On the direct detection of extragalactic weakly interacting massive particles

We consider the direct detection of weakly interacting massive particles (WIMPs) reaching the Earth from outside the Milky Way. If these WIMPs form a distinct population they will, although of much lower flux than typical galactic halo WIMPs, have a number of features which might aid in their ultimate detectability: a high and essentially unique velocity $(\sim 600\mathrm{k}\mathrm{m}/\mathrm{s}$ in the galactic rest frame) due to their acceleration in entering the Milky Way, and most likely one or two unique flight directions at the Earth. This high velocity may be experimentally advantageous in direct detection experiments, since it gives a recoil signal at relatively high energy where background is generally much reduced. For a density of extragalactic WIMPs comparable to the critical density of the universe the count rate expected is very roughly the same as that of fast galactic WIMPs. If there is an increased density relative to critical density associated with the Local Group of galaxies, say 10–30 times the critical density, there is a corresponding increase in rate and the extragalactic WIMPs would show up as a high energy shoulder in the recoil energy distribution. Evidence of such WIMPs as a separate population with these distinct properties would offer interesting information on the formation and prehistory of the galaxy.