In the immediate aftermath of the cosmic genesis event, the nascent Universe was characterized by an ultra-hot, incredibly dense plasma, a state likened to a trillion-degree ‘soup’. A significant experimental endeavor has now yielded the inaugural confirmation that this primordial, exotic fluid indeed exhibited dynamic motion, characterized by eddies and currents, much like a conventional liquid.
Technically designated as quark-gluon plasma (QGP), this effervescent mixture represents the earliest and hottest liquid substance known to have existed. Theoretical estimations posit that it maintained temperatures a billion times exceeding that of the Sun’s surface for brief intervals, on the order of millionths of a second, prior to its subsequent expansion, cooling, and eventual transformation into atomic structures.
As elucidated in a recent publication, a collaborative contingent of physicists from MIT and CERN undertook the task of replicating heavy-ion collisions analogous to those that generated QGP, with the objective of scrutinizing its inherent characteristics. A key question posed was whether a quark traversing this plasma experiences resistance and generates ripples, akin to a cohesive fluid, or if it undergoes random dispersion, as would be expected from discrete particles.
To ascertain this, researchers meticulously examined data pertaining to collisions involving lead nuclei, accelerated to velocities approaching the speed of light within the Large Hadron Collider (LHC) at CERN. These high-energy impacts generate a cascade of energetic particles, including quarks, alongside a fleeting droplet of QGP that once permeated the early Universe.
Employing a novel analytical approach that afforded a heightened resolution of the heavy-ion collisions compared to prior investigations, the physicists tracked the trajectories of quarks within the QGP and charted the energy distribution of the plasma in the wake of these interactions.
“Our findings reveal that the plasma possesses an extraordinary density, sufficient to impede the passage of a quark and induce fluid-like effects such as splashes and eddies. This substantiates the notion that quark-gluon plasma is indeed a primordial soup,” stated physicist Yen-Jie Lee of MIT.
The quarks propagating through the QGP transfer a portion of their kinetic energy to the surrounding fluid, thereby diminishing their speed and generating a wake reminiscent of that left by a moving vessel.
“By way of analogy, imagine a boat traversing a lake; the wake consists of the water displaced and moving in the boat’s direction. The boat has imparted momentum to a specific volume of water, which then ‘follows’ its path,” explained MIT physicist Krishna Rajagopal, a key figure in developing the theoretical model that predicted the fluid characteristics of QGP, in communication with ScienceAlert.
However, unlike the clearly defined wake observed in water, the researchers had to deduce the presence of this effect within the minuscule droplets of QGP from indirect evidence.
This investigative process necessitates the careful analysis of tens of thousands of particles engaged in vigorous interactions within a plasma existing at trillion-degree temperatures, typically persisting in the LHC for merely a quadrillionth of a second, in order to identify the subtle displacement of a limited number of particles caused by the wake.
This undertaking is inherently challenging. When quarks are generated in LHC collisions, they do not appear in isolation, as Rajagopal elaborated to ScienceAlert. They typically emerge in pairs with antiquarks, their antimatter counterparts possessing identical mass but opposite electric charge. The quark and its antiquark diverge in opposite directions at equal velocity, each generating a distinct wake that complicates the detection of individual wake effects.
Consequently, rather than focusing on quark-antiquark pairs, as had been the practice in previous research, the physicists shifted their attention to a different particle pairing. On occasion, LHC collisions result in the simultaneous production of a quark and a Z boson, a neutral fundamental particle that, crucially, does not generate a wake due to its inertness with respect to the QGP.
Nevertheless, these specific events are statistically infrequent. Out of the 13 billion LHC collisions examined in the research, only approximately 2,000 yielded a Z boson. However, owing to the Z boson’s lack of interaction with the QGP, the researchers were finally enabled to meticulously analyze the wake generated by a solitary, high-velocity quark. The findings comported with Rajagopal’s theoretical predictions, indicating that the QGP indeed behaved as a fluid, exhibiting dynamic movement in response to the quark’s passage.
Rajagopal conveyed to ScienceAlert that this constitutes “definitive, unambiguous evidence” of QCP’s fluid-like behavior; however, the longstanding debate concerning whether QGP exhibits fluid dynamics, including flow and rippling, may not be definitively resolved by this single study. It is anticipated that other scientific teams will conduct thorough examinations of these results.
Nonetheless, this innovative methodology establishes a precedent for exploring comparable phenomena in diverse classes of high-energy collisions, potentially shedding new light on one of the most enigmatic substances ever to have existed in the Universe’s history.
“In numerous scientific disciplines, understanding a material’s properties involves actively perturbing it and then observing how that disturbance propagates and dissipates,” Rajagopal remarked.
And this, in essence, is part of what imbues physics with its intrinsic fascination – when faced with an unknown mechanism, the solution often lies in accelerating particles to velocities approaching the speed of light.
