Physicists detect mysterious “X” particles from the beginning of time for the first time

Physicists detect mysterious X particles

X particles probably existed in the smallest fractions of a second after the Big Bang. MIT physicists hope to build the most accurate picture to date of the origins of the universe.

Physicists at the Large Hadron Collider (LHC), the world’s largest particle accelerator, located near Geneva at CERN, have detected for the first time a mysterious particle believed to have existed in the first millionths of a second after the Big Bang.

Called “X” particles, due to their unknown structure, they were found in a medium called quark-gluon plasma, generated at the LHC by the collision of lead ions. There, among the trillions of particles produced by these collisions, the physicists managed to extract 100 of the exotic specks.

By studying the primordial X particles in more detail, the scientists, who published their results in the journal Physical Review Letters, hope to build the most accurate picture to date of the origins of the universe. 

Probing the internal structure of particle X

“This is only the beginning of the story,” says physicist Yen-Jie Lee, from the Laboratory of Nuclear Sciences at the Massachusetts Institute of Technology (MIT), and a member of the CMS international collaboration, based at CERN (Switzerland), the European Organization for Nuclear Research.

“We have shown that we can find a signal. In the next few years we want to use quark-gluon plasma to probe the internal structure of the X particle, which could change our view of what kind of material the universe should produce.”

Quarks and gluons: elementary particles

Just moments after the Big Bang, the early universe was a superheated plasma at trillions of degrees, made up of elementary particles called quarks and gluons. These particles subsequently coalesced briefly in countless combinations before cooling and settling into more stable configurations, forming the neutrons that normal matter is made of today.

In that very short amount of time, the particles of the quark-gluon plasma collided, stuck together, and came apart again in different configurations. One of those configurations is the X particle, which we don’t know how it was formed.

A huge sign depicting the CMS detector in CERN building 40 in Meyrin, near Geneva.

Recreate the chaotic primordial soup of the Big Bang

To recreate the primitive conditions of the universe, the LHC researchers fired positively charged lead atoms at each other at high speed, causing them to collide to produce thousands more particles in a momentary burst of plasma that resembles the chaotic primordial soup of the Big Bang. 

Despite the complexity of the task, that was the easy part. The hard part, thanks in part to the very short-lived X particles, was sifting through data from 13 billion heavy ion collisions to find the X particles.

“In theory, there are so many quarks and gluons in the plasma that the production of X particles should be higher,” Lee said. “But people thought it would be too hard to search for them, because there are so many other particles produced in this quark soup.”

Algorithm helps to extract about 100 particles X

So, to examine the millions of collisions, the team developed an algorithm. Amid this ultra-dense, high-energy particle soup, the researchers were able to extract about 100 X particles, of a type known as X (3872), named for the particle’s estimated mass, the MIT statement details.

“It’s almost unthinkable that we can identify these 100 particles from this huge data set,” said Lee, who, along with co-author Jing Wang, an MIT physicist, ran multiple checks to verify their observation. “Every night I wondered: Is this really a signal or not?” Wang said. “And in the end, the data said yes.”

Decipher the structure of particle X

At the moment, the data is insufficient to learn more about the structure of the X particle. However, the researchers plan to gather much more data in the next one to two years, which should help elucidate the structure of the X particle.

While protons and neutrons are made up of three quarks each, physicists think that X particles can be made up of four, although they don’t know how they are bound together. Scientists believe that the new particle could be made up of four quarks bound together with the same force, which would make it an exotic particle called a tetraquark, or two pairs of quarks – called mesons – loosely bound together.

“Currently, our data is consistent with both [structures] because we don’t have enough statistics yet,” Lee said. “In the next few years, we’ll take a lot more data to be able to separate these two scenarios. That will broaden our view of the types of particles that were abundantly produced in the early universe.”