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 \textbf{842}, 48 (2010)] and show that the total absorption cross section,, $\sigma_{\mathrm A}\propto -\langle\Psi|W|\Psi\rangle$, decomposes as $\sigma_{\mathrm A}=\sigma_{\mathrm D}+\sigma_{\mathrm B}+\sigma_{\mathrm{int}}$, where $\sigma_{\mathrm{int}}$ is a coherent interference term between channel components. For the systems and fragment-target optical potentials considered, $\sigma_{\mathrm{int}}$ 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 $\sigma_{\mathrm{int}}$, which is required for flux-balance consistency. Calculations for $d+{}^{93}\mathrm{Nb}$ and ${}^{6}\mathrm{Li}+{}^{59}\mathrm{Co}/{}^{208}\mathrm{Pb}$ 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.