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.
