Cern discovered the predicted new particle, the "tetraquark," consisting of two quarks and two antiquark bindings.
CERN's LHCB institute announced the discovery of a new exotic particle: the so-called "tetraquark." The paper was written by more than 800 authors and has not yet been evaluated in a "peer review" by other scientists, but has been published in a symposium. At the symposium, the scientists also claimed common statistical thresholds for discovering new particles.
The discovery marks a major breakthrough in nearly 20 years of research conducted in particle physics laboratories around the world.
To understand what tetraquarks are, and why this discovery is so important, we need to go back to 1964, when particle physics was in the midst of a revolution. The Beatles had just exploded, the Vietnam War was raging, and two young radio astronomers in New Jersey had just discovered the strongest evidence ever for the Big Bang theory.
Two-particle physicists at the California Institute of Technology in the southwest and CERN in Switzerland on the other side of the Atlantic have published two separate papers on the same topic. Both are about understanding the vast array of new particles discovered over the past two decades.
Many physicists have a hard time accepting that there might be so many elementary particles in the universe, the so-called "particle zoo." George Zweig from CERN and Murray Gell-Mann from Caltech found the same solution. What if all these different particles were really made up of smaller, unknown basal blocks, like more than a hundred elements in the periodic table are made up of protons, neutrons, and electrons? Zweig called these base blocks "aces," while Gell-Mann chose the term we still use today: "quark."
We now know that there are six different quarks — upper quark, lower quark, cannabis quark, odd quark, top quark, and bottom quark. These particles also have their own counterparts of antimatter with opposite charges, which can be combined according to simple rules based on symmetry. Particles made up of quarks and antiquarks are called "mesons"; and three quarks combined together form "baryons". The familiar protons and neutrons that make up the nucleus of an atom are a type of baryon.
This classification scheme perfectly describes the particle zoos of the 1960s. However, even in Gell-Mann's original paper, he realized that other combinations of quarks were possible. For example, two quarks and two antiquarks may stick together to form "four quarks," while four quarks and one antiquark form a "five quark."
Particles with exotic properties
Fast forward to 2003, a belle experiment at Kek Labs in Japan reported an observation of a new meson called x(3872), which showed "singular" properties that were distinct from ordinary mesons.
Over the next few years, physicists discovered several new exotic particles, and they began to realize that they could only be explained if they were made up of four quarks instead of two. Then, in 2015, cernion's LHCB experiment discovered the first pentaquark particle composed of five quarks.
All tetraquarks and five quarks found to date contain two relatively heavy cannabis quarks and two to three lighter upper, lower quarks, or odd quarks. This particular combination is indeed the easiest to discover in experiments.
But LHCB's recently discovered tetraquark consists of four cannabis quarks, also known as x(6900). After observation, tetraquarks are produced by the collision of high-energy protons of the Large Hadron Collider, after which they decay into known meson particles j/psi, each composed of cannabis quarks and canon counter quarks. What's intriguing about tetraquarks is that it's made up not just entirely of heavy quarks, but also four of the same type of quarks — a unique sample that helps confirm how we understand quarks combined.
LHCB Explorer Bryce. j. Oudan, CERN
Currently, there are two different models that explain how quarks fit together: they may be strongly bound, combined to become what we call compact tetraquarks. Or it could be that the quarks are arranged into two mesons that are loosely glued to a "molecule."
Ordinary molecules are made up of atoms bound together by an electromagnetic force acting between a positively charged nucleus and a negatively charged electron. But the quarks in mesons or baryons are connected by a different force, the "strong force." Atoms and quarks, although they follow very different rules, can form very similar complex objects, which is really interesting.
Based on previous research, the new particles appear to be more in tune with dense tetraquarks than bimeson molecules. So it's different, and physicists have studied this new binding mechanism in detail. This also implies the presence of other heavy dense tetraquarks.
Enter the window into the microscopic universe
The strong action between quarks follows very complex rules — so complex that only approximations and supercomputers can be used to calculate their effects.
The unique properties of x(6900) will help to understand how to improve the accuracy of these approximations so that in the future we will be able to describe other more complex physical mechanisms that we cannot achieve today.
Since the discovery of X (3872), research on exotic particles has flourished, and hundreds of theoretical and experimental physicists have worked together to provide some clues to this exciting new field. The discovery of the New Tetraquark is a huge leap forward, showing that there are still many new exotic particles waiting for someone to unveil them.
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