Scientists are looking at the structure of rare tetraquarks

The latest study offers significant progress in understanding tetraquarks, a rare and sophisticated type of particle.

By developing a brand new approach that mixes advanced mathematical methods with an easier model of particle interactions, scientists have made necessary discoveries about the internal structure and mass of these particles.

Their results are consistent with each experimental data and former predictions. Moreover, the team made its own predictions about tetraquarks, which haven’t yet been observed and can likely be tested in future experiments at the Large Hadron Collider (LHC) and other particle accelerators.

What are tetraquarks?

Tetraquarks are exotic particles composed of 4 quarks, which distinguishes them from conventional, strongly interacting particles, collectively called hadrons. Hadrons consist of a quark and an antiquark (called mesons) or three quarks (called baryons).

Tetraquarks, on the other hand, are far more complex and consist of a mix of 4 quarks or antiquarks. Because they are much less understood than other particles, each experimentally and theoretically, higher understanding them is especially necessary for advancing our knowledge of particle physics.

The first tetraquark discovered experimentally was Z_C(3,900), observed in 2013 in two independent experiments – the BESIII experiment in China and the Belle experiment in Japan. This particle consists of a charm quark (), an anti-charm quark (c), an up quark () and an anti-down quark (d). Z’s discovery_C(3900) confirmed earlier theoretical predictions about the existence of such particles, which triggered a wave of experimental research and led to the discovery of several other tetraquarks with different quark contents.

Although the discovery of tetraquarks has generated rather a lot of interest, understanding their true nature stays a challenge. Quantum chromodynamics, the theory that describes the interaction of quarks and gluons, is incredibly complex. For this reason, accurately predicting the properties of tetraquarks directly from this theory has proven difficult, resulting in the development of various approximate models that have to be verified with experimental data.

Study of the structure of the tetraquark

In a recent study published in , a team of researchers from China and Germany simplified the calculations required to find out the structure and properties of tetraquarks by ignoring the constant creation and annihilation of virtual particles in a vacuum.

This effect, which occurs at subatomic scales and might influence particle behavior, makes calculations far more complex. By ruling out this factor, researchers could give attention to the direct interactions between quarks and gluons, simplifying the model and making it easier to administer. Although this approach requires some precision, it produces helpful predictions that will be tested against experimental data.

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Scientists used this structure to check double-heavy open-flavor tetraquarks — a term referring to species of quarks comparable to “up,” “down,” “charm,” or “down” that are not paired with their corresponding antiquarks. These tetraquarks, consisting of two heavy quarks (restrained or down), were recently discovered experimentally. Despite this breakthrough, their properties and internal structure remain poorly understood, making them an interesting subject for further theoretical research.

“Three years ago, the LHCb experiment discovered the first open-flavor doubly heavy tetraquark called T_cc(3875)⁺” Shi-Lin Zhu, a professor of physics at Peking University and one of the study’s authors, said in an email. “This discovery is each significant and intriguing, as previously just one other doubly heavy state with an open flavor, the conventional Ξ baryon_ccobserved. The latest tetraquark state represents a very different structure and is especially exciting.”

Using their approach, the team examined multiple tetraquarks consisting of two heavy quarks (or ) and two light antiquarks (u, d or s). They calculated the masses, sizes and determined how the 4 quarks are arranged in these exotic hadrons.

In particular, they calculated the mass of T_cc(3875)⁺which consists of two c-quarks, one anti-d and one anti-u. Their result was in close agreement with the experimentally measured value. Additionally, they determined the internal structure of the particle and located that it resembles a particle composed of two loosely coupled mesons, each composed of one charm quark and one light antiquark, consistent with experimental data.

“Generally speaking, tetraquark states are divided into meson particles and compact tetraquark states. These two different configurations reveal different internal structures and binding mechanisms for these exotic states,” Zhu explained. “Moreover, compact tetraquark states have three types of interesting spatial configurations. The so-called “compact, even tetraquark” is an analogue of the hydrogen molecule. “A ‘compact tetraquark concentrated in a diquark’ is an analogue of the helium atom.”

This diversity of structures is attributable to the larger number of quarks in tetraquarks in comparison with mesons and baryons, enabling deeper exploration of the subtle properties of the strong interaction. This diversity provides helpful insight into the complexity of the strong force and allows scientists to check its behavior in additional detail.

Future predictions

Most of the tetraquarks analyzed in the study haven’t yet been discovered experimentally, but researchers are optimistic about the near future. They consider that advances in experimental technology will enable the detection of these predicted particles, enabling further testing and validation of their model.

“We predicted the existence of T_B.C. and T_{bed and breakfast} states that are cousins ​​of the doubly enchanted tetraquark state T_cc(3875)⁺ discovered by the LHCb collaboration in 2021, but with one or two c quarks replaced by b quarks,” Zhu said. “These particles will likely be observed in the LHCb experiment at the Large Hadron Collider. RUN 3 – a new data collection period at the LHC – began in July 2022 and will last four years. With significant improvements, the LHCb experiment’s data collection rate is expected to increase tenfold. This increased capacity will greatly facilitate the potential discovery of T_{bed and breakfast} and T_B.C. tetraquark states.”

Beyond tetraquarks, scientists consider their theoretical framework will be applied to much more exotic particles consisting of 5 – 6 quarks. This could open latest frontiers in our understanding of strongly interacting particles and the fundamental forces acting on them.

“The current theoretical framework can be extended to various multiquark systems. In fact, we applied them to two other tetraquark systems and successfully described experimentally observed T states_cs(2870)^*X(6900) and X(7200) in a unified framework,” Zhu concluded. “In the future, we will use this framework to study pentaquark systems consisting of five quarks and even six-quark systems consisting of six quarks. We hope that our theoretical research will help promote experimental discoveries and understanding the nature of multiquark states.”