Neutrinos in IceCube/KM3NeT as probes of dark matter substructures in galaxy clusters
Galaxy clusters are one of the most promising candidate sites for dark matter (DM) annihilation. We focus on DM ($\ensuremath{\chi}$) with mass in the range of 10 GeV\char21{}100 TeV, annihilating through the channels $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}\ensuremath{\nu}\overline{\ensuremath{\nu}}$, $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}t\overline{t}$, or $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\nu}\overline{\ensuremath{\nu}}$, and forecast the expected sensitivity to the annihilation cross section into these channels by observing galaxy clusters at IceCube/KM3NeT. Optimistically, the presence of DM substructures in galaxy clusters is predicted to enhance the signal by 2\char21{}3 orders of magnitude over the contribution from the smooth component of the DM distribution. Optimizing for the angular size of the region of interest for galaxy clusters, the sensitivity to the annihilation cross section, $⟨\ensuremath{\sigma}v⟩$, of heavy DM with mass in the range of 300 GeV\char21{}100 TeV will be $\mathcal{O}({10}^{\ensuremath{-}24}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1})$ for full IceCube/KM3NeT live time of 10 years, which is about one order of magnitude better than the best limit that can be obtained by observing the Milky Way halo. We find that neutrinos from cosmic ray interactions in the galaxy cluster, in addition to the atmospheric neutrinos, are a source of background. We show that significant improvement in the experimental sensitivity can be achieved for lower DM masses in the range of 10\char21{}300 GeV if neutrino-induced cascades can be reconstructed to $\ensuremath{\approx}5\ifmmode^\circ\else\textdegree\fi{}$ accuracy, as may be possible in KM3NeT. We, therefore, propose that a low-energy extension ``KM3NeT-Core,'' similar to DeepCore in IceCube, be considered for an extended reach at low DM masses.