The building that houses the IceCube servers.
Neutrinos have little mass and no charge, which means that the usual ways to accelerate particles don’t work. Yet something somewhere in space was pushing someone to energies a thousand times higher than we can achieve in the Large Hadron Collider. And we only know that because we’ve finally built a detector that can detect high-energy neutrinos as they travel through the Earth.
In a recent article in the magazine nature physics, Francis Halzen, the principal investigator of the IceCube detector, discussed current efforts to learn more about the Universe using neutrinos. It turns out that neutrinos are surprisingly informative about the origin of cosmic rays and possibly dark matter as well.
Neutrinos are a fantastic tool for astronomy. Their properties – no charge and very little mass – mean they can arrive here on Earth unencumbered by almost anything between their source and Earth. For example, neutrinos generated in the sun can travel outward much faster than photons, which spend time interacting with the sun’s matter.
Neutrinos that form near black holes can escape their chaotic accretion disks. If they come from a part of the universe obscured by gas or dust, like the center of the Milky Way, they can travel through that material as if it weren’t there. Pretty much anything you put in their way, they just go through; even if you had lead mass a light year thick, it would only block half of the neutrinos trying to pass through it.
Once the neutrinos get here, researchers can use them to learn more 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 obstacles on their way to Earth, but they also pass right through any sort of conventional telescope or detector as if they weren’t there. (As well as the Earth itself, for that matter.)
Detect neutrinos
This is not an insurmountable obstacle. The reason the neutrinos can pass through solid matter is because they don’t interact through the electromagnetic force — the force that prevents solid objects from falling through the ground. But they’re not completely immune to interaction; occasionally, 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 big tanks of water lined with something to pick up these flashes of light. With a large enough tank, neutrinos will interact with the water statistically often enough for a consistent number 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 a series of detectors that run more than 1400 meters deep into the ice of Antarctica. Even at that depth, neutrinos produced when other particles hit the atmosphere are detected about 3,000 times per second. To learn more about the cosmos, the IceCube team routinely searches that sound to find neutrinos coming from elsewhere.
Cosmic rays
Something neutrino astronomy could explain is the cosmic rays constantly bombarding the Earth’s atmosphere. These “rays” are actually particles – specifically protons and other heavy atomic nuclei – but their origin is difficult to pinpoint. Unlike neutrinos, tracking 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 don’t move in a straight line, the direction they come from when they reach us is not necessarily the direction of their origin.
But some of the cosmic rays should collide with other matter and radiation near their birthplace, and those interactions should produce neutrinos. Detecting those neutrinos would therefore be indirect confirmation that cosmic rays are being generated there.
Possible sources of cosmic rays are gamma-ray bursts generated by massive stars collapsing into black holes. During these collapses, shock waves bounce back outward, accelerating charged particles, creating a possible source of cosmic rays. In 2013, IceCube detected neutrinos that matched what we predicted these events would generate.
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 out are not produce cosmic rays and neutrinos, but when they do, they contribute less than one percent of its observed flux.
So researchers began to focus on another explanation for the cosmic rays: active galactic nuclei. These are galaxies with central supermassive black holes that quickly eat up matter; the energetic environment of their accretion disks could produce both cosmic rays and neutrinos.
Dark Matter
Neutrino astronomy could also provide clues to the nature of the mysterious dark matter that holds galaxies together and to the large-scale structure of the Universe. While there are a plethora of dark matter candidates, the main 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, even though they lack mass for the effects we see.
But WIMPs can decay into neutrinos, so the IceCube team is also looking for sources in regions with a high density of dark matter. One such region, perhaps surprisingly, is the sun. Gravity is thought to have attracted many WIMPs over the lifetime of the sun.
Normally, WIMPs move too fast to be caught by the sun’s gravity. But since WIMPs have a weak interaction, they occasionally collide with a particle in the sun, which could slow them down enough to be caught by the sun’s gravity. Over time, the sun would accumulate enough WIMPs to balance their neutrino-producing decay, leading to some sort of equilibrium.
Neutrinos are produced in the sun in other ways, but neutrinos generated by WIMP would have more energy due to the relatively high mass of the WIMP particle. “The beauty of the indirect detection technique using neutrinos emanating from the sun is that the astrophysics of the problem are understood,” Halzen writes. Unlike looking for signs of WIMPs elsewhere, it doesn’t matter exactly how the dark matter is arranged.
If researchers look for such a signal elsewhere and don’t find it, it may be because the WIMPs in that part of the galaxy are not as dense as we assumed. But in the sun, not finding the signal would unequivocally mean that WIMPs of some type are not there.
Observations
We now have six years of IceCube data, compiled from about 100 detections of cosmic neutrinos. So far, these neutrinos appear to be isotropic: that is, they come in roughly equal numbers across the sky from all directions. 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 closely matches 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 largest producers of the diffuse gamma rays detected by Fermi are blazars. Blazars are active galactic nuclei with jets that happen to be aimed at 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 so far not picked up a signal from WIMPs from the sun, and thus has placed “leading edge” constraints on their properties, according to Halzen.
More generally, IceCube has proven that neutrino astronomy can be productive. It also validated the design of the instrument; the ability to space 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 therefore much more sensitive than IceCube. The future of neutrino astronomy looks bright.
nature physics2015. DOI: doi:10.1038/NPHYS3816 (About DOIs)