Exceptional Points and Observable Effects in Nuclear Phenomena

by David Cardona (3^rd year PhD student)

Exceptional points (EPs) are spectral singularities of non-Hermitian Hamiltonians at which two or more eigenvalues and their corresponding eigenstates simultaneously coalesce, rendering the Hamiltonian non-diagonalizable. While EPs have attracted broad interest across atomic physics, photonics, and quantum sensing, their manifestation in nuclear systems remains comparatively unexplored. Nuclear physics provides a natural setting for EP studies: nuclei are inherently open quantum systems spanning the full range from bound states to the scattering continuum, yet conventional reaction-theory tools such as R-matrix theory and Breit-Wigner parametrizations implicitly assume well-separated poles and fail qualitatively when two poles merge at an EP. The Gamow Shell Model in coupled-channel representation (GSM-CC) overcomes this limitation by treating bound, resonant, and continuum states on equal footing within a single microscopic framework, making it uniquely suited to describe resonance spectra and reaction observables in regimes of strong continuum coupling where EPs can emerge.

Using the GSM-CC, EPs are identified and characterized in the light nuclei ⁶Li, ⁷Li, ⁷Be, and ⁸Be, where doublets of resonances are shown to coalesce under variation of the spin-orbit interaction strength. In each system, the EP leaves clear imprints on multiple physical observables: the phase rigidity, elastic scattering cross sections, time-domain decay dynamics, and expectation values of operators all exhibit signatures that are qualitatively distinct from those of ordinary resonances. These findings demonstrate that EPs are physically accessible in the low-mass nuclear chart and that their effects are observable through both reaction cross sections and survival probability measurements.