In the immediate aftermath of the Big Bang, during the first millionths of a second, our universe existed as an incredibly hot, swirling plasma of quarks and gluons reaching temperatures of trillions of degrees. These fundamental particles momentarily combined in countless configurations before the universe cooled sufficiently to allow the formation of more stable structures that would eventually become the neutrons and protons constituting ordinary matter.
Amidst this primordial chaos, a small fraction of these quarks and gluons randomly collided to create ephemeral "X" particles—named for their enigmatic and previously unidentified structures. In today's universe, these X particles are exceptionally scarce, though theoretical physicists have long postulated that they might be recreated in particle accelerators through quark coalescence, where high-energy collisions can generate similar flashes of quark-gluon plasma.
Now, pioneering physicists from MIT's Laboratory for Nuclear Science and collaborating institutions worldwide have successfully uncovered compelling evidence of X particles within the quark-gluon plasma generated by the Large Hadron Collider (LHC) at CERN, the prestigious European Organization for Nuclear Research located near Geneva, Switzerland.
The research team employed cutting-edge machine-learning algorithms to meticulously analyze more than 13 billion heavy ion collisions, each producing tens of thousands of charged particles. Within this extraordinarily dense, high-energy particle soup, the investigators successfully isolated approximately 100 X particles, specifically of the type designated as X (3872)—named after the particle's estimated mass.
The findings, published this week in the distinguished journal Physical Review Letters, represent the first-ever detection of X particles in quark-gluon plasma—an environment that researchers hope will shed light on these particles' hitherto mysterious internal structure.
"This discovery merely marks the beginning of our exploration," remarks lead author Yen-Jie Lee, the Class of 1958 Career Development Associate Professor of Physics at MIT. "We've demonstrated that we can identify these elusive signals. In the coming years, we aim to utilize the quark-gluon plasma to investigate the X particle's internal architecture, which could potentially revolutionize our understanding of what kinds of matter the universe naturally produces."
The study's co-authors comprise members of the CMS Collaboration, an international consortium of scientists responsible for operating and collecting data from the Compact Muon Solenoid, one of the LHC's sophisticated particle detectors.
Unraveling the Nature of Exotic Particles
The fundamental constituents of ordinary matter—neutrons and protons—each consist of three quarks bound together with extraordinary force.
"For decades, physicists operated under the assumption that nature, for some reason, exclusively produced particles composed of either two or three quarks," Lee explains.
Only in recent years have researchers begun to observe indications of exotic "tetraquarks"—particles composed of rare combinations of four quarks. Scientists theorize that X (3872) might represent either a compact tetraquark or an entirely novel category of molecule formed not from atoms but from two loosely bound mesons—subatomic particles that themselves consist of two quarks.
X (3872) was initially observed in 2003 by the Belle experiment, a particle collider in Japan that smashes together high-energy electrons and positrons. Within this experimental environment, however, these rare particles decayed too rapidly for scientists to analyze their structure in detail. Researchers have hypothesized that X (3872) and other exotic particles might be more effectively studied within quark-gluon plasma.
"From a theoretical perspective, the abundance of quarks and gluons in the plasma should enhance the production of X particles," Lee notes. "However, many scientists believed it would be practically impossible to detect them amidst the tremendous number of other particles generated in this quark soup."
Confirming the Extraordinary Signal
In their groundbreaking study, Lee and his research team searched for evidence of X particles within the quark-gluon plasma created by heavy-ion collisions at CERN's Large Hadron Collider. Their analysis was based on the LHC's 2018 dataset, which encompassed more than 13 billion lead-ion collisions, each releasing quarks and gluons that scattered and merged to form more than a quadrillion ephemeral particles before cooling and decaying.
"Following the formation and subsequent cooling of the quark-gluon plasma, an overwhelming number of particles are produced, creating an exceptionally challenging background," Lee explains. "We needed to dramatically reduce this background noise to successfully identify the X particles within our data."
To accomplish this formidable task, the team developed a sophisticated machine-learning algorithm trained to recognize decay patterns characteristic of X particles. Immediately following their formation in quark-gluon plasma, particles rapidly break down into "daughter" particles that scatter in various directions. For X particles, this distinctive decay pattern, or angular distribution, differs significantly from all other particles.
The researchers, led by MIT postdoctoral fellow Jing Wang, identified crucial variables that describe the unique shape of the X particle decay pattern. They trained a machine-learning algorithm to recognize these distinctive variables, then fed the algorithm actual experimental data from the LHC's collision experiments. The algorithm successfully navigated the extremely dense and noisy dataset to identify the key variables most likely resulting from decaying X particles.
"We managed to reduce the background by several orders of magnitude, finally revealing the X particle signal," explains Wang.
The researchers focused their attention on these signals and observed a distinct peak at a specific mass, confirming the presence of X (3872) particles—approximately 100 in total.
"It's almost unimaginable that we could successfully isolate these 100 particles from such an enormous dataset," says Lee, who, along with Wang, conducted numerous verification tests to confirm their observation.
"Every evening I would question myself: is this truly a signal or merely background noise?" Wang recalls. "Ultimately, the data provided a resounding affirmative answer!"
In the coming one to two years, the researchers plan to collect substantially more data, which should help elucidate the X particle's structure. If the particle represents a tightly bound tetraquark, it should decay more slowly than if it were a loosely bound molecular structure. Now that the team has demonstrated that X particles can be detected in quark-gluon plasma, they intend to investigate this particle more thoroughly using the quark-gluon plasma environment to definitively determine the X particle's structure.
"Currently, our data is consistent with both possibilities because we don't yet have sufficient statistical evidence," Lee explains. "In the next few years, we'll collect significantly more data, allowing us to distinguish between these two scenarios. This will expand our understanding of the types of particles that were abundantly produced in the early universe."
This research received support, in part, from the U.S. Department of Energy.
