Physicists from the CMS (Compact Muon Solenoid) Collaboration at CERN’s Large Hadron Collider have detected about 100 particles of a type known as X(3872) in quark-gluon plasma, an environment that they hope will illuminate the particles’ as-yet unknown structure.
Sirunyan et al. found evidence of X(3872) particles in the quark-gluon plasma produced in CERN’s Large Hadron Collider. Image credit: Pete Linforth.
“The basic building blocks of matter are the neutron and the proton, each of which are made from three tightly bound quarks,” said MIT’s Dr. Yen-Jie Lee.
“For years we had thought that for some reason, nature had chosen to produce particles made only from two or three quarks.”
“Only recently have we begun to see signs of exotic tetraquarks — particles made from a rare combination of four quarks.”
“We suspect that X(3872) is either a compact tetraquark or an entirely new kind of molecule made from not atoms but two loosely bound mesons — subatomic particles that themselves are made from two quarks.”
X(3872) was first discovered in 2003 by the Belle experiment, a particle collider in Japan that smashes together high-energy electrons and positrons.
Within this environment, however, the rare particles decayed too quickly for physicists to examine their structure in detail.
It has been hypothesized that X(3872) and other exotic particles might be better illuminated in quark-gluon plasma.
“Theoretically speaking, there are so many quarks and gluons in the plasma that the production of X particles should be enhanced,” Dr. Lee said.
“But people thought it would be too difficult to search for them because there are so many other particles produced in this quark soup.”
In the study, Dr. Lee and colleagues looked for signs of X particles within the quark-gluon plasma generated by heavy-ion collisions in the Large Hadron Collider (LHC).
“This is just the start of the story. We’ve shown we can find a signal. In the next few years 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,” Dr. Lee said.
The CMS researchers based their analysis on the LHC’s 2018 dataset, which included more than 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.
“After the quark-gluon plasma forms and cools down, there are so many particles produced, the background is overwhelming,” Dr. Lee said.
“So we had to beat down this background so that we could eventually see the X particles in our data.”
To do this, the team used a machine-learning algorithm which they trained to pick out decay patterns characteristic of X particles.
Immediately after particles form in quark-gluon plasma, they quickly break down into ‘daughter’ particles that scatter away. For X particles, this decay pattern, or angular distribution, is distinct from all other particles.
The scientists 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 actual data from the LHC’s collision experiments.
The algorithm was able to sift through the extremely dense and noisy dataset to pick out the key variables that were likely a result of decaying X particles.
“We managed to lower the background by orders of magnitude to see the signal,” said Dr. Jing Wang, a postdoctoral researcher at MIT.
The CMS team zoomed in on the signals and observed a peak at a specific mass, indicating the presence of X(3872) particles, about 100 in all.
“It’s almost unthinkable that we can tease out these 100 particles from this huge dataset,” Dr. Lee said.
“Every night I would ask myself, is this really a signal or not? And in the end, the data said yes!” Dr. Wang added.
The team’s work was published in the journal Physical Review Letters.
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A. M. Sirunyan et al. (CMS Collaboration). 2022. Evidence for X(3872) in Pb-Pb Collisions and Studies of its Prompt Production at √sNN=5.02 TeV. Phys. Rev. Lett 128 (3): 032001; doi: 10.1103/PhysRevLett.128.032001
