“She was radioactive for seven days
How I wanted to be holding her anyways
But the doctors, they told me to stay away
Due to flying neutrinos and gamma rays”
-Andrew Bird “Puma”
Indifferent toward you and your goings on, ten trillion incredibly small particles called neutrinos pass through your body each second. You never knew this because neutrinos are equally disinterested in how your body’s particles are charged to attract and repel, and they are so tiny (a neutrino’s size is to a speck of dust as a speck of dust is to our solar system) that gravity is largely unable to divert them from their mission: to do nothing but travel in a straight line toward the edge of the universe.
(I’m kidding, they aren’t sentient.)
Neutrinos are subatomic particles produced during radioactive breakdown of atoms. The majority of the neutrinos in the universe have been here since the beginning. They are also created by stars, our own particle accelerators, and phenomena in the atmosphere.
These elusive particles are, as the name suggests, neutral in charge. Actually, they were originally named the neutron before what we now call the neutron was discovered. Enrico Fermi then decided to be cute and call them neutrinos, which means “little neutron” in Italian. Because they are neutral, they are immune to one of the strongest forces in our universe, the electromagnetic force. This means that other particles do not attract or repel them with positive or negative charge. The only force that has any significant effect on neutrinos is the weak subatomic force, and for this force to act on a neutrino would require the neutrino to be extremely close to other particles, a rare occurrence.
In fact, a neutrino could pass through light-years of lead and still never come in contact with another particle. This is because matter is mostly nothing. Though it appears that you, or your table, or your soup, or your grandma are made of something relatively substantial and dense, the atoms that make up these things are pretty much entirely vacuum. And so the neutrino presses on, traveling at very near the speed of light and rarely interacting with any matter at all.
So how do we know anything about them?
During a reaction called beta decay, a neutron becomes a proton and an electron. In 1930, Wolfgang Pauli studied these reactions and found that the energy and momentum the proton and electron gain is less than the original energy and momentum of the neutron. This is inconsistent with the fact that energy and momentum are always conserved, so Pauli hypothesized about a “Ghost Particle” that is also spit out during a beta decay and holds the missing energy.
Decades later in 1959, the electron neutrino was discovered by Clyde L. Cowan and Frederick Reines with a huge vat of heavy water. I would like to point out that part of their experiments were done at the Savannah River Plant in Augusta, GA. (I used to live in Augusta, and I am moving there again soon. Plus my name is Savannah, so I feel like I was part of this experiment.) They received the Nobel Prize for this work in 1995, one year before I was born. (Okay I’m done.)
The particle Cowan and Reines discovered, the electron neutrino, is just one of three different “flavors” of neutrinos. There are also muon and tau neutrinos, which correspond to the muon and tau particles. The flavor of a neutrino becomes apparent only when it goes through a decay process to create other charged particles. And here, just as it does with the entire “small realm” of the universe, quantum makes it weird. We can only either know a neutrino’s flavor or information about its relative mass, but not both.
Neutrino mass has been a topic of intrigue, especially as of late. The Nobel Prize in Physics 2015 was given to Takaaki Kajita and Arthur B. McDonald “for the discovery of neutrino oscillations, which shows that neutrinos have mass.” This discovery was ground-breaking in the world of particle physics, because the Standard Model of particle physics, which has been widely accepted by particle physicists for the past century, predicted that neutrinos had zero mass!
“Neutrino oscillation” is the changing of a neutrino from one flavor to another. In 2001, it was observed at the SNO detector in Canada that two-thirds of the electron neutrinos coming to the detector from the sun had oscillated into the other two flavors before arriving. These oscillations involve quantum mechanical phenomena, and they require a non-zero neutrino mass, though the value of the mass is still unknown.
The fact that neutrinos are massive (tiny, but massive) gives rise to questions about its nature in relation to other particles and the rules they obey. Is the neutrino exempt from the general symmetry rules of the universe? Unlike other particles, does it gain its mass from a source other than the Higgs field? Does the neutrino experience time the same way other particles do? Do neutrinos have anything to do with dark matter (matter that mathematically should be in the universe but has not yet been detected)? Could neutrinos’ neutral charge and non-zero mass be the reason matter has been able to overpower antimatter, leaving us with the universe as we know it?
I’ll expand on that last question. Particles have anti-particles associated with them, and just as particles make up matter, anti-particles make up anti-matter. These anti-particles have the same mass but opposite charges (except in the case of the neutron’s anti-neutron which has a neutral charge but is made of anti-quarks as opposed to quarks) as their matter partners. Neutrinos are unusual particles in that they are not made of any smaller particle that we know of, and they are also neutrally charged. So what could be different about the neutrino and the antineutrino? It could be that a neutrino its own antiparticle. This would raise deep questions about symmetry, but it may answer questions, too.
Any time anti-matter collides with matter, both are annihilated, leaving nothing. It is unknown why the anti-matter and matter in the universe haven’t cancelled each other out yet, or why there would be an unequal amount of each. The answer may involve dark matter, and neutrinos and antineutrinos could provide some insight.
Despite our many unanswered questions about neutrinos, we know an impressive amount about them considering how difficult they are to detect. Ice Cube is a neutrino observatory in the South Pole whose website describes their detectors as “Butterfly Nets for Ghosts.”
Note: Just today, June 22, 2016, two critically ill researchers were rescued from the Amundsen-Scott South Pole Station in which Ice Cube is located. The names of the researchers were not released, but there is a good chance they worked on Ice Cube. This is the third time ever that a successful rescue has been made during the South Pole’s winter season lasting from February to October. It is generally much too dark and cold to risk flying a plane there.
To detect these slippery particles, the observatories are built underground, shielded from the noise of cosmic rays. Some detectors, such as Super-Kamiokande in Japan, use enormous vats of water with photo-multiplier tubes lacing the sides to capture the effects neutrinos leave when they interact with the atoms in the tank. Super-K is filled with 50,000 tons of water and detects about 18 neutrinos a day. It was the primary detector used by the 2015 Nobel Laureates to prove the neutrino’s non-zero mass. The original Kamiokande detector, before Super-K, detected 19 out of the supposed billion trillion trillion trillion trillion neutrinos resulting from the explosion of a star in the Large Megallanic Cloud.
A similar detector called ANTARES lies 2.5 kilometers under the Mediterranean Sea. It consists of twelve long, floating strings of photomultiplier tubes sensing neutrinos that go through the water around it.
Those photomultiplier tubes detect what’s called Cherenkov Radiation. When a neutrino contacts other particles, their energy and mass will turn into different particles themselves. These new particles (electrons, muons, or taus depending on the neutrino’s flavor) travel at a speed faster than light inside that medium. These new charged leptons affect the electrons in the atoms around it as it travels, often leaving a blue ring around its path. This distortion is what the photomultiplier tubes detect. But all in all, only about a few thousand neutrinos are detected a year, and many of these detections have no information about the neutrino’s flavor.
As we discover more about the neutrino, I hope to write updates about them and post them here. These little buggers have opened my eyes to how mind-boggling particle physics can be, and I hope they’ve intrigued you all, too. Regardless though, they’ll keep passing through you by the trillions, slipping through the cracks of your matter and wandering the universe without an atom for a home.
“Do you see particles in the air?
Unguided particles in the air
Do you see particles in the air?
Nobody notices, nobody cares”
-Andrew Bird, “Puma”
I care, Andrew.
Super K Website:
Ice Cube Website:
Nobel Prize Info: