Coherent Absorption Dynamics: The Dual Role of Off-Diagonal Couplings in Weakly Bound Nuclei

Disentangling reaction mechanisms in weakly bound nuclei remains a long-standing challenge, often compounded by the treatment of absorption as an incoherent sum of channel contributions. Within the Continuum-Discretized Coupled-Channels (CDCC) framework, we apply the generalized optical theorem [Nucl.\ Phys.\ A $\mathbf{\text{842}}$, 48 (2010)] and show that the total absorption cross section, ${\sigma }_{\mathrm{A}}\propto -⟨\mathrm{\Psi }\mathrm{\mid }W\mathrm{\mid }\mathrm{\Psi }⟩$, decomposes as ${\sigma }_{\mathrm{A}}={\sigma }_{\mathrm{D}}+{\sigma }_{\mathrm{B}}+{\sigma }_{\mathrm{i}\mathrm{n}\mathrm{t}}$, where ${\sigma }_{\mathrm{i}\mathrm{n}\mathrm{t}}$ is a coherent interference term between channel components. For the systems and fragment-target optical potentials considered, ${\sigma }_{\mathrm{i}\mathrm{n}\mathrm{t}}$ is negative and comparable in magnitude to the direct absorption terms. The off-diagonal imaginary couplings play a dual role, redistributing flux among channels and generating ${\sigma }_{\mathrm{i}\mathrm{n}\mathrm{t}}$, which is required for flux-balance consistency. Calculations for $d+{}^{93}\mathrm{N}\mathrm{b}$ and ${}^6\mathrm{L}\mathrm{i}+{}^{59}\mathrm{C}\mathrm{o}\mathrm{/}{}^{208}\mathrm{P}\mathrm{b}$ show that retaining the full non-diagonal coupling matrix substantially enhances breakup-channel absorption for heavy targets while reducing the total absorption through interference effects. Neglecting off-diagonal imaginary couplings therefore leads to a systematically biased physical picture, overestimating total absorption and severely underestimating breakup contributions, implying that experimental analyses based on incoherent-sum models inherit this bias. Full-coupling CDCC calculations are thus essential for consistent, mechanism-resolved extraction of absorption cross sections in weakly bound systems.