How big is a neutrino? That is a very interesting question with a much more
complicated answer than you might think. So let's talk about it today on Even Bananas!
Hi I'm Kirsty Duffy and I'm a neutrino physicist at Fermilab. When we talk about
the size of a neutrino you often hear people say that neutrinos are very small or calling
them tiny particles. Neutrinos are the tiniest most intriguing... ...so small that they just,
Tiny, Tiny, Neutrinos are really small. But that's not technically true. Neutrinos
are what we call fundamental particles which means we think they aren't made up of anything
else smaller. They're the smallest building blocks that make up our universe. Electrons are probably
the best known of the fundamental particles. And technically, in the Standard Model of Particle
Physics, fundamental particles are defined as "point-like", which means they don't have a size,
they're just a point. It's a really tricky concept to think about. I imagine it like me dotting a pen
on a piece of paper. It's the smallest possible mark I can make on that paper. And compared to me,
say, drawing a circle on the paper and coloring it in, the dot has no size. It's just a point.
But that analogy isn't perfect. If we look close enough, the dot does have a size and it's much,
much bigger than a fundamental particle. Now, even though fundamental particles have no size we do
still have a way to talk about how big or small they are using a concept called "cross sections".
Think about this volleyball when you look straight at it, it looks like a circle.
And the area of that circle is what we call the cross-section of the ball. Now, a volleyball has
a bigger cross-section than a tennis ball, which has a bigger cross-section than a ping-pong ball.
Now imagine these balls being thrown at you.
The bigger the cross-section of the ball, the more likely it is to hit you.
So to figure out how likely a ball is to hit you you first figure out the cross section.
With particles we do it backwards. We look at how likely a particle is to hit or interact
with something when it moves towards it, and from there we can calculate the cross section.
Watch out! This is a nice way to visualize the concept of cross sections, but bear in
mind that particles are not actually spheres. Quantum mechanics is weird. So what determines
how likely a particle is to interact with another? Well, there are four fundamental forces: gravity,
electromagnetism, the weak force, and the strong force. Quarks, the particles that
make up protons and neutrons that make up all of our atoms, experience all four of those forces.
Electrons and their heavier cousins experience gravity, electromagnetism, and the weak force.
But neutrinos experience only gravity and the weak force. Neutrinos are extremely light, hundreds of
thousands of times lighter than electrons, so the pull of gravity is not strong and the weak force
is, well, weak. So neutrinos really don't like to interact. They generally just fly past things
without paying them any attention. And that's why we often say that neutrinos are really small
they're very unlikely to interact with other particles, so their cross-section is tiny.
To give you a sense of how minuscule neutrino cross sections are,
imagine that an electron's cross section was the size of a basketball court.
Then a neutrino with a typical energy would have the same cross section
as the smallest pencil dot I can draw. No wonder neutrinos are so hard to spot. But it gets even
weirder. A moment ago, you might have noticed I said a neutrino with a "typical energy".
That's because unlike the size of a pencil dot, the cross section of a fundamental particle
isn't fixed. It actually depends on the particle's energy a higher energy neutrino will behave
like it's a larger particle than a lower energy neutrino. In fact, the highest energy neutrinos
we've ever measured have a cross section 10 million times bigger than lower energy neutrinos.
But it's still several hundred times smaller than the cross section of a typical electron.
Now, there are three different types of neutrino, so you might wonder do the different types have
different cross sections? It turns out we actually don't know the answer to that, yet.
Because muon and tau neutrinos interact a bit differently than electron neutrinos, it's entirely
possible that their overall cross-section could be different, too. But that's an area of ongoing
investigation. In fact, it's something that I'm hoping to research myself in the future.
Even though we know that all neutrinos have tiny cross-sections, the details are still a mystery.
Thanks for watching this episode of Even Bananas. What's your favorite neutrino
fact that you've learned in the series so far? Let us know in the comments, and don't
forget to like share and subscribe so that you don't miss any upcoming neutrino adventures.
Fun fact: The very first experiment that detected neutrinos also provided the first estimate of
their cross-section and the value agreed with theory. Hooray! But in the next few years,
a reanalysis of the data actually revealed that the cross-section they measured
was twice as big as they thought. Oops! Luckily, theorists realized that the value
they had calculated should have been twice as large as well. So the experiment still agreed
with the theory. It just goes to show that the process of science is never finished.
blah blah blah blah. I don't know if that's true,
it's definitely not. You first figure out the cross section.
...first, figure out the cross section.
...You first figure out the cross section.