Heavy dibaryon baffles physicists

Exotic effect: according to current theory, pairs of two omega baryons – particles made up of three heavy quarks each – are only weakly bound to one another. However, simulations now suggest that this is not the case for one of these exotic binary particles: Baryon pairs with three bottom quarks each have a far higher binding energy than expected. This sheds new light on the action of the strong nuclear force in such combi-particles and also on the nature of exotic hexaquarks.

All atomic nuclei consist of baryons – particles made up of three quarks each, such as the nuclear building blocks proton and neutron. Understanding the interactions of such baryons is therefore of fundamental importance for particle physics, the understanding of the strong nuclear force, but also for cosmology. The easiest way to research these interactions is offered by dibaryons – pairs of two such quark triple combinations.

On the trail of baryon interactions

The problem, however, is that the only stable dibaryon is the deuteron – the nucleus of the heavy hydrogen isotope deuterium with one proton and one neutron. “Based on the theory of the strong nuclear force, however, there should be more dibaryons in nature, especially those made of heavy quarks and strange quarks,” explain Nilmani Mathur from the Tata Institute for Basic Research in Mumbai and his colleagues.

However, because these heavy dibaryons only exist for fractions of a second, they cannot yet be investigated experimentally. Physicists are therefore trying to track down their characteristics using simulations within the framework of so-called lattice gauge theories. On the basis of quantum chromodynamics in supercomputers, it is understood how the baryons interact with each other on the quantum field level.

Double pack of three bottom quarks each

So-called omega dibaryons – pairs of two baryons each with three identical heavy quarks – bottom or strange quarks, are considered particularly well suited for such basic tests. First theoretical models suggest that such pairs would have to repel each other and therefore can exist only very weakly bound or even not bound at all.

“However, there were also some lattice studies on heavy tetraquarks and dibaryons, according to which such systems can definitely be strongly bound,” explain Mathur and his colleagues. For their study, the physicists therefore carried out the first lattice gauge simulation with omega baryons each consisting of three bottom quarks (D _6b ). It is the first study of this kind on a dibaryon with six identical heavy quarks.

Stronger bond than expected

The result: “We find clear evidence of a strongly bound state with a binding energy of around 81 megaelectronvolts for D _6b ,” the researchers report. The dibaryon from the two heavy bottom triples is thus even 40 times more strongly bound than the two baryons in the deuteron. The results thus suggest that, contrary to expectations, dibaryons can be strongly bound.

“An interesting pattern emerges from this and other studies, according to which the presence of one or more bottom quarks strengthens bonding in such systems,” write Matur and his colleagues. According to the models, interactions of baryons from strange or charm quarks are significantly weaker. “The omega dibaryon with six bottom quarks could even be the most strongly bound heavy dibaryon in the entire universe,” the physicists state.

Insight also into exotic hexaquarks

These results could also provide insight into the nature of hexaquarks – particles composed of six quarks – detected in particle accelerators. It is not yet clear whether these hexaquarks are firmly connected dibaryons or more exotic structures made up of six quarks in one particle. It would be correspondingly exciting and important to observe and prove heavy dibaryons in reality.

“Our results for this heavy dibaryon now provide additional motivation to search for such particles,” say the researchers. “Even if the direct identification of D _6b will take some time, the detection of the double bottom baryons would be an important step to close the gaps in the cascade of hadronic reactions.” (Physical Review Letters, 2023; doi: 10.1103 / PhysRevLett.130.111901 )