Neutrinos have precious little mass and no charge, meaning the usual ways of accelerating particles won't work on them. Yet something, somewhere out in space pushed one to energies a thousand times higher than we can reach in the Large Hadron Collider. And we only know that because we finally built a detector that could spot high-energy neutrinos when they travel through the Earth.

In a recent paper in the journal Nature Physics, Francis Halzen, the principal investigator for the IceCube detector, discussed current efforts to learn about the Universe using neutrinos. As it turns out, neutrinos are surprisingly informative about the origins of cosmic rays and potentially about dark matter as well.

Neutrinos are a fantastic tool for astronomy. Their properties—no charge and very little mass—mean that they can arrive here on Earth unobstructed by almost anything in between their source and Earth. Neutrinos generated inside the Sun, for instance, can travel right out far faster than photons, which spend time interacting with the Sun's matter.

Neutrinos that originate near black holes can escape their chaotic accretion disks. If they’re coming from a region of the Universe that is obscured by gas or dust, such as the center of the Milky Way galaxy, they can travel through that material like it wasn't there. Pretty much anything you put in their way, they’ll go right through; even if you had a mass of lead a light-year thick, it would only block half of the neutrinos that tried to pass through it.

Once the neutrinos get here, researchers can use them to learn about the objects and processes that generated them. Unfortunately, the same thing that makes them such a good tool also creates an obstacle. Not only can they pass through obstacles on the way to Earth, but they'll also go straight through any kind of conventional telescope or detector as if it weren’t there. (As well as the Earth itself, for that matter.)

Detecting neutrinos

This isn’t an insurmountable obstacle. The reason the neutrinos can pass through solid matter is because they don’t interact via the electromagnetic force—the force that prevents solid objects from falling through the ground. But they’re not totally immune to interaction; every so often, by sheer chance, a neutrino traveling through solid matter will interact with a particle due to another force, the weak force. When it does, it produces a flash of light that can be detected.

Many neutrino detectors have essentially been large tanks of water lined with something to pick out these flashes of light. With a big enough tank, statistically, neutrinos will interact with the water often enough for a consistent rate of detections. IceCube takes a different approach. Instead of building an entire tank of water, it takes advantage of naturally occurring ice at the South Pole.

IceCube is actually an array of detectors running down to more than 1,400 meters deep within the ice of Antarctica. Even at that depth, neutrinos produced when other particles hit the atmosphere are detected roughly three thousand times per second. In order to learn about the cosmos, the IceCube team routinely sifts through that noise to find neutrinos originating elsewhere.

Cosmic rays

Something neutrino astronomy might be able to explain are the cosmic rays that constantly bombard Earth's atmosphere. These “rays” are actually particles—specifically, protons and other heavy atomic nuclei—but their origins are hard to pin down. Unlike neutrinos, tracing the path of a cosmic ray isn’t as simple as determining which direction it's coming from.

That’s because cosmic rays have a positive electrical charge, and so they’re affected by magnetic fields, which bend their trajectories. Since they haven’t been moving in a straight line, the direction they’re coming from when they reach us isn’t necessarily the direction of their origin.

But some of the cosmic rays should be bumping into other matter and radiation near their birthplace, and those interactions should produce neutrinos. Detecting those neutrinos, therefore, would be an indirect confirmation that cosmic rays are being generated there.

Possible sources for cosmic rays include gamma ray bursts generated by massive stars collapsing into black holes. During these collapses, shock waves rebound outward, accelerating charged particles, creating one possible source of cosmic rays. In 2013, IceCube detected neutrinos that were a match for what we had predicted would be generated by these events.

So IceCube then began searching its data for neutrinos coming from the same direction as known gamma ray bursts. But after checking over a thousand such events, there were no neutrino detections. That doesn’t mean the gamma ray bursts aren’t producing cosmic rays and neutrinos, but if they are, they're contributing less than one percent of the observed flux of them.

So researchers began to focus on another explanation for the cosmic rays: active galactic nuclei. These are galaxies with central supermassive black holes that are consuming matter quickly; the energetic environment of their accretion disks could be producing both cosmic rays and neutrinos.

Dark matter

Neutrino astronomy could also provide clues as to the nature of the mysterious dark matter that holds together galaxies and the large-scale structure of the Universe. While there is a plethora of candidates for dark matter, the leading one is WIMPS, or Weakly Interacting Massive Particles.

Like neutrinos, WIMPs would pass through most matter because they interact only weakly. In fact, neutrinos qualify as dark matter, though they have too little mass for the effects we see.

But WIMPs might decay into neutrinos, so the IceCube team also searches for sources in regions with a high density of dark matter. One such region, perhaps surprisingly, is the Sun. Over the Sun’s lifespan, its gravity is thought to have attracted a lot of WIMPs.

Normally, WIMPs go too fast to be captured by the Sun’s gravity. But since WIMPs are weakly interacting, they would occasionally bump into a particle in the Sun, which may slow them down enough to be caught by the Sun’s gravity. Over time, the Sun would collect enough WIMPs to balance their neutrino-producing decays, leading to a sort of equilibrium.

There are neutrinos being produced in the Sun by other means, but WIMP-generated neutrinos would have more energy because of the relatively high mass of the WIMP particle. “The beauty of the indirect detection technique using neutrinos originating from the Sun is that the astrophysics of the problem is understood,” writes Halzen. Unlike looking for signs of WIMPs in other places, it doesn’t matter precisely how the dark matter is arranged.

Elsewhere, if researchers look for such a signal and don’t find it, it could just be because the WIMPs aren’t as dense in that part of the galaxy as we’d assumed. But in the Sun, failing to find the signal would unambiguously mean WIMPs of a given type are not there.

Observations

We now have six years of data from IceCube, composed of roughly 100 detections of cosmic neutrinos. So far, these neutrinos seem to be isotropic: that is, they’re coming in roughly equal numbers from all directions in the sky. That doesn’t tell us much about their origins, but their energy does.

If these neutrinos are produced by cosmic rays, their energy should be roughly equal to that of cosmic rays. Extraordinarily, IceCube’s measured energy density of cosmic neutrinos is a close match for that of cosmic rays. Gamma rays, which would also be produced in the same reactions, are also a good match, based on data from the Fermi Gamma-ray Space Telescope.

This is interesting, Halzen notes, because recent research has suggested that the biggest producers of the diffuse gamma rays detected by Fermi are blazars. Blazars are active galactic nuclei with jets that happen to be pointed toward us. That doesn’t necessarily mean that blazars are the source of the cosmic rays and neutrinos, and in fact some searches have turned up nothing. But IceCube will be able to tell us definitively as more data comes in.

As for dark matter, IceCube has turned up no signal from WIMPs from the Sun as of yet and has thus put “world-leading” constraints on their properties, according to Halzen.

More generally, IceCube has proven that neutrino astronomy can be productive. It has also validated the instrument's design; the ability to place detectors more than a hundred meters apart and still reliably detect flashes of light between them makes it a powerful tool for studying the cosmos.

Because IceCube is so effective, work has already begun on designing a next-generation detector using the same principles. IceCube-Gen 2 will be much larger and hence much more sensitive than IceCube. The future of neutrino astronomy looks bright.

Nature Physics, 2015. DOI: doi:10.1038/NPHYS3816 (About DOIs)