Quenching of Cross Sections in Direct Reactions*

The strengths of single-particle transitions probed in direct reactions appear to be quenched by correlations. The degree of quenching is around 0.6 for stable nuclei. This factor seems to be independent of the probe used, the mass of the system, the orbital angular momentum of the nucleon added or removed from the system, and its separation energy. It is the same for protons and neutrons. Typical probes include single-nucleon transfer reactions carried out at a few MeV per nucleon above the barrier, intermediate energy heavy-ion induced knockout reactions, quasifree (p,2p) and (p,pn) scattering, and electron-induced knockout. The difference between the proton and neutron separation energies (ΔS) is relatively small for stable nuclei, varying between approximately –10 MeV and +10 MeV. The differences are much larger for nuclei far from stability, exceeding –/+20 MeV in some cases. Over the last two decades, a wealth of data using intermediate energy heavy-ion induced knockout reactions has revealed a striking trend in the degree of quenching, expressed as the reduction factor R (the ratio of experimental and theoretical inclusive cross sections), versus ΔS [1]. There are limited data available over such a range of ΔS using other probes, with only a handful of transfer-reaction and (p,2p) and (p,pn) results. However, there is a strong contrast between the R factors from intermediate energy heavy-ion induced knockout reactions and these other probes, for which there is not yet a robust theoretical explanation. I will give an overview of the status of this topic with a focus on results, opportunities, and techniques to explore quenching in exotic nuclei using transfer reactions in the coming decade.

[1] T. Aumann et al., Prog. Part. Nucl. Phys. 118, 103847 (2021).