The Neutrino: A Guide To The Invisible Particle That Has Astronomers So Excited

SNOLAB detector
The neutrino detector in the SNO lab in Ontario. SNOLAB

This month astrophysicists found new evidence of dark matter — the so far invisible substance that scientists believe makes up 80% of our known universe.

The astrophysicists detected strange X-ray emissions from neighbouring galaxies. They think the X-rays are from a decaying neutrino — but an entirely new kind of neutrino that only exists in theory.

Not only could they be the direct evidence of dark matter scientists have hunted for 80 years, neutrinos are getting a lot of attention in other fields. They could become the building block for some incredible new technology.

At the end of 2013, scientists detected neutrinos coming from outside the solar system for the first time ever, opening up a new frontier in “neutrino astronomy” and the editors of the journal Science just named neutrinos as an area of research to watch in 2014.

So, what is a neutrino?

Neutrinos are fundamental particles of matter that are created by a specific type of radioactive decay, called beta decay, and from nuclear reactions like the ones that take place in the sun and nuclear reactors. Neutrinos were also produced during the Big Bang.

Scientists know there are three different types of neutrinos — the different kinds exist because the particles spin in different patterns.

It seems like we know a lot about these particles, but really we don’t, considering we’re basically swimming in them.

“In seconds, trillions of these particles are streaming through your body,” astronomer Ray Jayawardhana, author of “Neutrino Hunters: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe,” said at a book signing on Dec. 10 hosted by the Secret Science Club.

Neutrinos are hard to pin down because they have almost no mass and interact weakly with other particles. It takes a giant chamber, like the one pictured above, full of extremely hot liquid hydrogen for scientists to be able to detect the presence of these particles.

The discovery of neutrinos.

Though it has been theorized since 1930, we didn’t discover the neutrino (in physical reality) until 1953. But its discovery was fundamentally important — it saved one of the most basic laws of science: The law of conservation of energy.

During beta decay, a neutron turns into a proton and spits out an electron. When scientists first measured what happens during beta decay they ran into a problem: The electrons were emitted with less energy than the reaction started with. It looked like energy was being lost somewhere in the beta decay process.

According to the law of conservation of energy, any reaction has to produce as much energy as it starts with. The scientists could not figure out an explanation for it, and some even considered giving up on the law.

But in 1930 the physicist Wolfgang Pauli came up solution, one that was considered insane for the time. He theorized that there must be another particle being emitted, it was just a particle that no one could see or detect.

According to Jayawardhana, Pauli had a great sense of humour about his crazy theory. He actually bet a case of champagne against himself and the ability of anyone to be able to detect the particle ever.

Finally, in the 1950s scientists confirmed that neutrinos exist.

Neutrino collision

Generally, neutrinos pass right through the ground. But a small fraction of neutrinos will actually hit something — sending out decay products that we can measure. There are several different types of neutrino detectors, but the basic idea is to catch a large number of neutrinos and hope that some of these will crash into atoms.

The image on the right shows the first recorded observation of a neutrino. The photograph was taken inside the chamber of a neutrino detector in the Argonne National Laboratory outside Chicago.

The spot where the invisible neutrino collides with a proton is circled in red.

Types of neutrinos.

Scientists know there at least three different kinds of neutrinos: electron, muon, and tau. The different types are referred to as “flavours” and they correspond with the way the particle is oscillating. Jayawardhana said the particles change flavours all the time.

“It’s as if chocolate flavored ice cream changed to vanilla, then to strawberry, then back to chocolate — it really is that strange,” Jayawardhana said.

The scientists detected the particle by mixing cadmium chloride in a tank of water. Cadmium absorbs neutrons really well. When the neutrinos collide with protons they produce neutrons and positrons. The positrons then collide with electrons and give off gamma rays.

Now that scientists have built more sophisticated neutrino detectors that can reliably detect all three flavours of these particles and study them, Jayawardhana says neutrinos have tons of potential.