Why Neutrinos, which barely exist, attract so much Nobel attention
Neutrinos take patience. They鈥檙e worth it, and the announcement of the recognizes that, following related prizes in , and . I could begin by telling you that neutrinos are elementary particles, but that sounds condescending. They鈥檙e not called elementary because they鈥檙e easy to understand 鈥� they aren鈥檛 鈥� but because they are seemingly point-like in size, and we can鈥檛 break them down into smaller constituents. There鈥檚 no such thing as half a neutrino.
The smallest things in the universe
Atoms, despite the Greek name (鈥渃annot be cut鈥�), are not elementary particles, meaning they can be disassembled. An atom is a diffuse cloud of electrons surrounding a tiny, dense nucleus composed of protons and neutrons, which can be broken into up and down quarks.
Particle colliders, which accelerate particles to near the speed of light and smash them together, help us discover new elementary particles. First, because of E = mc2 , the energy in the collision can be converted into the mass of particles. Second, the higher the accelerator鈥檚 beam energy, the more finely we can resolve composite structures, just as we can see smaller things with X-rays than with visible light.
We haven鈥檛 been able to take apart electrons or quarks. These are elementary particles, forming the basic constituents of ordinary matter: the Lego bricks of the universe. Interestingly, there are many heavy cousins of familiar particles that exist only for fractions of a second, and thus are not part of ordinary matter. For example, for electrons these are the muon and tauon.
What鈥檚 a neutrino?
How is this elementary particle 鈥� the neutrino 鈥� different from all other elementary particles? It鈥檚 unique in that it鈥檚 both almost massless and almost noninteracting. Those features are different, though often conflated (don鈥檛 take advice about neutrinos from a poet, even it is ).
It鈥檚 a mystery why neutrinos are almost, but not quite, massless. We do know why they鈥檙e almost noninteracting, though: They don鈥檛 feel the electromagnetic or strong forces that bind nuclei and atoms, only the aptly named (and gravity, but barely, because their masses are small).
Though neutrinos are not constituents of ordinary matter, they are everywhere around us 鈥� a trillion from the sun pass through your eyes every second. There are hundreds per every cubic centimeter left over from the Big Bang. Because they so rarely interact, it鈥檚 almost impossible to observe them, and you certainly don鈥檛 feel them.
Neutrinos have other weird aspects. They come in three types, called flavors 鈥� electron, muon and tauon neutrinos, corresponding to the three charged particles they pair with 鈥� and all of these seem to be stable, unlike the heavy cousins of the electron.
Because the three flavors of neutrinos are almost identical, there is the theoretical possibility that they could change into each other, which is another unusual aspect of these particles, one that can reveal new physics. This transformation requires three things: that neutrino masses are nonzero, are different for different types, and that neutrinos of definite flavor are quantum combinations of neutrinos of definite mass (this is called 鈥渘eutrino mixing鈥�).
For decades, it was generally expected that none of these conditions would be met. Not by neutrino physicists, though 鈥� we held out hope.
Doing astronomy with invisible particles
In the end, nature provided, and experimentalists discovered, supported by calculations from theorists. First came decades of searching by many experiments, with important hints to encourage the chase.
Then, in , the in Japan announced strong evidence that muon neutrinos produced in Earth鈥檚 atmosphere change to another type (now thought to be tauon neutrinos). The proof was seeing this happen for neutrinos that came from 鈥渂elow,鈥� having traveled a long distance through Earth, but not for those from 鈥渁bove,鈥� having traveled just the short distance through the atmosphere. Because the neutrino flux is (nearly) the same at different places on Earth, this allowed a 鈥渂efore鈥� and 鈥渁fter鈥� measurement.
In and , the in Canada announced strong evidence that electron neutrinos produced in the core of the sun also change flavors. This time the proof was seeing that electron flavor neutrinos that disappeared then reappeared as other types (now thought to be a mix of muon and tauon neutrinos).
Each of those experiments saw about half as many neutrinos as expected from theoretical predictions. And, perhaps fittingly, Takaaki Kajita and Arthur McDonald each got half a Nobel Prize.
In both cases, quantum-mechanical effects, which normally operate only at microscopic distances, were observed on terrestrial and astronomical distance scales.
As the front page of The New York Times in 1998, 鈥淢ass Found in Elusive Particle; Universe May Never Be the Same.鈥� These clear indications of neutrino flavor change, since confirmed and measured in detail in laboratory experiments, show that neutrinos have mass and that these masses are different for different types of neutrino. Interestingly, we don鈥檛 yet know what the values of the masses are, though other experiments show that they must be about a million times smaller than the mass of an electron, and perhaps smaller.
That鈥檚 the headline. The rest of the story is that the mixing between different neutrino flavors is in fact quite large. You might think it鈥檚 bad news when predictions fail 鈥� for example, that we would never be able to observe neutrino flavor change 鈥� but this kind of failure is good, because we learn something new.
International society of neutrino hunters
I鈥檓 delighted to see this recognition for my friends Taka and Art. I wish that several key people, both experimentalists and theorists, who contributed in essential ways had been similarly recognized. It took many years to construct and operate those experiments, which themselves built on slow, difficult and largely unrewarding work going back decades, requiring the effort of hundreds of people. That includes major US participation in both Super-Kamiokande and the Sudbury Neutrino Observatory. So, congratulations to neutrinos, to Taka and Art, and to the many others who made this possible!
When I first started working on neutrinos, over 20 years ago, many people, including prominent scientists, told me I was wasting my time. Later, others urged me to work on something else, because 鈥減eople who worked on neutrinos don鈥檛 get jobs.鈥� And, even now, plenty of physicists and astronomers think we鈥檙e chasing something almost imaginary.
But we鈥檙e not. Neutrinos are real. They鈥檙e an essential part of physics, shedding light on the origin of mass, the of the universe, and perhaps the existence of new forces that are too feeble to test with other particles. And they are an essential part of astronomy, revealing the highest-energy accelerators in the Universe, what鈥檚 inside the densest stars, and perhaps new and otherwise unseen astrophysical objects.
Tiny particles, big mysteries
Why should you care, beyond sharing our curiosity about revealing some of the weirdest things in the universe?
The weak force that neutrinos feel is what changes protons to neutrons, powering nuclear fusion reactions in the sun and other stars, and creating the elements that make planets and life itself possible.
Neutrinos are the only component of dark matter that we understand, and figuring out the rest will help us understand the structure and evolution of the universe. If the neutrino masses had been much larger, the universe would look much different, and perhaps we wouldn鈥檛 be here to see.
Finally, if you are purely practical, neutrino physics and astrophysics is one of the most difficult jobs, requiring us to invent incredibly sensitive detectors and techniques. This knowledge has other uses; for example, using a neutrino detector, we could tell if a purported nuclear reactor is on, what its power level is and even if it is producing plutonium. This may have some real-world applications.
The past decades in neutrino physics and astronomy have been great, but some of the most exciting things are just starting to happen. The at the South Pole is now seeing high-energy neutrinos from outside our galaxy. Super-Kamiokande has announced a plan, based on a from me and Mark Vagins, to improve their sensitivity to antineutrinos compared to neutrinos. And the international community hopes to build a major new neutrino facility, in which a powerful beam of neutrinos will be sent from Fermilab in Illinois to a detector deep underground in the Homestake mine in South Dakota. Who knows what we鈥檒l find?
And that鈥檚 what I鈥檝e really been waiting for.
John Beacom is a Professor of Physics, Professor of Astronomy, and Director of the Center for Cosmology and AstroParticle Physics (CCAPP) at Ohio State University. Read the at The Conversation.
