THESEUS insights into axionlike particles, dark photon, and sterile neutrino dark matter

Through a series of simulated observations, we investigate the capability of the instruments aboard the forthcoming THESEUS mission for the detection of a characteristic signal from decaying dark matter (DM) in the keV-MeV energy range. We focus our studies on three well studied Standard Model extensions hosting axion-like particles, dark photon, and sterile neutrino DM candidates. We show that, due to the sensitivity of THESEUS' X and Gamma Imaging Spectrometer (XGIS) instrument, existing constraints on dark matter parameters can be improved by a factor of up to around 300, depending on the considered DM model and assuming a zero level of systematic uncertainty. We also show that even a minimal level of systematic uncertainty of 1% can impair potential constraints by one to two orders of magnitude. We argue that nonetheless, the constraints imposed by THESEUS will be substantially better than existing ones and will well complement the constraints of upcoming missions such as eXTP and Athena. Ultimately, the limits imposed by THESEUS and future missions will ensure a robust and thorough coverage of the parameter space for decaying DM models, enabling either a detection of dark matter or a significant improvement of relevant limits.