The LHCb collaboration at CERN has announced the discovery of a new exotic particle: a so-called “tetraquark”. The paper of more than 800 authors has yet to be evaluated by other scientists in a process called “peer review”, but was presented at a seminar. It also meets the usual statistical threshold for the discovery of a new particle.
The finding marks a major breakthrough in a nearly 20 year search that was conducted in particle physics laboratories around the world.
To understand what a tetraquark is and why discovery is important, we have to go back to 1964, when particle physics was in the middle of a revolution. Beatlemania had just exploded, the Vietnam War was raging, and two young radio astronomers in New Jersey had just discovered the strongest evidence of the Big Bang theory ever.
On the other side of the USA, at the California Institute of Technology and on the other side of the Atlantic, at CERN in Switzerland, two particle physicists published two independent papers on the same subject. Both were about understanding the enormous number of new particles discovered in the past two decades.
Many physicists had trouble accepting that so many elementary particles could exist in the universe, in the so-called “particle zoo”
We now know that there are six different types of quarks – top, bottom, charm, strange, top, bottom. These particles also have corresponding antimatter companions with opposite charge, which can bind together according to simple rules based on symmetries. A particle consisting of a quark and an antiquark is called a “meson”. while three interconnected quarks form “baryons”. The known protons and neutrons that make up the atomic nucleus are examples of baryons.
This classification scheme very nicely described the particle zoo of the 1960s. However, Gell-Mann already recognized in his original work that other combinations of quarks could be possible. For example, two quarks and two antiquarks could hold together to form a “tetraquark”, while four quarks and one antiquark would form a “pentaquark”.
Fast forward to 2003, when the Belle experiment in the KEK laboratory in Japan reported the observation of a new meson called X (3872), which showed “exotic” properties that differ significantly from ordinary mesons.
Several new exotic particles were discovered in the years that followed, and physicists began to realize that most of these particles could only be explained successfully if they were tetraquarks made up of four instead of two quarks. In 2015, the LHCb experiment at CERN discovered the first pentaquark particles from five quarks.
All tetraquarks and pentaquarks discovered so far contain two relatively heavy charm quarks and two or three light quarks – top, bottom or strange. Indeed, this particular configuration is the easiest to discover in experiments.
The latest tetraquark discovered by LHCb, designated X (6900), consists of four charm quarks. The new tetraquark was generated during high-energy proton collisions at the Large Hadron Collider and observed through its decay into pairs of known particles, so-called J / psi mesons, each consisting of a charm quark and a charm antiquark. This makes it particularly interesting because it is not only made up entirely of heavy quarks, but also four quarks of the same type. This makes it a unique specimen to test our understanding of how quarks are connected.
At the moment there are two different models that could explain how quarks are connected: It could be that they are strongly bound, which creates a so-called compact tetraquark. Or it could be that the quarks are arranged so that they form two mesons that stick together loosely in a “molecule”.
Ordinary molecules consist of atoms that are connected by the electromagnetic force that acts between positively charged nuclei and negatively charged electrons. But the quarks in a meson or baryon are connected by another force, the “strong force”. It is really fascinating that atoms and quarks can form very similar complex objects according to very different rules.
The new particle appears to match a compact tetraquark rather than a two-meson molecule, which was the best explanation for previous discoveries. This makes it unusual because physicists can study this new binding mechanism in detail. This also implies the existence of other heavily compact tetraquarks.
Window in the microcosm
The strong force that acts between quarks follows very complicated rules – so complicated that the only way to calculate their effects is usually using approximations and supercomputers.
The uniqueness of the X (6900) will help to understand how the accuracy of these approximations can be improved so that in the future we can describe other, more complex mechanisms in physics that we cannot achieve today.
Since the discovery of the X (3872), the study of exotic particles has been successful, and hundreds of theoretical and experimental physicists have worked together to shed light on this exciting new field. The discovery of the new tetraquark is a great step forward and indicates that there are still many new exotic particles waiting for someone to reveal them.