Large Hadron Collider gives scientists their best look yet at conditions right after the Big Bang

The world's most powerful particle accelerator, the Large Hadron Collider, has given scientists their best look yet at quark-gluon plasma, the primordial matter that filled the universe moments after the Big Bang.

During the first fractions of a second of the universe's existence, the cosmos was filled with a hot and dense primordial soup called quark-gluon plasma. At the nearly 17-mile-long circular particle accelerator, the Large Hadron Collider (LHC) that sits deep below the French Alps, CERN scientists recreated the quark-gluon plasma by smashing together atomic nuclei of iron at near-light speed. The project is called ALICE (A Large Ion Collider Experiment).

The ALICE team obtained new information about the quark-gluon plasma (and thus the conditions in the early universe) when they spotted a pattern common to collisions between protons — the particles found at the heart of atoms — collisions between protons and lead nuclei, and collisions between lead nuclei themselves. This pattern could reveal how the quark-gluon plasma formed right after the Big Bang, indicating it could be forged by smaller particle collisions than previously thought.

When scientists first started smashing protons together at the LHC, it was theorized that collisions between protons as well as between protons and lead would be too small to generate quark-gluon plasma. However, tantalizing signs of this primordial matter have recently been seen in these small collisions as well as in the collisions between lead nuclei.

One of the signatures of quark-gluon plasma and its formation is the fact that particles aren't emitted evenly, but in a preferred direction, which scientists call anisotropic flow. At intermediate speeds, the anisotropic flow of particles depends on the number of quarks that compose them. Baryons, particles composed of three quarks, exhibit a stronger flow than mesons, which are particles composed of two quarks.

Scientists theorize that this is linked to the process that brings quarks together to form larger particles. Baryons have more quarks and thus gain greater flow.

In new research the ALICE Collaboration explained how they measured the anisotropic flow for different mesons and baryons created by proton-proton and proton-lead collisions. By isolating particles flowing together, the team confirmed that, just as is seen in heavy collisions, these lighter collisions give rise to baryons with stronger flow and mesons with weaker flow at intermediate speeds.

"This is the first time we have observed, for a large interval in momentum and for multiple species, this flow pattern in a subset of proton collisions in which an unusually large number of particles are produced," David Dobrigkeit Chinellato, Physics Coordinator of the ALICE experiment, said in a statement. "Our results support the hypothesis that an expanding system of quarks is present even when the size of the collision system is small."

The ALICE team compared the flow observations they made to models of quark-gluon plasma formation, finding the flow pattern closely fit models that account for the formation of baryons and mesons. Models that don't factor in this quark coalescence, however, failed to replicate the observed flow pattern.

The researchers also found that even the best-fit models couldn't completely account for the observed flow. There are still some lingering discrepancies, wrinkles that the team thinks other collisions between particles with sizes between protons and iron could help to iron out.

"We expect that, with the oxygen collisions that were recorded in 2025, which bridge the gap between proton collisions and lead collisions, we will gain new insights into the nature and evolution of the quark-gluon plasma across different collision systems," ALICE Spokesperson Kai Schweda said in the statement.

Then, scientists will edge even closer to understanding the conditions found at the very dawn of the universe.

A paper about this research was published on March 20 in the journal Nature Communications,