Lesson Video: Neutrinos Physics

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.