IceCube [here's my own description of the experiment], the big high-energy neutrino experiment cleverly embedded into the ice at the South Pole, announced a very interesting result yesterday, following on an already interesting result from a few weeks ago, one that I failed to cover properly. They have seen the highest-energy neutrinos ever observed, ones that, unlike previously observed high-energy neutrinos, appear not to be generated by cosmic rays hitting the top of the atmosphere. Instead, they apparently come from new sources far out in space. And as such, it tentatively appears that they’ve opened up, as long anticipated, a new era in neutrino astronomy, in which high-energy neutrinos will be used to understand astrophysical phenomena!
[The only previous example of neutrinos being used in astrophysics occurred with the discovery of neutrinos from the relatively nearby supernova, visible with the naked eye, that occurred in 1987. But those neutrinos had energies millions of times smaller than the ones discussed here. And there was hope that IceCube might see neutrinos specifically from gamma-ray bursts, including the one that occurred just two weeks ago; but that appears not to have happened.]
I don’t understand certain details well enough yet to give you a careful explanation — that will probably come next week — but here’s an early description (and expert readers are strongly encouraged to correct any errors.)
At present, there are various sources, one known and others suspected, of high-energy neutrinos (and anti-neutrinos) coming from the sky, as illustrated in Figure 1, taken from the IceCube talk that announced the result.
- When cosmic rays (mostly high-energy protons and some atomic nuclei created in natural particle accelerators in outer space) hit atoms in the atmosphere, they produce showers of hadrons, some of which are pions and kaons. Some of these in turn decay to muons (and anti-muons) and neutrinos (and anti-neutrinos). These “atmospheric neutrinos” take a wide range of energies, and (just like the cosmic rays that make them) become increasingly rare the higher-energy you go, the number falling like 1/(energy)3.7. They should be detectable by IceCube out to energies of a million GeV or so (the black curve in Figure 1), and in the early days of IceCube were already detected out to about 300,000 GeV (the blue dots in Figure 1).
- The cosmic rays give a second source of neutrinos that may be observable around a hundred thousand to a million GeV, from the production of charm quarks, which can create a small number of neutrinos that fall off more slowly with energy than do the neutrinos from other hadrons. One prediction for how many “prompt atmospheric” or “charm atmospheric” neutrinos should be present is the red curve in Figure 1.
- Neutrinos produced when the very highest-energy cosmic rays collide with photons from the cosmic microwave background (mainly through the process proton + photon –> “Delta” [an excited version of the proton] –> neutrino + pion, followed by pion –> anti-muon + neutrino, and also by anti-muon –> anti-electron + neutrino + anti-neutrino). These are called “GZK neutrinos” or “cosmogenic neutrinos”. Since the number and energy of high-energy cosmic rays is roughly measured, the number and energy of these GZK neutrinos can be roughly predicted.
- High-energy “astrophysical neutrinos” produced directly inside extremely energetic astrophysical objects, perhaps including the objects that make gamma-ray bursts. Since little is known about what objects are out there and how they work, the only clear thing that can be said about these neutrinos is that there can’t be too many of them (or we’d see more high-energy cosmic rays than we do). It is expected that the number of neutrinos from such sources will decrease as 1/(energy)2; since the plot in Figure 1 shows not the number of neutrinos but the number of neutrinos times their energy-squared, the (very rough) prediction from astrophysical sources is a flat green line in Figure 1. We don’t know how many of these neutrinos to expect, so the location of that line, though it cannot be higher than shown, but could well be lower.
Recently, in their data from 2010-2012, IceCube reported, in a pre-publication paper that appeared a few weeks ago, that they observed two neutrinos with energies of about one million GeV. [For some reason I don't know, they amusingly decided to call these neutrinos Bert and Ernie.] These are unusually energetic for atmospheric neutrinos, yet not energetic enough to be GZK neutrinos. This makes it likely (but not certain) that they are from new astronomical sources! But with just two events, it’s hard to say anything else about them. Until yesterday.
Yesterday, IceCube reported that, by using a technique that reduces the number of atmospheric neutrinos in their data, they were able to look for neutrinos from other sources at somewhat lower energies. They expected something like 10 (more precisely 10.6+4.5-3.9, or, including the charm atmospheric neutrinos in some model [??] 12.1±3.4 ) — about 5 from atmospheric neutrinos and 6 from muons from cosmic rays that give fake signals of neutrinos. But, as shown in Figure 2, they observed 28, including the two I mentioned in the previous paragraph. (To be clear, this means that 10 to 20 of them are probably neither atmospheric neutrinos nor fakes.) This is strong evidence (4.3 standard deviations) that IceCube is observing neutrinos that are not from atmospheric neutrinos… but they aren’t GZK neutrinos either.
What are these things? Are they astrophysical neutrinos, from some new, unknown class of sources? Well, it’s hard to say with so few of these neutrinos observed so far. On the one hand,
- They have some features expected from astrophysical neutrinos… they are consistent with coming uniformly across the sky (though with so few neutrinos it’s hard to tell), and their numbers appear to decrease with energy more slowly than atmospheric neutrinos do across the range of 20,000 – 1,200,000 GeV.
- There’s no sign that these neutrinos are associated with other particles simultaneously coming out of the sky, as would be expected for overhead cosmic rays that make atmospheric neutrinos.
- And, unlike the atmospheric neutrinos which are more often muon-neutrinos and muon-antineutrinos, these neutrinos seem to be more evenly distributed among the three types of neutrinos and antineutrinos.
But on the other hand, there are some possible challenges for this interpretation.
- If their numbers really decrease with as 1/(energy)2, as naively expected for most astrophysical sources, then IceCube should also have seen some additional neutrinos (something like five to ten of them) well above 1,000,000 GeV.
- Moreover, these neutrinos (which should have traveled straight across space from their source) don’t point back toward any known object (such as an active galaxy or a recent gamma-ray burst) so we don’t have any way to know what type of object may be producing them.
In short, these neutrinos appear to be from a new, unidentified, and perhaps unexpected type of source!
We must remain somewhat cautious about any new result that comes from a single experiment and involves so few neutrinos. But if IceCube’s result continues to hold up with more data and is confirmed by other similar neutrino experiments, or if in future this class of neutrinos can be linked with specific astrophysical objects, I suspect it will be seen as a major discovery — one that opens up the era of neutrino astronomy, and whose implications can today only be guessed at.