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They're the most
mysterious particles ever discovered,

tiny ghosts hidden in our world.

Now scientists are on a mission
to unlock their secrets.

They're called neutrinos.

The story of their discovery
is almost impossible to believe.

If they had bolted the detector

in place, the nuclear bomb
would've

just smashed it to smithereens.

With links to a
dramatic Cold War defection.

He disappeared
through the Iron Curtain,

and for five years,

disappeared off the face of the
planet.

And astonishing experiments

that keep defying
the laws of physics.

Even as someone
who builds these experiments for a living,

it just seems mind-blowing
that they ever work.

Today,
scientists are using neutrinos

to probe the edges
of our detectable universe.

They're on a mission to reveal
a hidden world

of "Particles Unknown."

Right now, on "NOVA."

We live in a world of matter...

a realm of tiny particles
far smaller than atoms

that build the universe
that we know.

But there is a mystery.

Scientists theorize there
exists a hidden, parallel world

of particles...
so-called dark matter.

So far, no one has managed
to detect a single one.

But now there might be a way.

Of all the particles scientists
have discovered,

the most elusive, on the very
edge of detectability,

are neutrinos.

Neutrinos are
really remarkable particles.

There are trillions
and trillions of them

streaming through our bodies,

and we don't even notice.

They are kind of ghost-like,
and yet they're everywhere.

Everywhere and nowhere.

Neutrinos are so ghostly,
they can pass

through solid matter as if
it didn't exist.

And yet they hold the secrets
to why the stars shine

and what our universe
is made of.

The reason
we care about these elusive particles

is because they do play a
fundamentally important role

in the universe,
in the nature of matter...

in some of the most violent
cosmic phenomena.

First theorized in the 1930s,

they would soon become linked
to nuclear secrets

and a dramatic Cold War
defection

behind the Iron Curtain.

He goes off to Europe

and never returns.

Now the
quest to detect neutrinos

has triggered vast experiments
all over the globe.

Even as someone who builds these
experiments for a living,

it just seems mind-blowing
that they ever work.

Today,
scientists are on the cusp

of an astonishing discovery.

Tantalizing evidence
suggests neutrinos

could be a doorway
between our world of matter

and the hidden world of
dark matter,

waiting to be discovered.

It would be a game-changer.

What exactly are these
particles?

What is its role in the
evolution of our universe?

The quest for answers

has driven scientists
to the edge

of what is experimentally
possible

to reveal a universe
we've never seen before.

Fermilab, in Batavia, Illinois.

World-renowned physics
laboratory.

Thousands of scientists
build enormous experiments

to probe the very smallest
particles

that make up our universe.

Leading one of the teams
is Sam Zeller.

Hi, team.

My interest in physics started
when I signed up

for a field trip to come
to Fermilab in high school.

It just blew my mind.

From that point on, I was
a particle physicist.

It turns out that
the universe can be described

by a small number
of subatomic particles.

Today,
scientists have discovered

17 basic particles
that make up our universe.

Some are the building blocks
of atoms.

Others are the things
that hold matter together.

It's an understanding of
our world that physicists call

the Standard Model.

The Standard
Model of particle physics

describes the most fundamental
constituents of matter

and how they interact
with each other.

It is in fact the most
mathematically well-defined

physical theory we as humans
have ever written down.

For 50 years,

the Standard Model
has withstood test after test,

confirming the hierarchy of all
the fundamental particles.

But one type remains
far more mysterious than others.

They're called neutrinos.

A neutrino is a

type of elementary particle,

a basic fundamental building
block of the universe,

and they come in
three different flavors.

Neutrinos are everywhere.

They are produced in the sun.

There are neutrinos that were
left over after the Big Bang.

Humans emit neutrinos.

Neutrinos have
got no electric charge.

They've almost got no mass
at all.

They're as near to nothing
as you can imagine.

They're so reluctant
to interact with stuff,

they pass through the Earth
as if it wasn't there.

And yet, at
Fermilab, scientists are constructing

a complex two-stage experiment

with the means to create them
and study them.

In its first stage,
a powerful ring of magnets

accelerates positively charged
particles called protons

to colossal speeds, sending
them smashing into a target.

The collision creates a shower
of new particles,

including a powerful beam
of neutrinos.

150 trillion per second pass
through the Earth

at nearly the speed of light,

racing towards the second
stage...

three giant neutrino detectors.

The largest is called ICARUS.

Once complete,

this immense tank filled with
a web of electronics

and cryogenic liquid

will be bombarded by hundreds
of trillions of neutrinos,

all in the hope of catching
just one each minute.

That alone will be a remarkable
achievement.

But the scientists
have even bigger ambitions.

One of the big goals
here at Fermilab is to try to search

for possibly a new type of
neutrino

that no one has yet observed.

Experiments
have hinted there could be

an even more elusive neutrino

beyond the three types already
known to exist.

Some have suggested
that it could be a link

to a hidden realm of particles

that could finally lead

to new discoveries beyond
the Standard Model.

If we found evidence
for a new type of neutrino,

that would be really astounding.

That's what gets me excited
in the morning.

That's what gets me coming
in to work.

It would be a major
and massive discovery.

Making that
discovery would be groundbreaking.

Because while ordinary neutrinos
are extremely hard to detect,

this fourth type of neutrino
could break the Standard Model.

What brought them to this
moment...

and possibly to the brink
of upending

one of the bedrocks of
modern physics?

That story begins almost 100
years ago

half a world away.

In Rome.

Physicist and historian
Professor David Kaiser

has traveled here,
to the place where,

in the 1930s,
scientists were investigating

the inner workings of the atom.

For millennia,
for thousands of years,

people had come to believe that
the world was made of atoms,

and those atoms were
the smallest thing there was.

In fact, the word atom
even means

"unbreakable" or "indivisible"...
the smallest piece.

But by the early 1900s,

scientists had revealed
a deeper hidden structure.

If you think about
an atom, it's about a nanometer,

about a billion times smaller
than a meter, roughly.

The inside, the deep core of
an atom, the nucleus,

is about 100,000 times smaller
than that.

So we're really zooming in
powers of ten, powers of ten,

getting to unimaginably
tiny scales.

During the early 20th century,

scientists discovered the atom's
tiny nucleus contained protons,

particles with
a positive electric charge.

These protons held in place

a cloud of negatively charged
electrons

that formed the atom's
outer limit.

It seemed that protons
and electrons

were the only two components
of all atoms...

permanent and fixed.

But scientists had also found
something shocking:

some types of atoms seemed
to break apart.

That was just jaw-dropping.

Literally, it contradicts
the name of the thing itself.

Atoms are supposed to not
break down.

It was as though
certain atoms had too much energy.

The nucleus would
spontaneously transform

and spit out an electron.

This phenomenon was
a type of radioactivity

known as beta decay.

It appeared to be

this sort of mysterious energy
leaking from or emanating from

certain atoms.

This process
was remarkable in itself,

but when scientists measured
the energy

of the electrons from beta
decay, something was wrong.

One of the
basic principles in all sciences

is that energy can change
from one form to the other,

but the total sum must be
conserved.

This is the
principle of conservation of energy.

From collisions in the
macro world

to the behavior
of tiny particles,

the principle states that
energy should never disappear.

But when scientists measured
the energy of the electrons

from beta decay, that's exactly
what seemed to happen.

So every time,
rather than having energy conserved,

what they were seeing is that

some amount of energy
would be missing.

Where was the energy going?

It seemed that the particles
themselves were breaking

the fundamental rules
of physics.

In 1926, a young Italian
physicist called Enrico Fermi

was working at the University
of Rome's Physics Institute.

It was here that Fermi probed

into the developing field
of nuclear physics.

Enrico Fermi
was really a towering figure

of 20th-century physics...

by any measure, one
of the greatest physicists

of the 20th century.

This is the site
where Fermi built what became

an absolutely world-class group
of researchers.

They were known

as the Via Panisperna Boys.

This is really
an iconic photograph.

It captures them in the middle
of what would become

world-changing research.

Fermi himself was remarkably
young...

he was just 26 years old,

and already he'd been made
the big senior professor

around which this young group
would come together.

They referred to Fermi as the
Pope, he was the great leader.

Rasetti was next in line,
he was a cardinal.

The person taking the
photograph,

the very young Bruno Pontecorvo,

the youngest member of the
group,

they called him the Puppy.

The group's ideas would
have a profound impact on the world.

In October 1931,

they invited a group of the
world's leading physicists

to a conference held at the
Physics Institute.

High on the agenda was
the problem

of the missing
radioactive energy.

One scientist at the conference,

the famous Wolfgang Pauli,
proposed a radical idea.

Wolfgang Pauli had
written a letter to colleagues.

And he put forward what
he called a desperate remedy,

a "versweifelten Ausweg"...
it was just ridiculous.

And he says so in his letter.

It's a really quite
strange-sounding idea.

What if there was a new type
of particle in the world

that no one had ever seen
or detected before?

Pauli suggested
that instead of just an electron,

perhaps there was an
unknown particle

that was carrying away the
missing energy.

Very few people
seem to have been convinced

that this was the right way
to go.

At that time,
physicists were quite confident

there existed two basic kinds
of particles,

electrons and protons.

But Pauli was suggesting,
"Let's make this enormous leap."

A new particle
of matter seemed a step too far.

But for Enrico Fermi,
the Pope of Via Panisperna,

he took the wacky idea
and ran with it.

Fermi dedicated the next
two years of his life

to describe the obscure
ghost particle.

It would be neutral,
and carry no electric charge.

It would be tiny,
far smaller than an electron.

And it would pass through atoms
as if they weren't there at all.

He named the particle
the neutrino,

Italian for
"little neutral one."

This was a
really quite remarkable step.

But many physicists,
Fermi included, thought

that it should be nearly
impossible...

perhaps impossible forever...

to detect such a particle
even if it really exists.

Outside the
intellectual fervor of the lab,

fascism was about to cast
a shadow

over the neutrino mystery.

In 1939, Fermi immigrated
to the U.S.A.

and was quickly put to work.

He helped to develop

the first operational
nuclear reactor

that led eventually
to the atomic bomb.

But not everybody had forgotten
about the elusive neutrino.

Bruno Pontecorvo, the Puppy of
the Via Panisperna Boys.

Upon moving to England
after the Second World War,

he continued to think
about neutrinos

until his life took
a shocking turn.

Pontecorvo was a
man who created big ideas.

The work that he did on
neutrinos alone

could have won him

certainly one Nobel Prize,

and been a candidate
maybe for two.

But it wasn't to be.

In 1950, in the midst
of the Cold War,

Pontecorvo and his family
mysteriously went missing.

Bruno Pontecorvo

disappeared through the Iron
Curtain in 1950,

and for five years,

disappeared off the face
of the planet.

Only after five years of silence

did he reappear
in the Soviet Union.

So, what happened?

Was he kidnapped?

Was he a spy?

Professor Frank Close
has spent years

researching Pontecorvo and his
mysterious disappearance.

He has come to the British
National Archives in London.

Earlier in his life,

Pontecorvo had been a member
of a communist party.

And there are now
British intelligence files

under his name.

Looking at these

old folders,
they're worn down the sides.

They have red stamps,
"top secret."

The case of Pontecorvo.

It is dripping with intrigue.

After the war,

while working for the
U.K.'s atomic energy program,

Pontecorvo devised a method
to try and detect neutrinos.

He reasoned that
nuclear reactors...

which derive energy
from splitting atoms...

should produce neutrinos in
vast quantities.

But the government classified
his paper.

Now, I conjecture that this
paper was classified secret

because, if you could indeed
detect neutrinos

coming from a nuclear reactor,

you would be able to work out

how powerful
the nuclear reactor was.

So they classified it.

As the Cold War escalated,

the U.S.A. became paranoid
of atomic espionage.

In 1950, the Rosenberg spy ring
was uncovered.

And it triggered
a communist witch hunt.

A secret letter reveals the FBI

wrote to a British
intelligence service

about Pontecorvo.

"The FBI now ask if
we can send them any information

"which would indicate that
Pontecorvo

may be engaged
in communist activities."

The letter was received in
London on the 19th of July.

Five days later,

Pontecorvo goes off to Europe
and never returns.

Flight manifests
reveal Pontecorvo and his family

flew from Rome, across Europe,
to Helsinki,

alongside two suspected
KGB agents.

Pontecorvo's son, just
12 years old at the time,

revealed they were then driven
across the border to Moscow...

with Bruno in the trunk.

He said to me,

"I knew something was up."

Frank believes a Soviet
mole passed the FBI letter to Moscow,

who then pressured Pontecorvo
to defect.

There's no clear evidence that
he had been a spy,

but whatever his reason
for leaving,

Bruno's time in the West
was over.

Was he a spy or not?

We don't yet know.

In any event, it was clear

that Pontecorvo
was a top-quality scientist

who had taken his
brain to the Soviet Union.

By 1950, the
U.S.A. and the Soviet Union

were engaged
in a nuclear arms race.

With it came a new opportunity
to hunt for neutrinos.

When a nuclear bomb goes off,

there is this huge cascade
of particles

that spews out:
protons, electrons,

a lot of light particles
carrying off energy.

And along with these particles
spewing out,

lots and lots of neutrinos
come out for free.

If neutrinos were
real, could a nuclear weapon finally be

the key to detect them?

In 1951, a young American
called Fred Reines

was working on the
U.S. nuclear program

at Los Alamos National
Laboratory.

It was here that Reines, along
with his colleague Clyde Cowan,

decided to take advantage
of destructive bomb tests

to investigate the mystery
of the missing neutrino.

Reines went back to a question

that had been kind of
abandoned in the decades

before the Second World War,

the question of, could
physicists ever actually detect

these very strange, elusive,
ghost-like particles?

They called
their mission Project Poltergeist.

For detecting the neutrino,
the good news was,

you could calculate the chance
of doing it.

And the bad news was,
it was almost zero.

Reines and Cowan
needed to tip the odds in their favor,

and knew a nuclear bomb test
could be the key.

An atom bomb should produce
thousands of times

more neutrinos than even
the biggest nuclear reactor.

But it also created a problem.

If they had bolted
the detector in place,

the nuclear bomb would've just

smashed it to smithereens.

So instead, the proposal

was to dig a shaft about
150 feet deep

right near where the bomb
would eventually

be detonated above ground.

The team planned to drop

a detector down the shaft to
avoid the shockwave of the bomb.

Inside that shaft, they
would pad the bottom with foam

and feathers and kind of, like,
mattress cushions.

It was, I mean...

a creative, ambitious,

and maybe slightly crazy kind
of idea

to try to catch these neutrinos
in the midst

of this very dramatic,
very worldly set of events

in the early years of the
Cold War.

Work
digging the shaft had begun,

but the head of physics
at Los Alamos was concerned

that the experiment
couldn't be repeated.

He urged the team
to find another way.

Couldn't they use
a nuclear reactor instead?

Late one evening, Reines
and Cowan had a realization.

In the same way that the nucleus
of an atom could decay

and release a neutrino,
they knew in theory

the process should be
reversible.

On the rare occasion a neutrino
could interact with a nucleus,

it should produce two new
particles,

called a neutron and a positron.

And if they traveled
through the right medium,

those two telltale particles
should produce

two distinctive flashes
of light.

So Reines
and Cowan built a detector,

essentially a big tank filled
with a solvent

that could pick up

this two coincident signal blip

deep under a nuclear reactor.

After five years of experiments,

in 1956,
finally, they got their answer.

They recorded the two
telltale flashes of light.

For the first time,
they saw evidence

of the elusive neutrino.

What they had done was
a remarkable achievement,

one that seemed impossible.

Neutrinos exist.

They're real and they're part
of the world.

They're not only a clever idea.

Knowing neutrinos exist

put a whole extra set
of investigations

on a kind of firmer path.

If neutrinos were
pouring from nuclear reactors on Earth,

then surely they would
be generated

in abundance in the largest
nuclear furnaces of all.

Stars.

For a long, long time,

scientists have been wondering,
what makes the stars shine?

What drives that enormous
output of energy?

People theorized that
our sun is like a giant nuclear reactor,

except, rather than heavier
elements breaking down

into smaller ones
and releasing energy,

you have lighter elements
that fuse together

through nuclear fusion.

In the heart of the sun,

tremendous heat and pressure
force hydrogen nuclei

to fuse together to make helium.

And, in theory,
vast quantities of neutrinos

that pass freely through the sun
and out into space.

So if we could detect neutrinos
from the sun,

we could learn about
the processes that fuel it.

We could peek inside the core
of our sun.

In the historic
gold mining town of Lead,

people descend into the depths
of the Earth.

But no longer
to mine precious metal.

They're hunting for neutrinos.

It was here in 1965

that an experimentalist
called Ray Davis

came to try and prove
what makes the sun shine.

Ray Davis got very excited

that there is this new thing
in the world called a neutrino.

He began realizing that other
kinds of nuclear reactors

that occur throughout
the universe, like stars,

they should be spewing out these
neutrinos all the time.

But catching
them wouldn't be easy.

Calculations showed
that neutrinos from the sun

would be so faint, a detector
near the Earth's surface

would be overwhelmed
by background radiation.

His only option was to go
to the bottom of a mine.

Beneath almost a mile of solid
rock, Davis's team built

a steel tank the size of a house

and filled it with
100,000 gallons

of dry-cleaning fluid.

In theory,
if a neutrino from the sun

collided with a chlorine atom
inside the tank,

it would cause a reaction
that Ray Davis could detect.

Here was something
that was completely fresh.

Nobody knew anything about it.

But the key thing was that
if neutrinos hit chlorine,

which you could get in
cleaning fluid,

it would turn the atoms
of chlorine

into a radioactive form
of argon.

And that's when Davis
got excited,

because he was a radiochemist,
and for him,

detecting radioactive forms
of argon was easy street.

Scientists had calculated

that around a million trillion
neutrinos from the sun

should pass through Davis's tank
each minute.

But the probability
of them hitting the fluid

and making an argon atom
was so small,

Ray Davis could only expect
to find

ten individual atoms of argon

from ten neutrino collisions per
week.

His task was almost impossible.

Many of his own physicist
colleagues doubted

this experiment would ever work.

He was having to convince people

that out of these millions
and millions and millions

and millions of atoms
inside this tank,

he could identify
the collisions of one or two

and convince you that these were
neutrinos coming from the sun.

Around each month,
Davis flushed out the giant tank

to extract the argon atoms.

To everybody's amazement,

he found them.

But there was a problem.

Instead of detecting the number
of atoms that theory predicted,

his measurements fell short.

They knew the
target number based on

the nuclear physics
theoretical explanation

of how stars shine,

and that led to a very
particular target number.

And Davis's remarkable
experiment

kept coming in not close to it,
not 80 percent,

but only at one-third
of that target number.

What happened?

Had the experiment gone wrong?

Another scientist carried out
a blind trial

to test the accuracy
of Ray's atom detection.

A colleague put
in 500 kind of rogue atoms

without telling Davis
the number.

And Davis was able to go through
the whole process,

sift it through,

and he counted exactly the
number that had been put in.

If the experimental
results were accurate,

then perhaps scientists
had gotten their theory

about neutrinos from the
sun wrong.

Everybody was
blaming everybody else.

There were even suggestions,

has the sun already burnt out
in the core?

It was just an enormous puzzle.

All these advances in
understanding how stars shine,

and then hitting this kind of
brick wall

where theory and experiment just
would not agree with each other.

The puzzle became
known as the solar neutrino problem.

1970,

20 years since Bruno Pontecorvo

defected to the Soviet Union.

Even after all that time,

his life behind the Iron Curtain
remained shrouded in secrecy.

But in a government lab outside
Moscow,

Pontecorvo worked tirelessly
to explain

the puzzling behavior
of neutrinos.

He suggested that instead
of just one,

there may be two or even three
different kinds of neutrino...

known as different flavors.

If this wasn't strange enough,
he calculated that something

peculiar might happen as they
traveled through space.

A neutrino would always be born
as one definite flavor,

but over time,
it would change its identity.

It would transform,
mixing back and forth

between the three different
types.

This was called
neutrino oscillation.

Pontecorvo's idea really is,
it's, it's sort of delicious.

These neutrinos could be not
taking one identity,

dropping that, adopting another
one, dropping that,

but going into this even
stranger mixture,

where they're in neither
and both states at once.

It was a bold idea.

No other fundamental particle

seemed to spontaneously change
its identity.

But if neutrinos were
transforming into flavors

that Ray Davis's detector
couldn't see,

it might explain why
two-thirds of the neutrinos

from the sun appeared
to be missing.

But there was a catch.

The Standard Model,

the most precise scientific
theory in human history,

made one important prediction
that stood in the way.

The Standard Model anticipated

neutrinos would be
completely massless.

They would have no mass at all,
much like the photon of light.

And if they had no mass,

that meant that they could not
oscillate.

If neutrinos had no mass,

one of Albert Einstein's most
important theories

predicted that neutrinos could
not possibly oscillate.

There is this
mind-boggling phenomenon

from Einstein's relativity

that says that a clock
that is moving closer

and closer to the speed of light

will tick at a slower
and slower rate.

If that clock were moving
literally at the speed of light,

it would never tick at all.

No time would pass
for that object

that moves at exactly
the speed of light.

According
to Einstein's theories,

the faster a particle travels,

the more its internal clock
slows down.

A particle with no mass can only
travel at the speed of light,

which is where time stops.

So if a neutrino had zero mass,

it would not experience
the passage of time,

and would never be able
to change.

If a particle has zero mass,

what that means is that its
internal clock is not ticking.

There's no way for that
particle to experience time.

If there's no passage of time,

then how could they change over
time from one identity

to another?

If neutrino
oscillation was real,

neutrinos must have some mass.

But could the Standard Model
really be wrong?

Throughout the 1950s and '60s,
clues from experiments

performed at CERN,
alongside Fermilab,

helped to lay the foundation
of the Standard Model.

What they found
revolutionized our understanding

of the particles that make up
our universe.

By means of
this machine, it is possible to see

the tracks
of sub-nuclear particles,

the smallest particles
known to man:

the electron, the positron,

the photon, and the neutrino...

Over the years, work at CERN

led to groundbreaking
new technologies:

medical advances like PET scans;

even the birth of
the World Wide Web.

Perhaps CERN's biggest success
came in 2012.

Nearly 50 years after the
Standard Model was proposed,

physicists detected the
final particle

it predicted... the Higgs boson.

I think we have it.

Finally, all the pieces needed

to describe the detectable
physical universe

seemed to be in place.

Along with the Higgs boson,
there are force carriers,

like the photon of light.

Quarks, which form
the nuclei of atoms.

Leptons, including the electron,
muon, and tau.

And three corresponding flavors
of neutrinos.

It is a map of what's out there,

what we're made of,
and how we fit... all of us.

We are made of these things.

And that is a kind of basic
understanding

of nature, of our own world,

that I, I think is, is just a
remarkable

human achievement.

And yet, for all its success,

the Standard Model had
no equations to explain

how or why the neutrinos
would have mass.

For Ray Davis
and his missing solar neutrinos,

it seemed an unsolvable paradox.

For decades, Davis persists,

but he still only finds
one-third of the neutrinos

that were supposed to be coming
from the sun.

Well, we've been carrying
on this experiment

for about 20 years right here.

But we're still observing a
low flux of neutrinos.

Eventually,
the problem is too big to ignore.

In the 1990s,
scientists in Canada and Japan

construct a new generation of
supersized neutrino detectors

to finally settle the mystery.

One of them lies deep beneath
Japan's Ikeno Mountain.

Scientists fit 11,000
light detectors

to the inside of
a gigantic container

and fill it with 50,000 tons of
ultra-pure water.

This $100 million detector
is named Super-K.

The Super-K experiment ended up
being a game-changer.

In the rare
event that a neutrino collides

with the liquid in Super-K,

the reaction produces
a trail of light

which the sensors can pick up.

Unlike Davis's detector,

this signal allows scientists
to calculate

which type of neutrino has hit

and the direction it came from.

Super-K allows scientists

to test the theory
of neutrino oscillation

by catching them from
a new source:

the Earth's atmosphere.

Theory suggests that
when radiation from space

hits the atmosphere,
it creates neutrinos

that travel directly through
the Earth.

Some travel a short distance,

but others will come from
the other side of the planet

to reach the detector.

If the neutrinos are not
changing,

the combination of flavors they
record

coming from a short distance
will be the same

as those coming from afar.

If they are changing over
a long distance,

the combination of flavors will
be different.

After two years
of recording data,

the team finally has an answer.

What they were seeing was that

one type of neutrinos was
depleting

when traveling through
the Earth.

The Super-K results combined
with results

from another experiment

were able to definitively show

that neutrinos can change
from one type to the other.

For that to happen,

you must have non-zero
neutrino mass.

The results are groundbreaking.

Neutrinos change their identity.

Neutrinos have mass after all.

And the Standard Model's
prediction

of the nature of neutrinos
must be wrong.

With the new input,

the evidence that neutrinos
really oscillate,

they really change their
identities,

therefore they really,
really have a mass,

this long-standing,
decades-long challenge

to understand the solar neutrino
problem

finally fell into place.

Nuclear fusion in the sun

produces one type of neutrino.

But on the long journey through
space,

the neutrinos oscillate,

and turn into a mixture of
all three.

On Earth,

Ray Davis's detector only
picked out one flavor.

His results had been accurate
all along.

37 years after
the experiment began,

Ray Davis was awarded the
Nobel Prize.

For Bruno Pontecorvo
and his theory of oscillations,

sadly, the discovery came
too late.

Nobel Prizes aren't everything,

but by the time the oscillations
had been sorted out

and the whole thing finally
understood,

Pontecorvo was dead.

So that's the final tragedy
of his life.

After almost 100
years of research and discovery,

today, neutrino physicists face

perhaps their biggest
puzzle yet.

The Standard Model's equations,

which are so precise for
other particles,

cannot explain why neutrinos
have mass or why they oscillate.

It's a sign that our
understanding of matter

is still incomplete.

Today, neutrino experiments are
in overdrive,

hunting for clues.

We're in the midst of, really,

a neutrino bonanza... I mean,
they're just, they're popping up

all over the field of physics.

At the South Pole,

scientists have built

the largest neutrino detector
on the planet.

It's made of more
than 5,000 sensors drilled into

a cubic kilometer
of Antarctic ice.

It's known as IceCube.

IceCube is in this,

this huge field around me...
I'm sitting,

kind of standing in the middle
of IceCube.

It's kind of amazing to think

that we were able to haul
something like

five million pounds of cargo

down to the South Pole...
this is

instrumentation,
cables, drill equipment,

fuel...

As well as
probing neutrino oscillations,

IceCube acts like
a neutrino telescope,

catching cosmic neutrinos

from billions
of light years away.

This is the universe that
has really

only been opened to our eyes
for the last 50 years.

There's all kinds of discoveries
that are waiting out there.

With new
experiments like IceCube,

scientists believe that
neutrinos may reveal discoveries

beyond the Standard Model.

Neutrinos could even help unlock

one of the biggest mysteries
in physics today.

It seems that most of what
our universe is made of

is missing.

The whole
quest of particle physics

is to explain the matter
contents of the universe.

And we seem to be doing
this phenomenally good job.

You crank through the math
of the Standard Model,

and everything makes sense.

And yet it only describes
some very small fraction

of what the universe is made
out of.

Looking into space,

cosmologists can see
the gravitational influence

of a material that binds entire
galaxies together,

but that is completely invisible
to their detectors.

Scientists call
this material dark matter,

because nothing in the Standard
Model can describe what it is.

And yet, it seems to be

what most of the matter
in the universe is made of.

The Standard Model
is very good at describing

about five percent
of the universe.

95% of the stuff is an utter,
complete mystery,

made of dark stuff, whether
it's dark matter or dark energy.

And what either of those are,
we don't know.

All we really know about
dark matter

is that it creates gravity,

but it's not interacting
with the instruments

that we have used to observe
the universe.

Whatever is filling space,

much more of it than the
ordinary matter

that makes up us
and our planet and our stars,

it's some other,
other kind of particle.

Whatever
dark matter particles are,

scientists must look beyond the
Standard Model to find them.

Neutrinos might be the key.

At Fermilab, for over 20 years,

scientists have been
investigating

neutrino oscillations.

What they've found

doesn't add up.

The first observation
that something was amiss

was in the late 1990s.

Something we don't quite
understand is going on.

At Fermilab,
scientists fired a beam of neutrinos

just 500 yards to their
detector.

Neutrinos oscillate too slowly

for the detector to see them
change

over such a short distance...

at least according to theory.

But the detectors saw
an increase in one type

of neutrinos.

Neutrinos seem to oscillate
faster

than is theoretically possible.

The strange thing

that we're seeing is that
neutrinos seem to be

changing from one type
to the other

much faster than expected.

In order for that to happen,

we think it's possible

that there are extra neutrinos
out there.

In addition to
the three flavors of neutrino

that the Standard Model
describes,

there could be a fourth neutrino
that affects them,

making them oscillate faster.

Scientists call it
a sterile neutrino,

and it's never been directly
detected.

So we call
it a sterile neutrino,

in essence, just because it
interacts even less

with other particles than the
regular neutrinos do.

A sterile neutrino
would be the ultimate ghost particle.

It would never collide with
atoms in our world.

No detector could ever see it.

But it may reveal itself

through its effects
on the neutrinos we can see.

The only
way that we can tell they exist

is through their effects
on neutrino oscillation.

If sterile neutrinos exist,

it would break the neat symmetry
of the Standard Model

that organizes particles
in groups of three.

What if there's a fourth kind
of neutrino,

a so-called sterile neutrino?

Well, where would you put
that on our map?

There's no room to kind of
shoehorn in,

to squeeze in a fourth neutrino.

So I think there really is a lot
riding on this.

If they're real,
sterile neutrinos would have mass,

but not interact with our
detectors...

just like dark matter.

They could be the first particle
of dark matter ever discovered,

and through their effects on the
neutrinos we can see,

they could give scientists
a window into another world.

The neutrino
might be a kind of link,

almost a kind of messenger
or portal

to this whole other possible
kind of stuff out there.

At Fermilab,
scientists are edging towards the truth.

I think we're
getting a lot closer.

Neutrino physicists are
incredibly patient.

It takes a long time for us
to collect our data,

and we really want to be sure in
what we're seeing before

we potentially make
a very important discovery.

We're trying to answer

some of the biggest questions
in physics.

I think it's really unique
that neutrinos

may hold all the answers.

What began
as a hypothetical particle

that no one thought possible
to detect

could now be a key that unlocks

what most of our universe
is made of and how it works.

Every time we look up,

there seem to be these
very curious neutrinos.

They are constantly bedeviling

our mental maps of how we carve
up nature

and try to dig in and study it.

And that's just
amazingly exciting.

So they've gone from, "Maybe
they exist, maybe they don't,

we might never know,"

to being our surest ticket
to the next step.

History has shown that

with every little bit
of progress,

we've learned huge, surprising
things about our cosmos.

To me, that's really exciting.

And I'm curious to know, where
else could we go?

Wherever we go,

neutrinos could be our guide.