Neutrinos are minuscule particles that rarely interact with other matter and skim unimpeded through Earth – and us – at a rate of trillions per second. We’ve detected the signatures of neutrinos from space with a vast range of energies, but we don’t know precisely where they come from.
Now, and Walter Winter at the German Electron Synchrotron in Zeuthen have devised a model that explains where neutrinos come from based on their energies and the directions they traveled to Earth.
They used data from IceCube, the world’s largest neutrino detector, which resides beneath the ice in Antarctica. Instead of assuming that all neutrinos are created in the same type of environment, as many have, Palladino and Winter found that four sources can account for the different energy levels and abundances of the particles.
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“The neutrinos that we’ve detected at IceCube are unlikely to have a simple one-shot explanation,” says at the University of Wisconsin-Madison.
The slowest of the bunch
The lowest energy neutrinos are created when protons in cosmic rays smash into Earth’s atmosphere. At slightly higher energies, there is a small discrepancy in where they land on Earth – more hit the southern hemisphere than the northern – implying they come from different locations. Because the southern hemisphere faces the centre of the Milky Way, Palladino and Winter claim that neutrinos at these energies originate within our galaxy.
But these aren’t the biggest population of neutrinos we detect. Most of the neutrinos not produced in our atmosphere come from distant galaxies. The same number arrive from every direction, but we’ve never been able to track their origin with telescopes. So, Palladino says they must come from an abundant type of celestial object that is not particularly bright, such as starburst galaxies.
Entering the fast lane
We’ve detected only one extraordinarily high-energy neutrino, which lit up IceCube’s detectors with 4.5 million billion electron volts. To carry that much energy, the particle would have to be created in an extreme environment like a supermassive black hole or a star being ripped apart. These areas do not have as much matter for cosmic rays to interact with, so Palladino says neutrinos could be formed there as cosmic rays hit photons of light.
at the Fermi National Accelerator Laboratory in Batavia, Illinois says we may not have strong enough data to make these conclusions. “I’m sceptical that some of these observations should be treated as hard facts,” he says. “Some of them are just unconfirmed hints.” He says that with more data there might be a simpler solution than the varied sources Palladino’s model suggests.
IceCube has plans for a bigger, more sensitive detector that should be able to help us figure it out. “In the next few years we’ll have a pretty good approximation of where neutrinos come from,” Hooper says. “We’re closing in on the answer.”
Reference: arXiv,
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