Galactic PeV neutrinos from dark matter annihilation

The IceCube Neutrino Observatory has observed highly energetic neutrinos in excess of the expected atmospheric neutrino background. It is intriguing to consider the possibility that such events are probing fundamental physics beyond the standard model of particle physics. In this context, OdPeV_ dark matter particles decaying to neutrinos have been considered, while dark matter annihilation has been dismissed invoking the unitarity bound as a limiting factor for the annihilation rate. However, the latter claim was done ignoring the contribution from dark matter substructure, which in a PeV cold dark matter scenario would extend down to a free- streaming mass of Od10-18 M. _. Since the unitarity bound is less stringent at low velocities, dsannv_ = 4p= m2. v, then it is possible that these cold and dense subhalos would contribute dominantly to a dark- matter- induced neutrino flux and easily account for the events observed by IceCube. A dark matter model in which annihilations are enhanced by a Sommerfeld mechanism can naturally support such a scenario. Interestingly, the spatial distribution of the events shows features that would be expected in a dark matter interpretation. Although not conclusive, 9 of the 37 events appear to be clustered around an extended region near the Galactic center, while 6 others spatially coincide, within the reported angular errors, with 5 of 26 MilkyWay satellites. However, a simple estimate of the probability of the latter occurring by chance is 35%. More events are needed to statistically test this hypothesis. PeV dark matter particles are massive enough that their abundance as standard thermal relics would overclose the Universe. This issue can be solved in alternative scenarios, for instance, if the decay of new massive unstable particles generates significant entropy reheating the Universe to a slightly lower temperature than the freeze- out temperature, TRH. Tf 4 x 104 GeV.