Shedding light on nuclear aspects of neutrinoless double beta decay by heavy-ion double charge exchange reactions

We review the status and prospects of heavy-ion double charge exchange (HI-DCE) reactions. Their important role for nuclear reaction, nuclear structure and double beta-decay investigations is outlined. From the experimental side the characteristically tiny cross sections for these processes and the high background generated by other more probable competing reactions is the main challenge, which has hindered HI-DCE spectroscopy until recent years. Modern magnetic spectrometers have proven to possess the right requisites to overcome past limitations, fostering the present and future development of the field. From the theory side, the description of the measured HI-DCE cross sections poses manifold challenges. Dealing with processes which involve composite nuclei, HI-DCE reactions can, in principle, proceed through several alternative paths. These, in turn, correspond to different reaction mechanisms probing competing aspects of nuclear structure, from mean field to various classes of nucleon–nucleon interactions and correlations. A powerful way to scrutinize the nuclear response to HI-DCE is to consistently link it to the information extracted from the competing direct reactions. Indeed, these complementary studies are mandatory in order to minimize the systematic errors in the data analyses and build a many-facets and parameter-free representation of the systems under study.