International research team achieves most precise measurement of hypertriton binding energy to date / New insights into the strong nuclear forces that hold matter together

An international research team of the A1 Collaboration at the Mainz Microtron (MAMI) of Johannes Gutenberg University Mainz (JGU) has succeeded in determining the binding energy of the hypertriton with unprecedented precision. This experiment provides crucial new insights into the interaction between hyperons and nucleons – an aspect of the strong nuclear force that has so far remained insufficiently understood. The results show that the hypertriton is significantly more strongly bound than many earlier experiments suggested. The renowned scientific journal Physical Review Letters has recently published the study.

Exotic nuclei as a key to understanding fundamental forces

The hypertriton is the lightest known hypernucleus. It is an artificially produced hydrogen isotope that, in addition to a proton and a neutron, contains a so-called Lambda hyperon. Although hypernuclei exist for only a few hundred trillionths of a second, they provide unique insights into the strong interaction – the fundamental force that binds atomic nuclei and underlies the structure of matter in the universe. The hypertriton plays a key role in this context: consisting of only three particles, it is ideally suited for precise tests of theoretical models of the hyperon-nucleon interaction.

"Precisely because the hypertriton has such a simple structure, its properties are highly sensitive to the underlying nuclear forces," explained Prof. Dr. Patrick Achenbach from the Institute for Nuclear Physics at JGU. "Our new measurement clearly shows that this interaction is stronger than long assumed – an important step toward resolving a puzzle that has persisted for many years."

Research on light hypernuclei has engaged physicists for more than a decade, because experimental data and theoretical predictions differ significantly in some cases.

Mainz infrastructure as a driving force in hypernuclear research

To address these open questions in a targeted manner, a comprehensive hypernuclear research program has been carried out at MAMI. At its core is a high-resolution three-spectrometer setup, complemented by a fourth spectrometer developed specifically for hypernuclear experiments. This unique combination enables a level of measurement precision that sets international benchmarks.

Earlier experiments at MAMI had already shown that the masses of hyperhydrogen-4 and hyperhelium-4 differ unexpectedly strongly – an indication of nuclear forces that are not yet fully understood. For the 2022 performed and newly published measurement of the hypertriton, the experimental setup was further optimized, including the use of a newly developed lithium target, that is hit by the electron beam. It has a very unusual long and thin geometry to provide minimum energy losses for the outgoing particles in direction of the high-resolution spectrometers.

In this experiment, the energy of the pion produced in the decay of the hypertriton was determined with high precision. This measurement is the crucial factor that allows the binding energy of the hypernucleus to be determined accurately. By directly comparing with the decay of the already very precisely measured hyperhydrogen-4, the experiment could be calibrated with exceptional precision. The data analysis was carried out in close collaboration with Japanese partners, in particular within the framework of the doctoral research of Dr. Ryoko Kino from Tohoku University, who has received multiple awards for her work.

Classification of the results and international relevance

The new study ranks among the leading results of major international experiments such as ALICE at CERN (Geneva, Switzerland) and STAR at the RHIC accelerator (Long Island, USA). The measured binding energy lies above values reported in some earlier emulsion and heavy-ion experiments, but is in good agreement with the most recent STAR data. This points to a stronger interaction between the Lambda hyperon and the remaining hydrogen nucleus than previously assumed.

The results place new constraints on theoretical models of the strong interaction and also influence discussions of exotic systems such as a hypothetical Lambda-neutron-neutron nucleus. At the same time, they make a significant contribution to resolving the long-standing "hypertriton puzzle", which arose from contradictory earlier measurements.

The puzzle of hyperhydrogen

The visible universe consists predominantly of hydrogen, the lightest and simplest element in the periodic table. The nuclei of stable hydrogen atoms contain either a single proton or a proton-neutron pair. If, instead of an additional neutron, an exotic nuclear constituent such as a hyperon is added, so-called hyperhydrogen nuclei are formed – fascinating, short-lived systems that are still not fully understood.

The unique infrastructure at MAMI has enabled major advances in the study of these systems and provides new impulses for understanding the fundamental forces in atomic nuclei. This includes the hypernuclear database hypernuclei.kph.uni-mainz.de, operated by the Mainz research group, which serves worldwide as a reference for comparing hypernuclear measurements.

The current study was funded by the German Research Foundation (DFG) within the framework of the project "Precision measurement of light hypernuclei masses".