First Detection of Exotic ‘X’ Particles in Quark-Gluon Plasma
In the initial millionths of a second after the Big Bang, our universe was an agitated pull of subatomic particles, trillion-degree plasma of quarks and gluons– elementary particles that were briefly interacting with each other on countless combinations before cooling down and settling into more stable configurations to make the neutrons and also protons of the matter we know today.
In the turmoil prior to cooling down, a fraction of these quarks and gluons collided randomly to form short-term “X” particles, so named for their enigmatic, unknown structures. Today, X particles are exceptionally rare, though physicists have actually theorized that they may be developed in particle accelerators with quark coalescence, where high-energy collisions can create comparable flashes of quark-gluon plasma.
Now physicists at MIT’s Laboratory for Nuclear Science and somewhere else have discovered proof of X particles in the quark-gluon plasma created in the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, based near Geneva, Switzerland.
A bright beginning
The group utilized machine-learning techniques to sort through more than 13 billion heavy-ion collisions, each of which generated tens of thousands of charged particles. Amid this ultra-dense, high-energy particle soup, the researchers were able to tease out approximately 100 X particles, of a type referred to as X (3872 ), named for the particle’s approximated mass.
The outcomes, published today in Physical Review Letters, mark the first time scientists have found X particles in quark-gluon plasma– an atmosphere that they hope will illuminate the particles’ as-yet-unknown structure.
” This is just the start of the story,” states lead author Yen-Jie Lee, the Class of 1958 Career Development Associate Professor of Physics at MIT. “We’ve shown we can find a signal. In the next few years, we wish to utilize 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 create.”
The research’s co-authors are members of the CMS Collaboration, an international team of scientists that runs and collects data from the Compact Muon Solenoid, one of the LHC’s particle detectors.
Particles in the plasma
The basic building blocks of matter are the neutron and also proton. Each of them is made from three firmly bound quarks.
” For years we had thought that for some reason, nature had chosen to produce particles made only from two or three quarks,” Lee states.
Just recently have physicists started to see signs of exotic “tetraquarks”– particles made from a rare combination of four quarks. Scientists believe that X (3872) is either a compact tetraquark or a completely new sort of molecule made from not atoms but two freely bound mesons– subatomic particles that themselves are made from 2 quarks.
X (3872) was very first found in 2003 by the Belle experiment, a particle collider in Japan that collides high-energy electrons and positrons. Within this environment, nonetheless, the rare particles decayed too rapidly for researchers to analyze their structure in detail. It has actually been hypothesized that X (3872) and also other exotic particles may be much better illuminated in quark-gluon plasma.
” In theory, there are numerous quarks and gluons in the plasma that the manufacturing of X particles need to be enhanced,” Lee states. “However, people thought it would be too hard to look for them because there are so many other particles created in this quark soup.”
‘Really a signal’
In their brand-new research study, Lee and his colleagues searched for signs of X particles within the quark-gluon plasma created by heavy-ion collisions in CERN’s Large Hadron Collider. They based their evaluation on the LHC’s 2018 dataset, that included over 13 billion lead-ion collisions, each of which launched quarks and gluons that spread and merged to create more than a quadrillion short-lived particles before cooling down and decaying.
” After the quark-gluon plasma forms and cools down, there are so many particles produced, the background is staggering,” Lee claims. “So we had to beat down this background to ensure that we could eventually see the X particles in our data.”
To do this, the group utilized a machine-learning algorithm which they trained to pick out degeneration patterns characteristics of X particles. Right after particles form in quark-gluon plasma, they quickly decompose into “daughter” particles that spread away. For X particles, this decay pattern, or angular circulation, stands out from all various other particles.
The scientists, led by MIT postdoc Jing Wang, identified key variables that detail the shape of the X particle decay pattern. They trained a machine-learning algorithm to acknowledge these variables. Afterward, they fed the formula actual data from the LHC’s collision experiments. The algorithm had the ability to sift with the incredibly dense and noisy dataset to select the crucial variables that were likely a result of decaying X particles.
” We managed to reduce the background by orders of magnitude to see the signal,” states Wang.
The scientists zoomed in on the signals and they observed a peak at a particular mass, indicating the presence of X (3872) particles, about 100 in all.
” It’s practically unimaginable that we can tease out these 100 particles from this huge dataset,” claims Lee, who together with Wang ran several checks to verify their observation.
” Every evening I would ask myself, is this actually a signal or not?” Wang recalls. “In the end, the data said yes!”
In the next year or two, the researchers intend to collect much more data, which should help to elucidate the X particle’s structure. If the particle is a securely bound tetraquark, it must decay much more gradually than if it were a loosely bound molecule. Since the team revealed that X particles can be identified in quark-gluon plasma, they prepare to probe this particle with quark-gluon plasma in much more detail, to determine the X particle’s structure.
” Presently our data follows both because we do not have an enough statistics yet. In the following few years we’ll take far more data so we can separate these 2 scenarios,” Lee claims. “That will expand our view of the kinds of particles that were created generously in the early universe.”
Read the original article on Scitech Daily.
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Reference: “Evidence for X(3872) in Pb-Pb Collisions and Studies of its Prompt Production at vsNN=5.02 TeV” by A. M. Sirunyan et al. (CMS Collaboration), 22 December 2021, Physical Review Letters.
DOI: 10.1103/PhysRevLett.128.032001
