Nucleons are social particles. Not only do they enjoy living in communities inside nuclei, but they also form couples within these communities. Indeed, one can observe protons and neutrons forming pairs inside nuclei. DPhN physicists have played a decisive role in the first measurement of such pairs of nucleons using a new method, that will pave the way to the study of these close (or short range) interactions in radioactive nuclei. The results have recently been published in Nature Physics [Pat21]. The study of these nucleon pairs in radioactive nuclei is the goal of the ANR project COCOTIER led by IRFU.
Understanding how the nuclear interaction emerges from the basic constituents of matter is one of the challenges of contemporary physics. The nuclear interaction between nucleons (proton or neutron) is seen as a manifestation of the strong force between quarks, which is mediated by the exchange of gluons and holds the nucleon together. In spite of the longstanding efforts, a unified nuclear interaction that allows predicting the properties of all nuclei does not exist yet.
1 Mean fields techniques have successfully explain the behaviour a nucleon inside a nuclei in many experiement albeit not all.
It has been observed that when an electron beam knocks-out a proton, about 20% of the time a correlated partner proton or neutron is simultaneously released instead of a unique nucleon (Fig 1). A peculiar feature is that 18% of the time, those are proton-neutron pairs, making them dominant on same species pairs (proton-proton or neutron-neutron). Nuclear physicists know how to detect and study these pairs of nucleons, and surprisingly the proton and the neutron escape mostly in opposite directions at high speed!
This observation has been interpreted as due to configurations with two nucleons sitting close together inside the nucleus, a bit like a couple in a crowd. These configurations have triggered the interest of physicists who have named them Short-Range Correlated (SRC) pairs. Indeed if they know quite well how a crowd of nucleons impacts an individual one1, the way two nucleons closed to each other behave remains not fully understood. Those pairs may very well shed a new light on this point.
To understand better these nuclear couples, DPhN physicists have contributed to a pilot experiment that took place in 2018 at the Nuclotron accelerator in Dubna, Russia. A 12C beam was sent on a proton liquid target, followed by a detection system designed for identifying all reaction products and being able to measure all their momenta (Fig. 2). The goal of the experiment was to validate the use of this technique, which appears to be quite original. Indeed in the past experiments the role of the 12C and proton would have been flipped, i.e. the beam would have been composed of protons while the target would have been made of 12C. This original configuration is a major step forward as it will allow in future to extend the study of SRCs to radioactive nuclei with lifetimes as short as few milliseconds. With such a short lifetime, it is not possible to build a target from them.
However, when colliding a beam of 12C on a proton target would produce a vast amount of data, among which only a tiny fraction corresponds to a knocked out nucleon or nucleon pair. Looking for those events is as difficult as looking for a needle in haystack because of all these parasitic reactions. Therefore, one needs to sort and select interesting events at very high frequencies. This is where the Dubna experiment has played a decisive role. It has validated the possibility to cleanly select the processes where the proton interacts only once with a pair of nucleons inside the 12C nucleus, extracting it out of the nucleus without breaking the latter. In such a case, we can say that the nucleus became transparent to the proton probe. The selection has been made possible thanks to the ability to detect the remaining unbroken nucleus amputated of a nucleon pair (called the fragment) in a dedicated spectrometer. Fragment selection made the trick, allowing physicists to catch the needle!
This experiment is the first step towards the extension of SRC studies to radioactive nuclei.
Interestingly, SRCs do not happen at the same rate for all nucleons. Indeed, considering nuclei with a different number of protons and neutrons, it is possible to show that proton-neutron pairs dominate among SRC pairs [Hen14]. Therefore, the nucleons belonging to the minority species (most of the time the protons, especially in the case of stable heavy nuclei and neutron rich unstable nuclei) become more “correlated” [Due18]. This can be illustrated considering 12C and 208Pb with a constant fraction of 18% of the nucleons forming a proton-neutron pairs. In the case of 12C containing 6 protons and 6 neutrons, such a fraction corresponds to 1 proton-neutron pair. Since 12C possesses the same number of protons and neutrons, the correlation rate is the same for both of them, equal to 18%. But in the case nuclei having a different number of proton and neutron, the correlation rate varies. As an example, for 208Pb, made 82 protons and 126 neutrons, just like before, 18% of the total amount of nucleons are involved in a proton-neutron pair, i.e. 19 protons and 19 neutrons. But surprisingly, one can easily see that the fraction of correlated protons (23%) becomes much higher in 208Pb!
The next step and the primary goal of the COCOTIER project, financed by the ANR in 2017, is to extend the study of SRCs to radioactive nuclei. An experiment lead by an IRFU-Massachusetts Institute of Technology team has been recently approved by the Program Advisory Committee of the Research Society on Heavy Ions (Gesellschaft für SchwerIonenforschung GSI) facility in Darmstadt, Germany and is expected to run in 2022. This experiment can only be carried out at the GSI accelerator thanks to
(i) its capability of producing radioactive nuclei at velocities of about 75% of the speed of light needed for this kind of studies
(ii) the availability of a detection system designed for measuring every kinematical variable.
This detection system is completed by a liquid hydrogen target built at IRFU (through the COCOTIER grant). It will be the first attempt to study SRCs in an radioactive nucleus: 16C, which contains 4 more neutrons than protons and whose mass is very similar to the well-studied reference system 12C. This will permit to analyze how the fraction of SRCs protons and neutrons evolves with increasing the neutron-proton asymmetry and may guide our efforts to better understand how nuclei are bound by the strong interaction.
[Pat21] Patsyuk, M., Kahlbow, J., Laskaris, G. et al. Nat. Phys. (2021).
[Hen14] O. Hen et al. (CLAS Collaboration), Science, 346 (6209):614, 2014.
[Due18] M. Duer et al. (CLAS Collaboration), Nature, 560:617, 2018.
Contact: Anna Corsi