Next year, scientists will cut the ribbon on IceCube, a neutrino observatory consisting of strings of detectors buried deep in Antarctic ice. But eager researchers have already used the unfinished detector to search for a different type of particle from space, called cosmic rays—mostly energetic protons and helium nuclei of cosmic origin. Both cosmic rays and neutrinos create the same particles—muons—when they collide with matter, and muons are what the observatory is designed to detect. Only neutrinos, however, can travel through Earth. So muons that come from below are from neutrino collisions inside the ice, whereas the vast majority of muons that come from above are created by cosmic ray collisions in Earth's atmosphere. Next month in The Astrophysical Journal Letters, researchers report that they used IceCube to study a longstanding puzzle: whether the distribution of cosmic ray arrivals is uneven across the southern sky, as scientists have previously observed in the northern hemisphere. Indeed, the team found, IceCube detected a disproportionate number of cosmic rays arriving from some parts of the sky. But the reason for this uneven distribution remains unclear.
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