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A Story of ‘Nothing’ Part II: DUNE
This article can be read on its own, but you may want to read Part I for a bit more context.
A few weeks back, I sat in on an academic group meeting, where a Professor involved in the Deep Underground Neutrino Experiment (DUNE), in South Dakota, gave a presentation on recent news in the physics community. One of his slides featured a photo of the British Minister for Science with his counterpart in the US, both signing an agreement at the White House stating the UK Government would commit £65m and academic support towards the development of DUNE. His offhanded assessment of this situation was that 65 million is “a big number” and “it’s probably all good.”
As we saw in Part I, neutrinos are bizarre particles that have a habit of oscillating into different *flavours *(types) depending on their energy and how far they’ve travelled. What’s more, this behaviour is only possible if neutrinos have a mass, an unfathomably tiny one compared to any other particle we know of. This strange identity-changing nature of neutrinos has deep implications for the laws of physics, and the universe as we know it.
So where does DUNE come in?
Out in Illinois USA, where bison graze, stands Fermilab research facility. Resembling a Bond villain’s lair (and performing appropriate sounding experiments), Fermilab houses a beam capable of producing trillions of (totally harmless) neutrinos every second. These neutrinos pass virtually unimpeded through the Earth to the Standford Underground Research Facility (SURF), 800 miles away where a multi-kiloton tank of liquid argon will reside, forming DUNE.
Like a gigantic dartboard for the world’s tiniest and most ethereal darts, the liquid argon at SURF will provide a target for some, lucky neutrinos to collide into. Particle detectors will then record their energies and use a variety of physics-inspired and machine-learning algorithms to recognise neutrinos and determine their flavour.
The next short paragraph has some tedious naming conventions involved. Persevere and you will be rewarded with an adorable cat photo.
Recall that neutrinos come in three varieties with the catchy names *electron neutrinos, muon neutrinos *and tau neutrinos. These names reflect each neutrino’s “partner” particle, which it can convert into during a nuclear collision. Unlike neutrinos, the electrons, muons and taus (collectively referred to as leptons) have electrical charges which means they can be measured directly with bog-standard physics tools like photomultiplier tubes (things that measure very weak light signals). That makes the overall process of measuring neutrinos a lot easier, since we can detect the particle they turn into, rather than the neutrino itself!
So Fermilab makes neutrinos and SURF measures them (or more specifically, the particles they decay into during nuclear collisions). We know that neutrinos oscillate into different flavours and that this, in physics parlance, is very freaky behaviour for a particle.
But so what? Why should so many of us be paying 70p each to help a particle with it’s identity crisis? After all, *I *have an identity crisis every week, and at no (direct) expense to the taxpayer. Here are some of the scientific mysteries DUNE and its funny little neutrinos could solve.
Supernovae
When certain stars much larger than our sun have burnt through all their fuel, they have a tendency to dramatically explode in a colossal, astrophysical fireball, called a supernova. That stellar explosion fuses atoms together in energetic nuclear reactions (which by the way, is where all natural gold is formed). From Part I, you might have picked up that where nuclear reactions are concerned, neutrinos can often be found too.
Supernovae emit so many neutrinos that even when they occur in galaxies in the far reaches of the universe, the detector at DUNE will be able to pick up on them. In fact, it will be the best instrument in the world to do so. Combined with (the recently Nobel Prize-winning) gravitational waves and observatory measurements, we can understand supernovae in greater detail and more thoroughly understand how they occur.
Since this is where neutrinos are formed in their largest numbers, the flip side is we can use our understanding of supernovae and in turn, check that the neutrino flux we measure is consistent with all the other measurements we make. Any discrepancy would be a potential indication of new physics.
What’s more, supernovae explode so dramatically they occasionally compact the inside of the parent star into a black hole. Measuring these neutrinos could allow us to witness the birth of a black hole for the first time.
Combining forces
Apart from gravity, which is a whole ‘nother kettle of fish, there are three forces that really concern particle physicists. One is our old friend electromagnetism, powering fridge magnets, electricity, chemical reactions and providing the light that allows you to read this article. The other two are associated with *nuclear *processes: the weak nuclear force (that governs things like neutrino interactions) and the strong nuclear force (that holds an atomic nuclear together).
One of the great discoveries in physics in the late 20th century was that rather than considering these forces as separate entities that knock particles about by totally independent means, the weak nuclear force and electromagnetism turn out to be different facets of the same unified force. This elegantly ties the two theories together into a single framework that allows us to make better sense of the forces of nature.
The hope is that the strong nuclear force can also be mathematically combined with the other two into a sort of super-force, which would beautifully describe all known fundamental interactions except gravity. Physicists love this idea so much, they give such theories the name Grand Unified Theories (or GUTs). A lot of these GUTs predict that protons, a hydrogen nucleus composed of other particles called quarks, should spontaneously break apart or decay.
This must be an incredibly rare process if it’s possible, since protons make up so much of the world around us and we don’t see any of them spontaneously decaying all the time (if you do, please consult your nearest doctor). So to spot such an incredibly rare event, you would need a lot of protons. It turns out the 10 kilotons of liquid argon in DUNE, also contains a *lot *of protons.
And since DUNE already has lots of detectors in place and the technological capability to screen out erroneous signals that might mimic proton decay, it’s well placed to try and measure proton decay if it’s possible (or else, set a limit on how long protons are likely to live, if we don’t see any decay!). If proton decay is measured, it could reveal how the forces of nature unite.
What’s the (anti-)matter?
Remember the classic, blockbuster film Angels and Demons starring Tom Hanks? Of course you don’t, it was bloody awful. Anyway, the major threat in that film is that CERN has managed to produce some anti-matter (in real life, only enough to mildly heat up a cup of tea) which if put into contact with ordinary matter will explode and destroy the Vatican, for some reason.
Asides from being a terrible film, the principle physics is true; when anti-matter meets ordinary matter it annihiliates and usually turns into two particles of light. We actually use this technology all the time in PET (Positron Emission Tomography) scans, which use antimatter particles called positrons to annihilate with the electrons in your body so that the light given off can be used to image your insides.
The laws of physics treat anti-matter and matter practically equally. In every given reaction they are produced in nearly exactly the same amounts. This poses a big problem for understanding the early universe! If matter and anti-matter were made in equal parts, all those particles intermingling in the heat of primordial plasmas should have met each other, annihilated and turned into light by now.
In short, as far as we know, there is no good reason why you, the Earth or anything in the universe other than light should still be here. All there should be is electromagnetic radiation. Bummer.
Clearly there must be some sort of imbalance that tipped the scales in favour of matter (the stuff that makes us) over anti-matter. In the equations that describe neutrino oscillations, it turns out that there is such a see-saw tipping parameter (with the stylish name θ13). If this turns out to be large, it would go some way in explaining why nature favoured matter over anti-matter in the early universe, enough to allow us to exist. DUNE will provide one of the best measurements of this parameter and potentially help resolve the conundrum behind our existence, as well as narrowing down the mass measurements of the neutrino.
And more
As well as filling in the gaps in our knowledge of supernovae, black holes, antimatter and quantum forces, DUNE is set to flesh out our understanding of neutrino oscillation. This is the only phenomena* not* entirely predicted by the Standard Model, the current best theory for how the subatomic world works. To summarise, if neutrinos offer a window into new mysteries of nature, then DUNE is our best bet to look through it.
I framed the problem of oscillation as an “identity crisis” for the neutrino; a particle constantly changing in nature and travelling with a mass whose mechanism we don’t fully understand. But the questions that DUNE will tackle are fundamental to all of physics; they touch on the laws that have shaped the universe, from particles to galaxies, and allow it to exist in its present state. These are questions pertaining to our own existence. With any luck, DUNE won’t just fix the neutrino’s existential crisis, it may very well resolve our own.