Nucleons are social particles. Not only do they like to live in communities inside nuclei, but they also form pairs within these communities. Indeed, we can observe protons and neutrons forming pairs inside nuclei. Physicists from Irfu/DPhN of LabEx P2IO have played a decisive role in the first measurement of these nucleon pairs using a new method, which will pave the way for the study of these close (or short-range) interactions in radioactive nuclei. The results were recently published in Nature Physics [Pat21]. The study of these nucleon pairs in radioactive nuclei is the objective of the ANR COCOTIER project and to carry out this project, P2IO is funding a 1/2 thesis grant.
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 considered as a manifestation of the strong force between quarks, through the exchange of gluons that hold the nucleons together. Despite long-standing efforts, there is still no unified nuclear interaction that can predict the properties of all nuclei.
It has been observed that when an electron beam strikes a proton, about 20% of the time, an associated proton or neutron is simultaneously released instead of a single nucleon (Fig 1). A special feature is that in 18% of the cases, these are proton-neutron pairs, which makes them dominant compared to pairs of the same species (proton-proton or neutron-neutron). Nuclear physicists know how to detect and study these nucleon pairs, and surprisingly, the proton and neutron most often escape in opposite directions at high speed!
This observation has been interpreted as being due to configurations in which two nucleons are close to each other inside the nucleus, much like a pair in a crowd. These configurations have aroused the interest of physicists who have named them short-range correlated pairs (SRC). Indeed, if they know well the impact of a crowd of nucleons on an individual nucleon, the behavior of two nucleons close to each other is not yet fully understood. These pairs could well bring a new light on this point.
Figure 1: Artistic view of electron-nucleus scattering producing a correlated proton and neutron pair. From [Hen14].
To better understand these nuclear couples, DPhN physicists contributed to a pilot experiment that took place in 2018 at the Nuclotron gas pedal in Dubna, Russia. A beam of 12C was sent onto a liquid proton target, followed by a detection system designed to identify all the products of the reaction and to be able to measure all their moments (Fig. 2). The aim of the experiment was to validate the use of this technique, which seems quite original. Indeed, in previous experiments, the role of 12C and proton would have been reversed, i.e. the beam would have been composed of protons while the target would have been 12C. This original configuration is a major advance because it will allow in the future to extend the study of SRC to radioactive nuclei with a lifetime of only a few milliseconds. With such a short lifetime, it is not possible to build a target from them.
However, the collision of a 12C beam on a proton target would produce a large amount of data, of which only a tiny fraction corresponds to a ripped-off nucleon or nucleon pair. Searching for these events is as difficult as finding a needle in a haystack because of all the parasitic reactions. It is therefore necessary to sort and select the interesting events at very high frequency. This is where the Dubna experiment played a decisive role. It validated the possibility of cleanly selecting processes where the proton interacts only once with a pair of nucleons inside the 12C nucleus, extracting it from the nucleus without breaking it. In such a case, one can say that the nucleus has become transparent to the proton probe. The selection was made possible by the ability to detect the remainder of the unbroken nucleus minus a pair of nucleons (called fragment) in a dedicated spectrometer. The selection of fragments made all the difference, allowing the physicists to catch the needle!
Figure 2: Schematic of the detection system used for the SRC measurement at Dubna. Ionized carbon is detected by MWPCs and hits the target. The reaction products (two protons and the fragment) are detected by TC, GEM, RPC and Si, DCH, respectively. From Nature Physics.
This experiment is the first step towards extending CRS studies to radioactive nuclei.
It is interesting to note that SRCs do not occur at the same speed for all nucleons. Indeed, considering nuclei with different numbers of protons and neutrons, it is possible to show that proton-neutron pairs dominate among the SRC pairs [Hen14]. Consequently, the nucleons belonging to the minority species (mostly protons, especially in the case of stable heavy nuclei and unstable neutron-rich nuclei) become more "correlated" [Due18]. This can be illustrated by considering 12C and 208Pb with a constant fraction of 18% of nucleons forming a proton-neutron pair. In the case of 12C containing 6 protons and 6 neutrons, such a fraction corresponds to 1 proton-neutron pair. As 12C has the same number of protons and neutrons, the correlation rate is the same for both, equal to 18%. But in the case of nuclei with different numbers of protons and neutrons, the correlation rate varies. For example, for 208Pb, composed of 82 protons and 126 neutrons, as before, 18% of the total amount of nucleons are involved in a proton-neutron pair, that is 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 primary objective of the COCOTIER project, funded by ANR in 2017, is to extend the study of CRS to radioactive nuclei. An experiment conducted by an IRFU-Massachusetts Institute of Technology team was recently approved by the Program Advisory Committee of the Heavy Ion Research Society (Gesellschaft für SchwerIonenforschung GSI) facility in Darmstadt, Germany, and is scheduled to take place in 2022. This experiment can only be performed at the GSI gas pedal thanks to
(i) its ability to produce radioactive nuclei at speeds up to about 75% of the speed of light required for this type of study and
(ii) the availability of a detection system designed to measure each kinematic variable.
This detection system is completed by a liquid hydrogen target built at Irfu (thanks to the COCOTIER grant). This will be the first attempt to study SRCs in a radioactive nucleus: 16C, which contains 4 more neutrons than protons and whose mass is very close to the well studied reference system, 12C. This will allow us to analyze how the fraction of proton and neutron SRCs evolves with increasing 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
