Neutrinos have a reputation for playing hard to get. Tiny, sub-atomic particles that have no charge and close to zero mass, they usually slip past Earth's atoms leaving barely a trace of their presence.
But not all run past unscathed. New research confirms highly energised 'ghost particles' can be stopped dead in their tracks, a discovery that doesn't surprise theorists but could open new ways toward seeing the most hidden parts of our planet.
An international group of researchers making up what's known as the IceCube collaboration has set a world first in directly measuring Earth's absorption of neutrinos.
The fact that neutrinos can interact with other bits of matter, and therefore theoretically be absorbed, isn't exactly a revelation.
In fact, the Standard Model of physics – that awesome theory that predicts how all particles should behave – tells us exactly what to expect.
Neutrinos are certainly hard to catch, with literally tens of trillions of them flying through our bodies every second as they radiate out of the Sun and down from the cosmos without so much as pause to offer an atomic handshake.
But we know that the higher the energy of the neutrino, the greater the chance it has of being absorbed by a proton or neutron in an atom's nucleus.
"We always say that no particle but the neutrino can go through the Earth," says physicist Francis Halzen from the University of Wisconsin-Madison, and principal investigator with IceCube.
"However, the neutrino does have a tiny probability to interact, and this probability increases with energy."
This chance of interaction is called its cross-section. The problem isn't that we don't know about it – it's that we can't produce neutrinos with enough energy to push them into a cross-section we can study.
Particle accelerators on Earth just won't cut it. The only accelerators capable of producing such energetic neutrinos are the ones operating at a cosmic scale – we're talking supermassive black holes and the cores of star-forming galaxies.
Neutrinos showering down from space, as well as a bunch produced by cosmic rays smacking into our atmosphere at high speed, leave the paltry particles we can produce in the dust.
So the researchers used 5,160 detectors the size of basketballs scattered throughout a cubic kilometre of ice beneath the South Pole to catch signs of these supercharged neutrinos.
The detectors work by measuring faint flashes of radiation that occurs when the occasional neutrino bumps into another particle.
This isn't a common event. Remember how trillions are slipping through you right this moment? The researchers analysed 10,784 flashes detected across an entire year.
They were able to determine the energies of these particles, and found there were fewer high-energy neutrinos pushing their way through the planet from the direction of the northern hemisphere.
This allowed them to work out the cross-section for neutrinos in the field of 6.3 to 980 teraelectronvolts (TeV). To put it into perspective, the best we've managed to generate with human-made accelerators have been neutrinos at around 0.4 TeV.
"What we measure is consistent – up to now – with what is expected," says Halzen.
"We were of course hoping for some new physics to appear, but we unfortunately find that the Standard Model, as usual, withstands the test."
Oh well. No discoveries of hidden dimensions for us, then.
But let's look on the bright side. It might lead to is a way to better visualise our planet's core. In spite of the vivid artistic renditions in our text books, we're still a little foggy on what Earth looks like far beneath our feet.
There's no room for disappointment when it comes to getting a better grip on these so-called ghosts of the particle zoo.
This research was published in Nature.