Transparent View on Nucleons inside Atomic Nuclei

Strongly interacting many-body quantum systems are difficult to investigate experimentally. Such a system in nature is the atomic nucleus with its constituents, the nucleons, bound together by the strong force. Exploring the properties of this interaction among nucleons in the dense nuclear medium poses a particularly difficult challenge. High-energy proton beams with short wave length, for instance, are an often used probe by shooting them on a target containing the atomic nuclei of interest. Due to the strong interaction, however, the resulting picture is blurred by complex multiple interactions.

Resting protons under fire

Here, the scientists of the collaboration set in. For the measurements published in “Nature Physics”, they quasi reversed the principle of the experimental apparatus – they apply inverse kinematics. Instead of shooting a proton beam on atomic nuclei, the carbon nuclei to be explored were shot onto protons at rest. That is how they succeeded to demonstrate that a selection of the final state is possible that allows to exclude more complex interactions – provided inverse kinematics is used. This enables the investigation of the nuclear forces in the atomic nucleus without perturbation caused by other physical processes.

Key of the experiment described in “Nature Physics” was inverting the kinematics, where the nuclei of interest are accelerated and directed as a high-energy beam (here approximately 3 GeV/nucleon 12C beam) towards protons at rest (provided as a liquid-hydrogen target). This enables besides the detection of the scattered nucleons, as well as the measurement and identification of the final state of the atomic nucleus after the scattering process, which is not feasible in normal kinematics. The analysis of the experiment has demonstrated that a proper selection of the final state allows the identification of direct one-step scattering processes and with that the extraction of information on the properties of the nucleons in the ground state of the atomic nucleus.

Thanks to this selectivity, the scientists succeeded already in this first experiment to gain new insight into the short-distance component of the nucleon-nucleon interaction. The properties of the nucleon-nucleon and more-nucleon interactions at short distances is thereby of particular importance for the understanding of the properties of neutron-rich nuclear matter at high density, as present as neutron stars in our universe.

Dr. Meytal Duer, scientist at the Institute for Nuclear Physics (AG Aumann) at the TU Darmstadt, and responsible for a significant fraction of the data analysis of the experiment published in “Nature Physics”, explains: “This experimental break-through paves the way to explore the properties of the short-range nucleon-nucleon interaction in neutron-rich short-lived nuclei in detail.” Such nuclei could be produced as intense beams at the new accelerator facility FAIR in Darmstadt, says Duer. The collaboration would plan already for the next year a first experiment with an unstable nucleus at the R3B facility (Reactions with Relativistic Radioactive Beams) at the GSI Helmholtz Center for Heavy-Ion Research in Darmstadt.

Aumann/sip