LHC Reveals Early Universe Matter Behaved as a 'Near-Perfect Liquid'
Scientists at CERN's Large Hadron Collider (LHC) have presented experimental evidence indicating that quark-gluon plasma (QGP), a state of matter believed to have existed in the early universe, behaved as a liquid. By observing "wakes" created by high-energy quarks moving through this plasma, researchers have identified properties consistent with fluid dynamics, offering unprecedented insights into the universe's initial moments.
This groundbreaking research provides the first direct experimental evidence that the universe's primordial matter was a "near-perfect liquid," flowing smoothly with minimal friction.
The Primordial Plasma
Quark-gluon plasma (QGP) is an exotic state of matter composed of quarks and gluons, the fundamental constituents that later cooled to form particles like protons and neutrons. This plasma is believed to have existed for just a few millionths of a second after the Big Bang, at mind-boggling temperatures reaching trillions of degrees – billions of times hotter than the surface of the Sun.
Often referred to as a "primordial soup," QGP is described as the universe's first and hottest liquid.
Recreating Early Universe Conditions
To investigate the properties of QGP, researchers from institutions including the Massachusetts Institute of Technology (MIT) and CERN utilized the LHC. They recreated this fleeting state of matter by colliding heavy lead atoms at nearly the speed of light within the LHC's Compact Muon Solenoid (CMS) detector.
These high-energy collisions generate temporary, extremely small, and short-lived droplets of QGP. In this extreme environment, the intense density and temperature cause atomic structures to break down, allowing quarks and gluons to move freely and collectively.
Unveiling Quark Wakes
A key objective of the experiment was to understand how energetic particles interact within the QGP. Theoretical predictions, including those from a "hybrid model" of QGP and models developed by physicists like MIT's Krishna Rajagopal, suggested that a high-energy quark traversing the plasma would leave a detectable wake. This phenomenon is analogous to a boat moving through water, with the wake signifying the transfer of energy and momentum to the surrounding plasma.
Previous attempts to observe these elusive wakes encountered significant difficulties. Quarks are typically produced alongside antiquarks, and their respective wakes would obscure each other, making clear observation impossible.
To overcome this challenge, the research team devised a novel strategy. They focused on a rarer combination: a quark produced in conjunction with a Z-boson. Z-bosons are neutral elementary particles that interact minimally with the plasma and, crucially, do not create wakes. This allowed Z-bosons to serve as an unaffected indicator of the quark's original direction and energy.
By tracking the Z-boson, scientists could precisely determine the initial path of the quark, enabling a clear observation of the quark's wake undisturbed by other interactions.
In these scenarios, where a Z-boson and a high-momentum quark are generated, they recoil in opposite directions. The Z-boson's unique lack of interaction with the plasma thus permitted the clear observation of the quark's wake.
Experimental Breakthroughs
The team meticulously analyzed data from an astounding 13 billion LHC collisions. Among this vast dataset, approximately 2,000 instances of Z-boson production were successfully identified. In these specific events, fluid-like splash patterns, or wakes, were consistently observed traveling in the direction opposite to the detected Z-bosons.
The detailed analysis revealed a less than 1% suppression in particle production in the backward direction relative to the quark's motion. This critical observation is entirely consistent with a quark transferring energy and momentum to the surrounding plasma, creating a depleted region and exhibiting liquid-like behaviors such as splashing, rippling, sloshing, and swirling. Professor Yen-Jie Lee of MIT confirmed that the plasma's density is sufficient to slow down a quark and produce these liquid-like effects. The observed patterns closely aligned with the ripple predictions of the hybrid model for quark-gluon plasma.
Rajagopal described the experimental evidence for these quark wakes as "definitive."
Cosmological Significance
The findings unequivocally indicate that QGP is a "near-perfect liquid," meaning its quark and gluon components flowed smoothly with minimal friction. This discovery marks the first direct experimental evidence that quarks drag additional plasma as they move through it.
This research offers profound cosmological implications. The early universe is believed to have been filled with QGP before it cooled to form the matter observed today. Since this initial era is not directly observable through telescopes, heavy-ion collisions at the LHC provide a unique window into the universe's behavior during its earliest moments. The experimental technique developed establishes a crucial framework for investigating similar high-energy collision processes, potentially offering further insights into the fundamental properties of the universe's earliest matter.
The research was published in the prestigious journal Physics Letters B.