First detection of exotic 'X' particles in quark-gluon plasma

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In the first millionths of a second after the Big bang, the universe was a roiling, trillion-degree plasma of quarks and gluons that briefly coalesced together in countless combinations before cooling and settling into more stable configurations.

The short-lived "X" particles were formed when a fraction of the quarks and gluons collided. Physicists theorize that X particles may be created in particle accelerators through quark coalescence, where high-energy collisions can generate similar flashes of quark-gluon plasma.

Physicists at MIT's Laboratory for Nuclear Science and elsewhere have found evidence of X particles in the quark-gluon plasma produced in the Large Hadron collider.

The team used machine-learning techniques to sift through 13 billion heavy ion collisions, each of which produced tens of thousands of charged particles. The researchers were able to tease out about 100 X particles, of a type known as X (3872), named for the particle's estimated mass.

The first detection of X particles in quark-gluon plasma was published this week in Physical Review Letters.

"This is just the beginning of the story," says lead author Yen-Jie Lee, the Class of 1958 Career Development Associate Professor of Physics at MIT. We have shown we can find a signal. 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 members of the CMS Collaboration, an international team of scientists that operate and collect data from the Compact Muon Solenoid, one of the LHC's particle detectors.

There are particles in the air.

The two basic building blocks of matter are the neutron and the protons.

For a long time, we thought nature had chosen to make only two or three quarks.

Physicists are starting to see signs of exotic "tetraquarks", particles made from a rare combination of four quarks. Scientists suspect that X is a new kind of molecule made from not atoms but two bound mesons, which are made from two quarks.

The first discovery of X was made in 2003 by the Belle experiment, a particle collider in Japan that smashes together high-energy electrons and positrons. Scientists were not able to examine the structure of the rare particles within this environment. Exotic particles might be better illuminated in quark-gluon plasma.

The production of X particles should be enhanced because there are so many quarks and gluons. People thought it would be hard to find them because there are so many other particles in the quark soup.

It's a signal.

Lee and his colleagues looked for signs of X particles within the quark-gluon plasma generated by heavy-ion collisions. Their analysis was based on the 13 billion lead-ion collisions, each of which released quarks and gluons that scattered and merged to form more than a quadrillion short-lived particles before cooling and decaying.

The background is overwhelming after the quark-gluon plasma forms and cools down. We had to beat down the background so that we could see the particles.

The team trained a machine-learning algorithm to pick out decay patterns of X particles. After particles form in quark-gluon, they break down into daughter particles that scatter. The decay pattern for X particles is different from the other particles.

Key variables that describe the shape of the X particle decay pattern were identified by the researchers. They fed the data from the collision experiments to the machine-learning program. The key variables that were likely a result of decaying X particles were picked out by the algorithm.

Wang says that they lowered the background by orders of magnitude to see the signal.

The researchers observed a peak at a specific mass, indicating the presence of X particles, about 100 in all.

"It's almost unthinkable that we can tease out these 100 particles from this huge dataset," says Lee, who along with Wang ran multiple checks to verify their observation.

I would ask myself if this was really a signal. Wang remembers. The data said yes in the end.

The X particle's structure should be better understood in the next year or two as the researchers gather more data. The particle should decay more slowly if it is tightly bound. The X particle's structure will be pinned down by the team after they probe this particle with quark-gluon plasma.

We don't have enough statistics yet so our data is consistent with both. In the next few years, we will take more data so we can separate the two scenarios. "That will help us understand the types of particles that were produced in the early universe."

Evidence for X(3872) in Pb-Pb Collisions and Studies of its Prompt Production can be found in the Physical Review Letters. There is a book titled "PhysRevLett.128.032001."

Journal information: Physical Review Letters.

The first detection of exotic 'X' particles in quark-gluon plasma was retrieved on January 21, 2022.

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