Physicists are hunting for a particle that they hope could clue us in on some of the biggest mysteries in the universe. Questions like:
Why the heck do we exist?
But after scouring an entire year’s worth of data from the largest particle detector on the planet, scientists at IceCube Neutrino Observatory have some bleak news. They’re 99% sure the particle doesn’t exist.
Let’s start at the beginning
You might not know it, but you’ve probably met a neutrino before. One hundred trillion of these tiny, speedy particles shoot (harmlessly) through your body every second.
But neutrinos, nicknamed the “ghost particle,” are known for being kind of aloof — they rarely interact with matter. In fact, they can sprint through an entire light year of lead without stopping.
To get to know these elusive neutrinos, scientists have built some of the largest experiments in the world. One lesson they have learned so far is that neutrinos come in three “flavours,” or types, which the particles oscillate between as they move through space.
But in 1995, an experiment called the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory detected hints of a mysterious fourth flavour of neutrino lurking just out of reach.
Meet Elvis, or the sterile neutrino
Now, meet the sterile neutrino, a prize scientists have had their eyes on for two decades.
Unlike other neutrinos, the hypothetical sterile neutrino wouldn’t interact with matter at all, and would only feel the force of gravity. So detecting this particle would be a little bit like chasing the shadow of a ghost.
“Like Elvis, people see hints of the sterile neutrino everywhere,” Francis Halzen, a University of Wisconsin-Madison professor of physics and the principle investigator for the IceCube Neutrino Observatory, said in a University of Wisconsin press release. “There was this collection of hints and theorists were convinced it exists.”
Now, in a study published Monday in the journal Physical Review Letters, IceCube researchers come to a pretty disappointing conclusion. Their results very strongly suggest that the sterile neutrino hinted at in LSND does not exist. The researchers were able to state this with 99% confidence, which means that there is only a 1% chance that their findings are a result of random luck. The researchers combed through two independent sets of a year’s worth of data, each set consisting of about 100,000 neutrino events. There was no trace of the sterile neutrino anywhere.
A beast of a detector
IceCube isn’t the first experiment in which scientists have turned up empty handed in the hunt for the sterile neutrino. But it
is the most powerful experiment of its kind, putting stronger constraints on the hypothetical particle than any previous experiment has ever done before.
IceCube is a beast of a detector. It consists of 5,160 light detecting sensors frozen in a billion tons of Antarctic ice — all of it tucked deep (more than a mile) beneath the South Pole.
When neutrinos crash into water molecules in the ice, they release high-energy eruptions of subatomic particles that can stretch as far as six city blocks, the physics publication Symmetry reported recently. These particles move so quickly that they emit a brief cone of light, called Cherenkov radiation. That’s what IceCube’s detectors pick up.
In its hunt for sterile neutrinos, IceCube investigates neutrinos coming from the opposite end of the earth, at the North Pole. Since neutrinos behave differently when they pass through dense matter, when these neutrinos pass through the tightly packed core of the earth, they act a little extra funky, allowing IceCube’s detectors to pick up a signal that would have been lost otherwise.
But detecting a sterile neutrino is tricky work. The only way to observe one is by paying attention to its infamous oscillation between its three known flavours. Any neutrino changing into this fourth flavour would fall off the radar.
So, since researchers at IceCube know how many neutrinos they should expect to see in a given period of time, they know they’d some neutrinos disappear, “vanishing into nothingness,” Carlos Argüelles, a postdoc at MIT and one of the leading researchers in this study, told Business Insider, if sterile neutrinos were real.
“We’re looking for a very small effect,” Ben Jones, a postdoc at the University of Texas at Arlington and another one of the leading researchers in this study, told Business Insider. “But IceCube happens to be looking at neutrino in the right energy range coming from the right places that there would a huge amplification of the effects of the sterile neutrino, if it existed.”
But no neutrinos turned up missing in IceCube’s data. Which means if there are sterile neutrinos out there, they’re wildly different from what we’ve imagined.
“Of course it’s more exciting to find things than not to find them,” Argüelles said. “But as we try to find new physics somewhere out there in the neutrino field, we have to look in many directions. And if you don’t find it in one direction, it narrows down where you can look. You have to come up with ways to make neutrinos survive new constraints.”
Why do we exist?
So what would this obscure flavour of neutrino mean for the field of physics?
A lot, it turns out. These neutrinos might clue us in to where dark matter, the invisible form of matter making up more than four-fifths of the mass in the universe, comes from.
And they might even explain why we live in a world dominated by matter.
During the Big Bang, which created our universe as we know it, there should have been equal amounts of matter and antimatter. But when matter and antimatter collide, they annihilate each other leaving behind nothing but energy. And yet here we are: Beings made of matter on a planet made of matter in a universe made of matter.
Neutrinos shouldn’t have mass, but they do. Scientists suspect that the itsy, bitsy mass of the neutrinos might have something to do with matter winning the battle of the Big Bang. And they’d been hoping that the existence of a fourth flavour of neutrino would have led them to answers about where the neutrino mass comes from and why it’s so small.
A crack in the pillar of particle physics
Sterile neutrinos might also reveal a crack in the pillar of particle physics, the Standard Model, which only allows for three flavours of neutrinos.
“For the longest time since about the 1970s we thought we had a very well understood picture of particle physics called the Standard Model,” Jones said. “Many of the particles in it were predicted before they were discovered and almost like clockwork they were discovered where we expected them.”
The most recent example of this, Jones said, was the Higgs Boson. Scientists knew what they were looking for and they went out and found it.
“But the sterile neutrino was a particle that no one expected,” Jones said. “If it had been confirmed it would have been the first example of a particle outside the Standard Model. It would have been an insight into new physics and potentially a path forward for the entire field after the final completion of the Standard Model.”
More work to be done
Although IceCube’s experiment rules out the possibility of a sterile neutrino with a light mass, which is what was suggested by LSND, Argüelles said there are still some surviving islands of possible solutions that need to be targeted.
“It’s an extremely powerful result and it puts a very strong constraint on sterile neutrinos,” David Schmitz, co-spokesperson for an upcoming sterile neutrino stake-out called the Short-Baseline Neutrino (SBN) program, told Business Insider. “But there’s more work to be done.”
Sterile neutrinos with higher masses are interesting to phsyicists since they naturally lead to the small neutrino masses, Jordi Salvado Serra, a postdoc at the Institute of Corpuscular Physics and another leading researcher in this study, told Business Insider in an email. He added that there are many theories that use heavy sterile neutrinos to explain the abundance of dark matter in the universe and how we ended up in a world ruled by matter.
Experiments like the SBN program will hunt in the small remaining mass region where the sterile neutrino might be hiding.
These experiments will also look for slightly different signals — while IceCube only looked for the disappearance of one flavour of neutrino, the SBN program will also be looking for an excess of a different flavour.
Nature is what nature is
But according to Jones, the sterile neutrino is now disfavored more than it ever has been. And the more experiments that reject it, and the stronger those experiments are, the more remote the possibility of it existing becomes.
Don’t let that get you down though, because the fact that we’re even able to detect any of these elusive particles is still pretty darn amazing.
And, according to the researchers, there’s still a lot of other questions to be answered, and a lot of great neutrino physics to be done at IceCube. The researchers are excited to investigate high energy neutrinos in other ways, as well as to continue improving their constraints on sterile neutrinos with even more data.
“We’re physicists, so we’re not allowed to be disappointed by our results,” Jones said. “Nature is what nature is and it’s a privilege to be able to understand it.”
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