated the calculation for plasmas made up of different particles and found the same skewed curve, regardless of the composition considered. They also identified a similar structure in an earlier calculation of a strongly coupled plasma, despite differences in the microscopic physics. Du Plessis described this pattern as apparently universal and said the work formalizes it. The source presents these results as theoretical calculations, not as independent experimental corroboration. MIT

The possible usefulness of the result depends on how heavy quarks behave in experiments. If the frequency of large changes depends on the plasma’s microscopic nature, measuring them could provide additional information about the medium. The authors propose comparing their predictions with experiments to characterize the plasma and study what kind of fluid existed in the early moments of the universe. This comparison is presented as a future possibility, not as a result already obtained.

Du Plessis also interprets the primordial plasma as possibly more like a liquid than a gas and more strongly interacting than the Brownian model would suggest. The report attributes this interpretation to the researcher; it does not present it as a direct observation of the primordial plasma. MIT

Research and institutional context

MIT reports that Du Plessis and Scheihing-Hitschfeld reported their results that day in Physical Review Letters. The research received partial support from the Nuclear Physics Office of the U.S. Department of Energy’s Office of Science, the U.S. National Science Foundation (NSF), and the Simons Foundation. MIT

As institutional context, the NSF states that it supports basic and applied research, primarily through grants, as well as supporting collaborations and scientific infrastructure. This information describes the agency’s general role, but does not independently confirm the study’s results. NSF: research support

The NSF reference on materials research describes programs focused on understanding materials, their properties, and potential applications. It provides no specific information about the quark-gluon plasma and does not validate the calculation reported by MIT. NSF: materials research