When you get right down to it, all of particle
physics is completely fascinating, but generally
each scientist develops expertise about a
specific subject, so they can then advance
humanity’s understanding of the laws of
nature.
At Fermilab, researchers decided that studying
the behavior of neutrinos provides the best
chance of making a breakthrough.
Fermilab is already the flagship accelerator-based
neutrino laboratory and by building the international
Deep Underground Neutrino Experiment, we’re
working hard to stay that way for the next
several decades.
Neutrinos are fascinating particles, which
can change their identity from one form to
another.
That’s pretty crazy-sounding and needs some
explanation.
For instance, if a neutrino were an animal,
it would be like a cat turning into a jaguar,
then into a tiger, before turning back into
a cat again.
Studying that morphing behavior of neutrinos
is key to making discoveries.
But before you can study the identity-changing
behavior of neutrinos, you first have to be
able to detect neutrinos.
And that takes some doing.
Why is that?
It’s because neutrinos don’t interact
very much.
In fact, neutrinos from the sun can pass through
the Earth very easily.
In very rough numbers, for every ten trillion
neutrinos hitting the surface of the Earth
and passing through the thickest part, one
of them interacts somewhere in the Earth,
and the rest pass through unscathed.
Now our detectors are big, but they aren’t
as big as the Earth.
One of the bigger detectors is a tank of water
weighing 50,000 tons.
And then there is Fermilab’s even more massive
DUNE detector, which, when completed, will
have a total mass of nearly 70,000 tons.
They’re both tiny compared to the Earth.
So that means that most neutrinos will pass
through detectors, well- undetected.
So how do we detect them?
Actually, it’s extremely easy.
To all intents and purposes, neutrinos interact
exclusively via the weak nuclear force.
The weak nuclear force is the weakest of the
three known subatomic forces.
Gravity is weaker, but it’s so much weaker
that it’s entirely negligible.
At the subatomic level, interactions occur
when a matter particle emits or absorbs a
force carrying one.
For the weak force, there are two force carrying
particles, with the unimaginative names of
the W and the Z bosons.
Both particles are very heavy- just shy of
a hundred times heavier than a proton.
The Z boson is electrically neutral, while
the W boson comes in two varieties- one negative
and one positive.
The way a neutrino interacts is that it is
just traveling along and, when it gets near
a nucleus of an atom- wham!
The neutrino emits either a W or a Z boson.
The W or Z boson that was emitted by the neutrino
then goes and plows into the nucleus of an
atom, breaking it up and creating all sorts
of particles.
Stray protons and neutrons can come out, but
so can other particles that are created from
the energy of the collision.
It’s an E equals mc squared thing.
Every collision is different.
Sometimes lots of particles come out.
Sometimes a few.
We can predict how often we’ll see each
kind, but individual collisions are random.
However, those particles that come flying
out of the destroyed nucleus fly by the atoms
in the detector.
Depending on the particle detector technology
used in an experiment, those secondary interactions
make electrical signals or blinks of light
that can be detected.
There are lots of different technologies employed
at Fermilab to detect neutrinos.
So that’s all there is to it.
Neutrinos emit W or Z bosons and those W or
Z bosons break apart atomic nuclei.
The debris then smashes into atoms and those
interactions are detected.
Easy peasy.
Of course, it’s possible that you’re confused
at this point.
Scientists make a big deal about how hard
it is to detect neutrinos and yet I just told
you how easy it was.
Something clearly isn’t adding up.
And that’s because, while I’ve answered
the question that some viewers asked, perhaps
that isn’t exactly the right question to
ask.
The proper question is both “how do we detect
neutrinos?” and “why are neutrinos so
hard to detect?”
To simplify the explanation, I’m not going
to distinguish between the W and Z bosons.
While they are different in detail, they have
two common features.
They're both very massive- nearly 100 times
heavier than a proton, and they both live
for a very short amount of time.
Focusing on those similarities and ignoring
the differences, we can simply combine them
and treat them as a single particle and call
it a weak boson.
Now there are a couple ideas from early 20th
century that matter here.
One is Einstein’s equation of E = mc squared,
which says that mass is energy and vice versa.
The other is the Heisenberg Uncertainty Principle,
which says that for very short periods of
time, energy doesn’t have to be conserved.
Since mass is energy and energy doesn’t
have to be conserved in fleeting subatomic
interactions, that means that weak bosons
don’t have a single and unique mass.
While their favored mass is about 100 times
the mass of a proton, you can find them with
masses that are larger or smaller.
What you’re seeing here is the range of
masses that a weak boson can have.
Here where the bump is high at 100-ish is
the most common mass.
Off to the side a little bit, close to but
a little different from the 100-ish place
are less common.
Further away is even less common.
But you’ll notice that the curve is smooth
and ranges all the way down to zero.
Thus it’s super-duper unlikely, but it’s
possible to find weak bosons with tiny masses.
Now, in neutrino scattering using the Fermilab
beam, the energies are generally much, much,
smaller than this 100-ish number.
In fact, they tend to be way down here.
And if you’re looking at the neutrinos from
the Sun that go through the Earth, they’re
even closer to zero.
So this is key to why the weak force is weak.
Weak bosons prefer to have a mass near 100-ish,
but the neutrino interactions at Fermilab
need to be very small.
That means that an interaction caused by the
weak force is very, very, unlikely.
And you can use the Heisenberg Uncertainty
Principle to get some insights into what is
going on.
In order for a weak boson to exist with a
mass so far away from the normal mass, it
can only exist for, in round numbers- for
one ten thousandth of a trillionth of a trillionth
of a second.
If you’re interested in how far such an
unusual weak boson can move, it can travel
a distance less than one, one thousandth the
size of a proton, which is a crazy short distance.
And that is key to understanding why neutrinos
are so hard to detect.
In order for a neutrino to interact in matter,
it has to plunge into the center of an atom
and essentially get a direct hit on one of
the various subatomic particles inside the
core of the atom.
If it misses just a little bit, there is no
interaction.
However, on the extremely rare occasion in
which there actually is an interaction, the
weak boson just jumps out, smacks into the
subatomic particle in the center of the nucleus
of the atom and breaks the nucleus apart.
That's the easy part.
And, like I said, detecting the particles
that come out of the collision is really pretty
easy, whether we look at blinks of light or
electrical signals.
In the DUNE experiment, which will be Fermilab’s
flagship experiment for the next few decades,
liquid argon- which is argon that has been
chilled to about 300 degrees Fahrenheit below
zero- is what will be used to detect the signals.
So, there you have it.
Neutrinos don’t interact very often but,
when they do, detecting them is very easy.
And now, you’re a neutrino detection expert.
Okay, that was fun.
Neutrinos really are crazy particles to work
with.
Getting them to interact is the hard part.
And if you want to learn more about detecting
them, there’s a link on your screen that
tells you more.
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and elusive particle, be sure to subscribe
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