Just millionths of a second following the Big Bang, everything in the known physical universe was in motion. Already, the cosmos was brimming with a trillion-degree plasma of quarks and gluons, which are elementary particles that only existed for comparatively short periods before cooling and changing into more stable particles.
From these came the neutrons and protons that make up our modern-day, conventional matter. But, before they cooled, a tiny fraction of these gluons and quarks randomly collided, forming "X" particles that don't last long.
And, despite the rarity of these mysterious, unknown particle structures, MIT scientists working with CERN have found evidence of X particles in the quark-gluon plasma generated by the Large Hadron Collider (LHC), according to a recent study published in the journal Physical Review Letters.
And this "is just the start of the story," said Yen-Jie Lee, lead author of the study and a 1958 Career Development Associate Professor of Physics, at MIT, in a press release. Crucially, this could be the first chance scientists have to examine the X particles in precise detail, building a better picture of the Big Bang.
The structure of particles from the Big Bang
X particles are rare because we don't see a Big Bang every day. But physicists think have suspected that they would emerge inside particle accelerators through a process called quark coalescence, when high-energy collisions lead to flashes of plasma that might emulate the chaotic, raw conditions of the unconscionably young universe. And now the MIT physicists at the institution's Laboratory for Nuclear Science, and other places, have discovered evidence that X particles can be produced in the LHC at CERN, in Geneva, Switzerland.
The discovery was accomplished via machine-learning techniques, which enabled the physicists to analyze more than 13 billion heavy-ion collisions, all of which then created tens of thousands of charged particles. And, probing this ultradense, high-energy cocktail, the MIT researchers sussed out roughly 100 X particles, specifically the X (3872) kind, which are named according to the estimated mass of the particle. This marks the first time scientists have successfully detected X particles in this quark-gluon plasma in a manner that scientists think might reveal their mysterious structure.
The X particle could be one of an entirely new kind
"We've shown we can find a signal," said Lee, the lead author, in the press release. "In the next few years we want to use the quark-gluon plasma to probe the X particle's internal structure, which could change our view of what kind of material the universe should produce." The study's co-authors are part of the CMS Collaboration, which features an international team of scientists working to gather data from one of the LHC's particle detectors, called the Compact Muon Solenoid.
We've known for a long time that neutrons and protons encompass the basic building blocks of atoms, and matter. But these are, in turn, comprised of three quarks, bound very tight. "For years we had thought that for some reason, nature had chosen to produce particles made only from two or three quarks," said Lee, in the release. Since X (3872) was discovered in 2003 during the Belle experiment (in Japan), scientists have speculated that X (3872) is either a compact tetraquark, or a completely new type of molecule that comes from mesons, instead of atoms, the former of which is composed of two quarks. "Currently our data is consistent with both," explained Lee. For now, a few more years of study are needed to distinguish between either scenario, and "broaden our view of the kinds of particles that were produced abundantly in the early universe."