The Universe is made up of numerous fundamental particles – fundamental, meaning, they cannot be broken down any further – the building blocks of reality. You may have heard of some of them: electrons, quarks, photons, bosons, and so on. One of these particles, however, is called a neutrino. And despite my biases, I’d argue they’re the most interesting of them all!
Neutrino physics – a crash course
Before we get into neutrinos, we have to understand this important concept – the Forces of Nature. Right now, there’s four, but if the work ongoing in a lab in Chicago turns fruitful, we may very well have five, but let’s not overcomplicate ourselves with my excitement for new discoveries. The four forces, simplified, are:
- gravity (causes objects with mass to be attracted to one another – like me and a buffet breakfast),
- electromagnetism (which you can think of as light, electricity and magnets),
- weak (which, in a very crude sense, you can think of as radiation),
- and strong (the force which holds particles together to form stuff like atoms).
As you can tell, we’re very imaginative as a profession when we come up with names like ‘weak force’ and ‘strong force’, but we try, I suppose.
So, how can we describe neutrinos? The neutrino (represented by the Greek letter Nu ‘ν’) is the smallest particle in the universe with mass. If you weigh something on a sensitive enough scale, and a number greater than 0 appears on the scale, then it has mass. They also only interact with the weak force and gravity – this makes them incredibly difficult to detect. Lastly (at the time of writing), neutrinos can be one of three different types (or, as we prefer to say, three different flavours) – electron-neutrino (νe), muon-neutrino (νμ) and tau-neutrino (ντ).
In the time it took for you to read that last paragraph, trillions of neutrinos have passed through your pinkie nail. That’s how small yet abundant they are!
A quantum tub of Neapolitan ice cream
So, hopefully we all understand what a tub of Neapolitan Ice Cream looks like (if you don’t, a quick google will jog your memory)? Well, let’s make this a bit more interesting, turning it into a quantum tub. Imagine if, instead of seeing three rows of flavours – the usual chocolate, strawberry and vanilla – that whenever you opened the lid, you only ever saw one flavour at a time. That’s more or less how neutrinos work. Neutrinos exist as all three of their flavours simultaneously. However, when you observe a neutrino, you only ever see it as one flavour at a time (either e, μ, or τ– just like our quantum tub of Neapolitan, being only chocolate, strawberry, or vanilla at a given time).
For the sake of simplicity, let’s call the e-flavour ‘chocolate’, the μ-flavour ‘vanilla’, and the τ-flavour ‘strawberry’.
This has a certain probability attached to it. For example, a random neutrino may, when observed, appear as chocolate half of the time, and either strawberry or vanilla a quarter of the time each. Effectively, it’s not ‘guaranteed’ that you will see a neutrino as a specific flavour.
Let’s take this even further. Imagine if you took a quantum tub of Neapolitan, saw that it was strawberry, closed the lid, bought it and walked home, only to open the lid and find out it was now vanilla. You’d be a bit miffed. Well, this phenomenon of flavour changing with movement applies to neutrinos too! For the fellow nerds who fancy a Wikipedia dive, this is called ‘Neutrino Oscillation’.
DUNE – better than the movie
The Deep Underground Neutrino Experiment, or DUNE (the acronym makes up for the unimaginative name), is a state-of-the-art experiment being built to be able to detect and measure these Neutrino Oscillations. It spans 1300km, between Illinois and South Dakota in the USA, and is located roughly 1.5km underground.
It works by counting the number of neutrinos of a given flavour in Illinois, fires neutrinos to South Dakota, and then measures the flavours again. Because these neutrinos will have travelled 1300km, the flavours will have had a chance to change. Maybe nine in ten neutrinos in Illinois were vanilla, but when measuring these neutrinos in South Dakota, only three in ten are now vanilla.
The question is, why does this matter? This is actually a really stupid pun, which is why I love it!
The Universe, and everything in it, is comprised of matter – but there also exists something called antimatter. Antimatter is the same as matter in every way except that: the charge of the particle changes (for example, from negative to positive – if the matter particle has no charge, then its antimatter equivalent also has no charge); and then, flip everything in the Universe, in a mirror (in a crude sense, antimatter uniquely behaves as if it’s moving backwards through time). For Swifties, however, you can think of antimatter like the Anti-Hero.
Crucially, when matter and antimatter come into contact with each other they annihilate – they basically destroy each other, releasing a bunch of energy in an explosion. This is a problem, because, based on our current understanding of physics, there should be an equal amount of matter and antimatter in the universe – so where is all the antimatter?
If matter and antimatter annihilate each other, and they were created in equal quantities, this implies that there should either be:
- No matter nor antimatter, as they all annihilated each other;
- Or, there’s an equal amount of antimatter across the universe to make up for the fact we’re all made of matter.
Neither of which is true – if the first scenario was true, we wouldn’t exist because we’re made of matter; if the second scenario was true, we’d be able to see significant pockets of antimatter across the Universe, which we don’t. So, what’s going on?
Well, the rate at which flavours change with distance might not be the same for matter and antimatter. This might explain why there’s more matter than antimatter in the Universe – because they’re not equal and opposite, like we currently think. This is where my own research begins.
Car crash investigation – but smaller
Neutrinos are really difficult to see, in that, we can’t see them. That’s a problem. As a result, experiments, like DUNE, work by not looking at neutrinos themselves, but rather, looking at what they crash into. Neutrinos are like ‘hit and run’ cars, causing a bunch of visible destruction and collateral damage, and then disappearing without being caught up in the aftermath.
Being able to track what caused the damage, to make sure it was a neutrino, and making sure we don’t miss any of the hard-to-see collateral, is essential for these experiments. In fact, we’ve gotten so good at this, that we can reconstruct information like the energy of the neutrino! My job, more specifically, relates to making this reconstruction as close to reality as possible.
So, we never actually directly detect the neutrinos, we simply wait for them to be terrible enough at moving and crash into enough things, that we can see they’ve passed through our detector, because of the damage they’ve wreaked in their pathway of destruction. Aren’t they just delightful wee things?
Why bother with this research?
It’s one thing to say there’s high-level physics applications, like solving some of the biggest unanswered questions in physics or proving Einstein right… again. But, what will this do to help us in our everyday lives?
The reality is that technologies we develop, in order to perform these experiments, are precisely what the most important revelations for humanity are. The by-products of our research endeavours are what become the most useful for society, whether it’s X-Rays, MRIs, the internet (world wide web), cancer treatments, touchscreen technology and so on. These are all technological by-products of physics research, like the very research I do. DUNE, and many neutrino experiments like it, will follow this exact same pattern.
To that end, this has been a very condensed introduction into some of my work as a PhD student in astroparticle physics – and hopefully, it not only helps legitimise the importance of research that I, and many others are working on, but also sparks a wee bit of light onto the hidden world of fundamental particles.
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