Neutrinos are the enigmatic ghosts of the subatomic world.
They only rarely interact in our detectors and have surprised scientists more than once.
For instance, while the original idea of neutrinos supposed that there was only a single kind
of neutrino, it turns out that there are actually three distinct variants.
One class of neutrino is associated with electrons and is called the electron type neutrino.
The other two kinds of neutrinos are associated with cousins of the electron, the muon and
the tau.
Accordingly, these other types are called muon neutrinos and tau neutrinos.
Scientists use the lower case Greek letter nu to indicate a neutrino, with a subscript
to tell you what kind it is.
The way scientists discovered that there were different kinds of neutrinos is that neutrinos
seemed to remember their origins.
For instance, an experiment in 1962 created neutrinos in tandem with a muon.
If one of those neutrinos was then collided into an atomic nucleus, only muons were generated
in the collision...never electrons and never tau particles.
The neutrino remembered how it was made.
This observation led to a Nobel Prize in physics in 1988.
With the observation that neutrinos have distinct types, scientists thought they understood
neutrinos reasonably well.
However, they continued to study them.
In 1964, one scientist wanted to study neutrinos originating in the biggest nuclear reactor
around; the Sun.
Raymond Davis was a chemist by trade.
He knew that neutrinos could interact with chlorine and make argon.
He also knew that neutrinos interacted very weakly and he'd need a huge number of chlorine
atoms to make it work.
So he took a huge vat of perchloroethylene, which is just a scientific name for dry cleaning
fluid.
The vat contained one hundred >>thousand<< gallons of liquid.
That's about the size of an Olympic sized swimming pool.
He calculated that for every week of operation that he could expect to create ten atoms of
argon.
Yes, you heard me right.
Ten atoms.
1, 2, 3, 4, 5, 6, 7, 8, 9, 10.
Ten
Now that number should have blown your mind.
His vat contained about 9 million-million, million-million [pause] million atoms of chlorine
and ten of them should converted to argon.
This just sounds impossible and yet it turned out that Davis could do just that.
So, what did he find?
Well it turns out that he didn't find the ten atoms he expected.
He only found three.
The easiest explanation was that either the prediction or the measurement was wrong and
yet many follow-on experiments confirmed his result.
He was detecting fewer neutrinos than expected.
This came to be called the solar neutrino deficit.
You should be forgiven if you now believe that somehow the entire field of neutrino
physics made a huge mistake, however, another class of experiments told a similar tale.
Another source of neutrinos come from cosmic rays, which is a constant pelting of high
energy protons from the deepest of space slamming into the atmosphere.
Because of how the cosmic rays interact, each electron-type neutrino should be accompanied
by two muon-type neutrinos.
While the solar neutrino deficit could have been due to improper measurement or calculation,
it is very difficult to imagine how neutrinos from cosmic rays could occur in any ratio
other than 1 electron type to 2 muon type.
So, what was measured?
Well, different experiments observed different results, but it was generally true that there
were fewer muon neutrinos than expected.
Another mystery had appeared, this one called the atmospheric neutrino problem.
In the mid- to late- 1950s, Italian-born physicist Bruno Pontecorvo hypothesized that it would
be possible for the different flavors of neutrinos to oscillate into one another.
If the idea was true, then a bunch of electron neutrinos could gradually morph into muon
neutrinos and then back again to electron neutrinos.
At the time he came up with the idea, only one kind of neutrino was known to exist, so
his proposed oscillations were of neutrinos into their antimatter equivalents.
However, we now know that the right way to think about neutrino oscillation is between
the three distinct types.
The first compelling evidence for neutrino oscillations came in 1998 using the SuperKamiokande
experiment in Japan.
This detector is a huge, underground cavern, filled with 50,000 tons of water, surrounded
by detectors called phototubes.
Rare neutrinos would interact in the water and give off a blink of light.
Using that blink of light, you could identify the trajectory of the neutrino.
By separating out neutrinos created in the atmosphere directly above (which was about
12 miles away) from neutrinos created on the other side of the Earth (which was about 8,000
miles away), they proved that it was the neutrinos that travelled a large distance that had changed
identity the most.
The SNO experiment in a mine deep under Sudbury Ontario clinched it.
Neutrinos were changing their identity.
In the past decade, scientists have studied neutrino oscillations using beams of neutrinos,
made at particle physics laboratories.
The Fermilab accelerator near Chicago, the CERN accelerator near Geneva, Switzerland
and the KEK accelerator in Japan all fire beams of neutrinos through the Earth to targets
hundreds of miles away.
These experiments have made tremendous headway in understanding the phenomenon of neutrino
oscillation.
However, there remain significant questions in the details of neutrino oscillations.
The next decade is expected to be the era in which neutrinos tell us their enigmatic
tale.