The findings could redefine the types of particles that were abundant in the early universe. -ScienceDaily

In the chaos that preceded the cooling, a fraction of these quarks and gluons randomly collided to form short-lived “X” particles, so named because of their mysterious and unknown structures. Today, X-ray particles are extremely rare, although physicists have speculated that they could be created in particle accelerators by coalescence of quarks, where high-energy collisions can generate similar flashes of quark plasma. -gluon.

Today, physicists at MIT’s Nuclear Science Laboratory and elsewhere found evidence of X-ray particles in quark-gluon plasma produced in the Large Hadron Collider (LHC) at CERN, the European Research Organization nuclear, based near Geneva, Switzerland.

The team used machine learning techniques to sift through more than 13 billion heavy ion collisions, each producing tens of thousands of charged particles. Amid this soup of ultra-dense, high-energy particles, the researchers were able to spot about 100 X particles, of a type known as X(3872), named after the particle’s estimated mass.

The results, published in Physical examination letters, it’s the first time researchers have detected X-ray particles in a quark-gluon plasma, an environment they hope will shed light on the particles’ yet-unknown structure.

“This is just the beginning of the story,” says lead author Yen-Jie Lee, a 1958 associate professor of career development physics at MIT. “We showed that we could 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 the type of material the universe should produce.

The study’s co-authors are members of the CMS Collaboration, an international team of scientists that operates and collects data from the Compact Muon Solenoid, one of the LHC’s particle detectors.

Particles in plasma

The building blocks of matter are the neutron and the proton, each of which is made up of three closely related quarks.

“For years we thought that for some reason nature chose to produce particles that were made up of only two or three quarks,” Lee explains.

Only recently have physicists started seeing signs of exotic ‘tetraquarks’ – particles made up of a rare combination of four quarks. Scientists suspect that X(3872) is either a compact tetraquark or an entirely new type of molecule made up not of atoms but of two loosely bound mesons – subatomic particles themselves made up of two quarks.

X (3872) was first discovered in 2003 by the Belle experiment, a particle collider in Japan that crushes high-energy electrons and positrons. In this environment, however, the rare particles decayed too quickly for scientists to examine their structure in detail. It has been hypothesized that X(3872) and other exotic particles may be better illuminated in the quark-gluon plasma.

“Theoretically speaking, there are so many quarks and gluons in the plasma that X-particle production should be improved,” Lee says. “But people thought it would be too difficult to search for them because there are so many other particles being produced in this soup of quarks.”

“Really a signal”

In their new study, Lee and his colleagues looked for signs of X-ray particles in the quark-gluon plasma generated by heavy ion collisions in CERN’s Large Hadron Collider. They based their analysis on the 2018 LHC dataset, which included more than 13 billion lead ion collisions, each releasing quarks and gluons that scattered and merged to form more than one quadrillion short-lived particles before they cool and decay.

“After the quark-gluon plasma forms and cools, there are so many particles produced that the background noise is overwhelming,” Lee says. “So we had to knock that background down to eventually be able to see the X particles in our data.”

To do this, the team used a machine learning algorithm they trained to identify the characteristic decay patterns of X-ray particles. Immediately after the particles form in the quark-gluon plasma, they rapidly decay into particles. “girls” who scatter. For X particles, this decay pattern, or angular distribution, is distinct from all other particles.

The researchers, led by MIT postdoc Jing Wang, identified key variables that describe the shape of the X-particle decay pattern. They trained a machine learning algorithm to recognize these variables, then fed the algorithm with real data from the LHC collision experiments. The algorithm was able to sift through the extremely dense and noisy dataset to select key variables that likely resulted from the decay of the X particles.

“We managed to reduce the background noise by orders of magnitude to see the signal,” says Wang.

The researchers zoomed in on the signals and observed a peak at a specific mass, indicating the presence of X particles (3872), about 100 in all.

“It’s almost unthinkable that we can disentangle these 100 particles from this huge dataset,” says Lee, who along with Wang performed several checks to verify their observation.

“Every night I wondered, is this really a signal or not?” remembers Wang. “And in the end, the data said yes!”

Over the next two years, the researchers plan to collect much more data, which should help elucidate the structure of the X particle. If the particle is a tightly bound tetraquark, it should decay more slowly than if it is was a weakly bound molecule. Now that the team has shown that X particles can be detected in quark-gluon plasma, they plan to probe this particle with quark-gluon plasma in more detail to determine the structure of the X particle.

“Currently, our data is consistent with both, because we don’t have enough statistics yet. Over the next few years, we will be taking a lot more data so we can separate these two scenarios,” says Lee. “It will broaden our view of the types of particles that were produced in abundance in the early universe.”

This research was funded, in part, by the US Department of Energy.