Transfer at sub-barrier energies and its role in understanding the suppression of fusion

Fusion cross section measurements in nuclear reactions of heavy nuclei consistently fall below model predictions at energies both well below and above the fusion barrier. Understanding the physical mechanisms leading to this fusion suppression has been one of the major challenges in nuclear reaction physics [6, 7, 1]. It is crucial for a consistent understanding of nuclear reactions involving both light nuclei (e.g. in astrophysical scenarios) and heavy nuclei (e.g. in the formation of super-heavy elements). The suppression of fusion at above-barrier energies has been associated with deep inelastic collisions (DIC) [3, 7]. Kinetic energy dissipation into nucleonic degrees of freedom as part of the DIC is believed to lead to a reduction of the fusion probability. Fusion at energies well below and close to the fusion barrier occurs through quantum tunnelling of the projectile nucleus through the fusion barrier, which in turn is affected by couplings to few inelastic and/or transfer channels which are most dominant at these energies [2]. We expect in reality a smooth transition from nucleon transfer to low-lying discrete states in sub-barrier nuclear reactions on one end, to (multi-)nucleon transfer leading to highly excited nuclei in DIC at energies above the barrier on the other end.