Bit of old news here — well, the existence of the Moon is extremely old news, but even this new result is slightly non-new. But it was new to me.
Ice Cube is a wondrously inventive way of looking at the universe. Sitting at the South Pole, the facility itself consists of strings of basketball-sized detectors reaching over two kilometers deep into the Antarctic ice. Its purpose is to detect neutrinos, which it does when a neutrino interacts with the ice to create a charged lepton (electron, muon, or tau), which in turn splashes Cherenkov radiation into the detectors. The eventual hope is to pinpoint very high-energy neutrinos coming from specific astrophysical sources.
For this purpose, it’s the muon-creating neutrinos that are your best bet; electrons scatter multiple times in the ice, while taus decay too quickly, while muons give you a nice straight line. Sadly there is a heavy background of muons that have nothing to do with neutrinos, just from cosmic rays hitting the atmosphere. Happily most of these can be dealt with by using the Earth as a shield — the best candidate neutrino events are those that hit Ice Cube by coming up through the Earth, not down from the sky.
It’s important in this game to make sure your detector is really “pointing” where you think it is. (Ice Cube doesn’t move, of course; the detectors find tracks in the ice, from which a direction is reconstructed.) So it would be nice to have a source of muons to check against. Sadly, there is no such source in the sky. Happily, there is an anti-source — the shadow of the Moon.
Cosmic rays rain down on the Earth, creating muons as they hit the atmosphere, but we expect a deficit of cosmic rays in the direction of the Moon, which gets in the way. And indeed, here is the map constructed by Ice Cube of the muon flux in the vicinity of the Moon’s position in the sky.
There it is! I can definitely make out the Moon.
Really this is a cosmic-ray eclipse, I suppose. We can also detect the Moon in gamma rays, and the Sun in neutrinos. It’s exciting to be living at a time when technological progress is helping us overcome the relative poverty of our biological senses.