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ABSTRACT Reticulum II (Ret II) is a satellite galaxy of the Milky Way (MW) and presents a prime target to investigate the nature of dark matter (DM) because of its high mass-to-light ratio. We evaluate a dedicated INTEGRAL observation campaign data set to obtain γ-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the γ-ray band 25–8000 keV, and we derive a flux limit of $${\lesssim}10^{-8}\, \mathrm{erg\, cm^{-2}\, s^{-1}}$$. The previously reported 511 keV line is not seen, and we find a flux limit of $${\lesssim}1.7 \times 10^{-4}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$$. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of e+s produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses $${\lesssim}8 \times 10^{15}\, \mathrm{g}$$. Our limits on the velocity-averaged DM annihilation cross section into e+e− are $$\langle \sigma v \rangle \lesssim 5 \times 10^{-28} \left(m_{\rm DM} / \mathrm{MeV} \right)^{2.5}\, \mathrm{cm^3\, s^{-1}}$$. We conclude that analysing isolated targets in the MeV γ-ray band can set strong bounds on DM properties without multi-year data sets of the entire MW, and encourage follow-up observations of Ret II and other dwarf galaxies. 

Abstract The existence of nonzero neutrino masses points to the likely existence of multiple Standard Model neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as heavy neutral leptons (HNLs). In this white paper, we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding of key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.