Mysterious rare particles
In a millionth of a second after the Big Bang, the universe was a turbulent, chaotic, hot quark and gluon plasma. Later, as the universe expanded and cooled, the quarks began to combine with each other to form a composite particle called a hadron. Hadrons include mesons composed of one quark and one antiquark, as well as baryons combined by three quarks, and the protons and neutrons we are familiar with are baryons.
Ordinary hadrons. | Image design: Wen Wenzi
However, during the chaotic period before cooling, that is, before protons and neutrons are formed, these elementary particles will briefly stick together in countless combinations. Some of these quarks and gluons collide randomly, forming short-lived "X" particles. As its name suggests, these particles have mysterious, hitherto unknown structures.
Now, an international team of physicists has found evidence of the existence of an X particle using quark gluon plasma data generated by cern's (CERN) Large Hadron Collider (LHC). They published the findings in a recent Physical Review Letters, marking the first time scientists have detected the X particle in a quark gluon plasma.
Unknown structure of X (3872).
Neutrons and protons are the basic components of ordinary matter, and they are all made up of three tightly bound quarks. For a long time, physicists believed that, for some reason, there were only particles formed by two or three quarks in nature.
In the 1960s, however, physicist Murray Gell-Mann realized that quarks might be combined in other ways. For example, two quarks and two antiquarks may stick together to form a tetraquark state, while four quarks and one antiquark can stick together to form a pentaquark state.
In recent years, physicists have begun to spot traces of bizarre tetraquark states. In 2003, physicists at the KEK Laboratory in Japan discovered a new particle called X (3872) while conducting the Belle experiment. Named for their estimated mass, the particles exhibit singular properties that are completely different from those of ordinary mesons.
The Belle experiment is a particle collision experiment that collides high-energy electrons with positrons. In this environment, these rare X particles decay so rapidly that scientists simply don't have time to study their structure in detail. So to this day, scientists still don't know exactly the internal structure of X (3872), and they suspect that X (3872) is either a compact tetraquark or an entirely new molecule that is not made up of atoms, but of two loosely bound mesons.
There are two possible compositions of the tetraquark state. | Image design: Wen Wenzi
It has been suggested that perhaps in the quark-gluon plasma environment, the mysteries of X (3872) and other exotic particles could be better revealed. Because theoretically, the large number of quarks and gluons contained in the plasma can enhance the production of X particles, but the reality is that there will be a large number of other particles in such a quark soup. Therefore, it will be an extremely difficult task to find X particles through this method.
Machine learning algorithms sift through massive amounts of data
In the new study, the team tried to look for signs of X particles by searching for data on quark gluon plasma from the LHC's heavy ion collisions. They analyzed a dataset generated by the LHC in 2018 that covered more than 13 billion collisions of lead ions, each of which released quarks and gluons. Before cooling and decaying, these quarks and gluons scatter and merge to form more than 10 short-lived particles.
After the quark-gluon plasma is formed and cooled, each such collision generates a large number of particles, which means that the background noise required to find X particles in such an ultra-dense soup of high-energy particles is overwhelming. Researchers must try to crack down on this problem before it is possible to finally see the X particle in the data.
To do this, they used a machine learning algorithm that attempts to identify the decay pattern characteristics of X particles by training the algorithm.
After the particles form in the quark-gluon plasma, they immediately break down into "sub" particles and then disperse. Among these particles, the decay pattern, or angular distribution, of the X particle is different from all other particles. The researchers identified the key variables that describe the shape of the X particle decay pattern, then trained a machine learning algorithm to identify those variables, and then fed the actual data generated by the LHC collision experiment into the algorithm. The algorithm is able to sift through extremely dense and noisy data sets to pick out key variables that may represent the decay of X particles.
Eventually, the researchers succeeded in reducing the background noise by several orders of magnitude to see the signal. They amplified the signal and observed a peak at a specific mass location, indicating the presence of X (3872) particles, which numbered about 100 in total.
The story has only just begun
Tinging out 100 particles from such a huge data set is almost an unimaginable challenge. The researchers examined their results several times to make sure they actually observed such a signal. Now, they have successfully confirmed that X particles can be detected in quark gluon plasmas.
Still, physicists say that will only be the beginning of the story. While it has been shown that they can look for X particles in this way, in the next few years, they hope to use quark-gluon plasma, which will help them probe X particles in more detail to determine the internal structure of X particles.
Are X particles tightly bound tetraquarks or loosely bound molecules? At present, the available data support for both possibilities is equivalent, and only the continued collection of more data will make it possible to distinguish between the two cases. By then, we may have a new understanding of the mass-produced particles that arose in the early universe.
#创作团队:
Author: Light rain
Typography: Wenwen
#参考来源:
https://news.mit.edu/2022/x-particles-quark-gluon-plasma-detection-0121
https://theconversation.com/cern-physicists-report-the-discovery-of-unique-new-particle-142315
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.032001
#图片来源:
Cover image: geralt/Pixabay
First image: CERN