Video Transcript
In this video, we’re looking at
neutrinos. Neutrinos are a type of fundamental
particle, and they come in three main varieties. Each of these varieties of neutrino
also has its own associated antiparticle. This gives us six different types
of neutrino in total. In this video, we’ll talk about the
properties of neutrinos, how we can classify them, and how we can represent them
using symbols. Let’s start by talking about what
exactly neutrinos are.
Well, firstly, we can say that
neutrinos are elementary or fundamental particles. This means that neutrinos are
indivisible. In other words, they’re not made up
of smaller particles. Neutrinos are also leptons. This puts them in the same category
as the electron, muon, and tauon. The three varieties of neutrino are
actually named after these three other leptons, giving us the electron neutrino, the
mu neutrino, and the tau neutrino.
Now, we can recall that every
particle has an associated antiparticle. For example, the antiparticle of
the electron is the positron, the antiparticle of the muon is the antimuon, and the
antiparticle of the tauon is the antitauon. Neutrinos are no exception to this
rule, and each one of these neutrinos has its own corresponding antiparticle, which
together are known as antineutrinos. So the electron neutrino’s
antiparticle is the electron antineutrino, the mu neutrino’s antiparticle is the mu
antineutrino, and the tau neutrino’s antiparticle is the tau antineutrino.
And at this point, we can note that
we’ve got all of the leptons and antileptons on the screen. We can see that each of the
neutrinos is represented by the Greek letter 𝜈, with a subscript letter either 𝑒,
𝜇, or 𝜏 that corresponds to the type of neutrino. The same convention is used to
represent antineutrinos, except there’s a bar over the top to signify that they’re
antiparticles. Even though these neutrinos, even
though the neutrinos and the antineutrinos are named after these other leptons,
their properties are very different. And there’re clues for a couple of
these properties in the name neutrino.
The first part of the word neutrino
signifies that neutrinos, much like the neutron, are electrically neutral. This means the three neutrinos and
three antineutrinos have zero charge. This is often used to draw a
distinction between the neutrinos and the other types of lepton on the left. The neutrinos and antineutrinos are
the neutral leptons, while the electron, muon, and tauon and their antiparticles are
together known as the charged leptons. So in this diagram, all of the
negatively charged particles are blue, all of the positively charged particles are
red, and all of the neutral particles are green.
The second clue as to the
properties of neutrinos comes from the second part of the word, -ino, meaning
small. Now, generally, particles are
small, but neutrinos are really small. Now, to be clear, when we say
small, we’re not talking about their physical size, rather their mass. Out of all particles with mass,
neutrinos have the lowest mass. We can also say that they’re the
lightest particles. In fact, neutrinos have such low
masses that for a long time physicists thought they had no mass at all. We now know that they have very
tiny masses. However, they’re so small that we
haven’t yet been able to figure out what they are.
Because of their incredibly low
mass and the fact that they’re electrically neutral, neutrinos very rarely interact
with anything else. This makes them really difficult to
detect and study, so there’s a lot that physicists don’t yet know about these
particles. However, despite being difficult to
study, neutrinos are incredibly common. Neutrinos are a common product in
reactions between other particles, for example, in 𝛽 minus decay. In this process, a neutron, shown
in green, which is often part of an atomic nucleus, decays into a proton, shown in
red. And in doing so, it emits an
electron and an electron antineutrino. The equation for such a reaction in
the case of, say, a carbon-14 nucleus decaying into a nitrogen-14 nucleus would look
like this.
It’s important to note that
neutrinos can only interact via the gravitational force and the weak nuclear force,
meaning they can’t interact by electromagnetism or the strong force. Because of their low mass,
neutrinos interact very weakly with gravity. But neutrinos are very commonly
produced in interactions that involve the weak force. As well as decay processes, such as
𝛽 minus decay, neutrinos are commonly produced in nuclear reactions in stars,
including the Sun. In fact, neutrinos are produced by
our Sun in such large quantities that the neutrino flux on the surface of the Earth
is around seven times 10 to the power of 10 per centimeter squared per second. This means that 70 billion solar
neutrinos pass through every square centimeter of the Earth’s surface every
second.
The reason we don’t notice this
constant barrage of neutrinos from the Sun is because of their small mass and their
very limited interactions with matter. This means that most neutrinos
actually just pass through us and through the planet as if we’re not there. So neutrinos are incredibly common
particles but ones that physicists still have a lot to learn about. Now that we know the basics, let’s
get some practice with these ideas by looking at some questions.
List the following particles in
order from the greatest to the least mass: positron, neutron, helium nucleus,
photon, and neutrino.
So here we’ve been given a list of
six different particles, and we need to arrange them in order from greatest to least
mass. A good place to start is with the
photon. We can recall that a photon has
zero mass. So we know that we can put photon
last on our list as we know that nothing can have less than zero mass. Next, let’s think about the
neutrino. We can recall that there are three
different types of neutrino: the electron neutrino, the mu neutrino, and the tau
neutrino. And each of these neutrino types
has an associated antiparticle, which is known as an antineutrino. These are known individually as the
electron antineutrino, the mu antineutrino, and the tau antineutrino.
Now we might recall that scientists
don’t actually know the masses of the different neutrinos. However, what we do know is that
neutrino masses are incredibly small. In fact, neutrinos have the least
mass of all massive particles. There’s actually a clue for this in
the name of the neutrino. The suffix -ino means small. Even though the type of neutrino
that we’re thinking about in this question hasn’t been specified, the fact that
neutrinos have the lowest mass of any massive particle means that we can write
neutrino just before photons at the end of our list.
We now have three particles left to
put into our list: the positron, neutron, and helium nucleus. Of these, the neutron and helium
nucleus are familiar to us from thinking about atoms. A helium nucleus is effectively an
atom of helium but without the orbiting electrons. And neutrons are the electrically
neutral particles which make up an atomic nucleus along with positively charged
protons.
Since a helium nucleus contains two
neutrons and two protons, we know that it must have more mass than just a single
neutron. But what about the positron? Well, we can recall that the
positron is the antiparticle of the electron. This means it has the opposite
charge to the electron but exactly the same mass. We can also recall that the
electron has a much lower mass than a neutron. In fact, the neutron has around
2000 times the mass of an electron. And since the positron has the same
mass as an electron, this means that a neutron has around 2000 times the mass of a
positron. This means that, of these three
remaining particles, the one with the least mass is the positron. Then weighing in at around 2000
times heavier, we have the neutron. And finally, the heaviest particle
on the list is the helium nucleus.
So now we have our completed
list. In order from greatest to least
mass, we have the helium nucleus, the neutron, the positron, the neutrino, and the
photon.
Now let’s look at a second example
question.
Which of the following symbols does
not represent a real neutrino?
So here we have a list of four
answer options, and each one consists of a lowercase Greek letter 𝜈. This is a character that looks kind
of like a curly letter V, and each one is followed by a different letter written in
subscript. This first one is the Greek letter
𝜇. This one of course we know as the
letter 𝑒. This character looks like an E, but
it’s actually a lowercase Greek letter 𝜖. And finally, this is the Greek
letter 𝜏. What we need to do is determine
which of these does not represent a real neutrino.
We can recall that all neutrinos
are represented by the Greek letter 𝜈. We can also recall that there are
three varieties of neutrino: the electron neutrino, the mu neutrino, and the tau
neutrino. Each of these also has its own
antiparticle. These are the electron
antineutrino, the mu antineutrino, and the tau antineutrino. As we can see, these particles are
named after the negatively charged leptons: the electron, muon, and tauon. And these negatively charged
leptons are represented by the symbols 𝑒 minus, 𝜇 minus, and 𝜏 minus. Because the neutrinos are named
after these three leptons, they also make use of the symbols 𝑒, 𝜇, and 𝜏. So we represent an electron
neutrino with a letter 𝜈 to signify that it’s a neutrino, followed by a subscript
𝑒 to signify that it is an electron neutrino.
Similarly, the muon neutrino is
represented by a 𝜈 followed by a subscript 𝜇. And the tau neutrino is represented
by a 𝜈 followed by a subscript 𝜏. The antineutrinos follow the same
pattern, but with a bar over the top to signify that they are antineutrinos. So looking again at our answer
options, we can see that option (A) corresponds to a mu neutrino, option (B)
corresponds to an electron neutrino, and option (D) corresponds to a tau
neutrino. So we can see that it’s option (C),
𝜈 sub 𝜖, which does not represent a real neutrino.
Now let’s recap some of the key
points that we’ve learned about neutrinos. Firstly, neutrinos are elementary
particles. This means that they cannot be
subdivided into smaller particles. Neutrinos are electrically neutral,
meaning they have no charge. Neutrinos also have the lowest
masses of all massive particles, although their exact masses are unknown. Neutrinos belong to the group of
particles known as leptons. And the three main neutrino types
are named after the negatively charged leptons: the electron, the muon, and the
tauon. This gives us the electron
neutrino, the mu neutrino, and the tau neutrino. Each of these also has its own
associated antiparticle. These are the electron
antineutrino, the mu antineutrino, and the tau antineutrino. And finally, we’ve learned that
neutrinos interact only via the gravitational force and the weak nuclear force. This is a summary of neutrinos.