A Mysterious 'X Particle' Could Help Explain the Birth of Reality

Scientists achieved breakthrough detections of an elusive particle from the dawn of time at the Large Hadron Collider.
Scientists achieved breakthrough detections of an elusive particle from the dawn of time at the Large Hadron Collider.
Image: Ramberg via Getty Images
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In a breakthrough that opens a window into the birth of the universe, scientists have detected elusive “X particles” in the otherworldly aftermath of high-energy collisions at the Large Hadron Collider (LHC), the most powerful particle accelerator on Earth.

These short-lived particles, which are called “X” because their internal structure is unknown, existed in the chaotic microseconds after the Big Bang, when the universe was filled with a churning subatomic soup called quark-gluon plasma. They are, however, exceedingly rare in the modern universe, leaving many of their properties shrouded in mystery. 


That’s why scientists are so thrilled to have recorded the decays of about 100 of these X particles using the Compact Muon Solenoid (CMS), a particle detector at LHC. The “result provides a unique experimental input to theoretical models” of X particle production and “of the nature of this exotic state,” according to a new study published in Physical Review Letters

X particles were initially detected in a 2003 experiment, but they decayed too quickly to be analyzed in detail. The new discovery marks the first time that they have been captured inside quark-gluon plasma, enabling scientists to hone in some of the properties that have, so far, evaded characterization.

The discovery of X particles in quark-gluon plasma “is very difficult and new!” said Yen-Jie Lee, who is the Class of 1958 Career Development associate professor of physics at MIT and lead author of the new study, in an email. 

“First of all, quark-gluon plasma produces tens of thousands of particles,” he noted, which is orders of magnitude more than the 2003 experiment recorded by the Belle detector in Japan’s KEKB facility, which produced decay signatures from a few X particles.  

In addition, the “experiment done at CMS with relativistic heavy-ion collisions is much more similar to how nature produces X particles in the early universe,” Lee continued. For this reason, the CMS technique has opened up a new way to probe the structure and behavior of these particles in unprecedented detail, which will help shed light on the murky beginnings of the cosmos.


Both experiments generated X particles by smashing tiny bits of matter together at high speeds, a process that creates a stew of weird subatomic particles that are difficult to observe in other environments. For instance, the CMS experiment monitored the fallout of billions of collisions between lead ions, which created the bizarre quark-gluon plasma.

 Because X particles decay almost instantly after they form, it is incredibly challenging to capture them. Jing Wang, an MIT postdoc and member of the CMS collaboration, developed a technique to overcome this obstacle: a machine-learning algorithm designed to spot the decay signatures of an X particle known as X (3872). 

This innovation was based on previous studies that have used machine learning to detect particles such as the Higgs boson. When Lee and his colleagues applied the algorithm to a 2018 dataset of heavy-ion collisions, they were thrilled with the results.

“We were surprised to see that we got such a big signal in heavy-ion collisions (which produce quark-gluon plasma) because this was almost 10 times larger than what we could see in proton-proton collisions,” Lee said. “It was predicted that X(3872) production could be greatly enhanced in the presence of quark-gluon plasma, but we finally managed to actually see it in our experimental data taken with CMS and CERN colleagues.”

 “The fact that we could detect the X signal with our colleagues in CMS at CERN so quickly was very exciting to us!” he added.


Lee noted that the team needs more data to estimate the exact size of the enhancement of X particle production in quark-gluon plasma, but those results should be forthcoming soon. The CMS collaboration also hopes to build on their findings by investigating the structure of  X(3872). For instance, the researchers want to see if they can hone in on actual X particles in the quark-gluon plasma, as opposed to detecting the decay products of X(3872). 

The team is also looking to answer a fundamental question about X(3872): What the heck is it? One hypothesis suggests the particle is tetraquark, which is an extremely rare type of particle made of four quarks. But some scientists have suggested that it may be a mesonic molecule, a never-before-seen type of particle made from two mesons.  

 “X(3872) is the best candidate for mesonic molecules!” Lee said. “If X(3872) turns out to be a mesonic molecule, we show that there must be many different kinds of mesonic molecules in the early universe,” he added, which would mean this era “was filled with not only the ordinary hadrons but also many other exotic particles.”

“It is going to be exciting to follow up this line of study with a much larger amount of data to be taken by the experiments at the Large Hadron Collider,” he concluded.

Indeed, these X particles can yield broader insights about the type of environment that existed in those searing and turbulent moments after the Big Bang. Since we cannot observe this ancient era directly, scientists need to recreate simulations of it in powerful facilities like the LHC to understand what was possible during the universe’s birth—and how it all led to the very different cosmic environment we live in today.