Evolving and interacting dark energy: photometric and spectroscopic synergy with DES Y3 and DESI DR2

We investigate the Dark Scattering (DS) interacting dark energy scenario, characterised by pure momentum exchange between dark matter and dark energy, combined with a time-dependent equation-of-state for dark energy described by the Chevallier-Polarski-Linder (CPL) parametrisation. This class of models is weakly constrained by CMB observations and can exhibit distinctive late-time suppression of structure growth. We derive constraints on cosmological, DS, and CPL parameters using three two-point correlation functions from the Dark Energy Survey Year 3 data, combined with baryon acoustic oscillation measurements from DESI, Type Ia supernovae from DES Year 5, and CMB data from Planck. We find the dark-sector interaction parameter \(A_\mathrm{ds}\) to be consistent with zero for all data combinations, and that CPL provides a statistically preferred fit over DS for the selected probes. From the full data combination we obtain \(w_0=-0.76\pm0.06, \, w_a=-0.77^{+0.23}_{-0.20}\) for CPL, and \(w_0=-0.79^{+0.05}_{-0.06}, \, w_a=-0.56^{+0.24}_{-0.15}, \, (A_\mathrm{ds}=9.8^{+2.8}_{-9.5}\,\mathrm{bn/GeV} )\) for DS. The inclusion of DES photometric information improves the Figure-of-Merit on \((w_0,w_a)\) by \(\sim12\%\) for CPL and \(\sim25\%\) for DS relative to DESI+SN+CMB alone. We find no evidence for an \(S_8\) discrepancy between the low-\(z\) and high-\(z\) measurements in either model. These results provide the most stringent pre-Euclid constraints on DS from a combined photometric and spectroscopic analysis.