The Universe just after the Big Bang was similar to... a liquid. Physicists today are managing to recreate this state in the laboratory, revealing unexpected behaviors that inform us about the first instants of the cosmos.
To achieve this, scientists at CERN use the Large Hadron Collider to collide heavy ions at prodigious speeds. These impacts briefly liberate quarks and gluons from their usual confinement, producing tiny droplets of this primordial plasma. This substance filled the Universe during its first microseconds.
A quark zooms through quark-gluon plasma, creating a wake in the medium.
Credit: Jose-Luis Olivares, MIT
To observe this plasma, an MIT team devised an innovative strategy. Instead of searching for quark pairs, the researchers focused on events where a quark is produced opposite a Z boson. Since this neutral particle barely interacts with its environment, it serves as a perfect reference point to isolate the quark's effects.
The analysis of data, from billions of collisions, revealed characteristic patterns in the plasma. Swirls and splashes, aligned in the direction opposite the Z boson, were observed and correspond to the wake left by a single quark. These results constitute the first direct evidence that the plasma behaves as a dense, collective fluid, and not as a simple cloud of independent particles.
These observations are in agreement with existing theoretical models, including one developed by Krishna Rajagopal and his collaborators. This model predicts that the plasma should ripple like a liquid when a particle moves through it at high speed. The experimental confirmation thus strengthens our perception of the primordial medium and its properties.
Thanks to this advance, new possibilities are opening up for examining the characteristics of the quark-gluon plasma with increased precision. By measuring the size and duration of the wakes, physicists will be able to better determine the viscosity and other parameters of this exotic fluid. This work helps to piece together the history of the Universe's first moments.
This study was conducted within the CMS Collaboration, an international group of particle physicists. The results were published in the journal
Physics Letters B, offering open access to all the data and methods used for this discovery.