Abstract
This work presents a detailed theoretical analysis of the single charge exchange reaction 40 Ca( 18 O, 18 F) 40 K at an incident energy of 275 MeV, with the aim of exploring mechanisms relevant to neutrinoless double beta decay. The contributions of the direct one-step meson-exchange process and the sequential two-step nucleon transfer mechanisms (np and pn ) were investigated within the framework of the distorted wave Born approximation and its second-order generalization. Nuclear structure input was derived from shell-model calculations, and the form factors were generated using folding procedures with microscopic optical potentials. The calculated integral cross sections were compared with the experimental spectrum, showing good agreement in the energy regions of interest. The analysis revealed that the dominant contribution in the first region of interest arises from the channel at 0.8 MeV, corresponding to the 18 F(1 + 1 ) + 40 K(2 - 1 ) configuration, while additional contributions appear in the second and third regions of interest with different ordering depending on the density of states. The results confirm earlier findings that the smallest transferred angular momenta provide the leading contributions to the cross section, whereas two-step processes remain comparatively weak but competitive with each other. These results not only validate previous conclusions but also highlight the need to include higher-lying excited states in future studies. The methodology established in this work provides a solid basis for extending the analysis to other nuclear targets and contributes to the ongoing effort of constraining nuclear matrix elements relevant to neutrinoless double beta decay.