Scalar-Coupled Particle-Like String Loops as an Interacting Dark Sector: Dynamics and Constraints in Light of DESI DR2 Observations

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Abstract
A string-motivated interacting dark sector cosmological model generated by scalar-coupled, particle-like string loops is investigated using dynamical systems techniques and observational constraints. The scalar dependence of the loop energy density naturally induces an interaction between the scalar field and the loop sector, providing a mechanism for energy exchange and establishing a connection between string-inspired microphysics and late-time cosmology. The dynamics are reformulated as an autonomous system and analyzed for both constant and dynamically evolving slope realizations of the scalar potential and loop coupling. The resulting phase space exhibits interacting loop--scalar scaling solutions together with a self-regulating interaction mechanism associated with a dynamically invariant hypersurface. The model is constrained using Pantheon+ SNe Ia, DES Y5, DESI DR2 BAO, and compressed CMB data. Both realizations are consistent with current observations while allowing controlled departures from the standard $\Lambda$CDM scenario. The constant-coupling realization provides the strongest overall fit to the combined datasets, whereas the dynamical-coupling realization remains observationally viable and exhibits a richer dynamical structure. These results demonstrate that scalar-coupled particle-like string loops constitute a viable and well motivated interacting dark sector framework.

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