Searching for the neutrino floor: Why matter matters

There’s a deep connection between the universe on the largest scale and the universe on the smallest scale.

Astrophysics and elementary particle physics may seem like the complete opposite ends of the spectrum, but to understand one you need to understand the other.

I have the privilege of having a job that involves asking questions such as, “How does the universe work?” It has always appealed to me to be able to ask the blue-sky questions.

Neutrinos are one of my favourite particles. Everything about neutrinos is incredibly tiny – including their masses, which are so very close to zero that for a very long time we thought they were exactly massless. But it turns out they do have tiny masses. And those masses allow neutrinos to change identity via a quantum mechanical process that occurs over macroscopic distances.

Of all the particles that we know about, neutrinos are the least understood – there are more things to figure out about neutrinos than any of the other known particles, and the open questions we have about them are very important. We want to be able to explain why their mass is tiny, but not absolutely zero. All other fundamental particles get their mass by coupling to the Higgs boson, but there is reason to suspect that something different is going on with neutrinos.

Perhaps the biggest open question is whether the neutrino is its own anti-particle, or whether there are distinct neutrinos and anti-neutrinos. The theoretical prejudice is very much that they are their own anti-particle. But it’s an incredibly difficult question to probe experimentally, although people are certainly trying. The answer to that would give us an important clue about another question: why does the universe have more matter than antimatter?

We have good reason to expect that the very early universe had equal amounts of matter and antimatter.

We have good reason to expect that the very early universe had equal amounts of matter and antimatter. So there must have been some process early on that left us with an excess of particles over anti-particles and led to the universe that we see today – a universe filled with matter, with no antimatter equivalent of me walking around. We think there were particle physics processes that were biased in the favour of particles versus anti-particles, which took us from a symmetric universe to the anti-symmetric one we see today. It has to involve physics that breaks the symmetries between particles and anti-particles, and we suspect that neutrinos hold those ingredients.

I also work on unknown particles, in particular, dark matter. My colleagues in the Dark Matter Centre are building Australia’s first underground dark matter experiment in the new Stawell Underground Physics Laboratory, which is an exciting development for Australian science, while I run the Centre’s Theory Program, which seeks to put together the clues to understand what dark matter actually is.

In fact, there are intrinsic connections between dark matter and neutrinos. Together, they can be thought of as the known and unknown components of the “hidden universe”. Both are frustratingly hard to detect and both played a major role in shaping the universe we live in. Interestingly, neutrinos and dark matter are increasingly going to merge in terms of detection techniques.

As our detectors become more and more sensitive, one of the fundamental challenges we are going to face is that neutrinos will interact with our detectors in a way that looks identical to the interactions of dark matter. We typically put dark matter detectors deep underground, to shield them from interference. But you can’t shield against neutrinos. They go straight through the Earth. The problem is that a neutrino can bump into a nucleus in the detector and deposit energy in exactly the same way as a dark matter particle. We can’t tell the difference.

We typically put dark matter detectors deep underground, to shield them from interference. But you can’t shield against neutrinos. They go straight through the Earth.

We call this the “neutrino floor” or the “neutrino fog”. This is a problem: how can we see through the neutrino fog to find the dark matter we are looking for? However, it is also an opportunity, because we will be able to study neutrinos not only with dedicated neutrino detectors but also with dark matter detectors. So our dark matter detectors will become multipurpose dark matter-neutrino observatories. Reaching the neutrino floor is the aim of the proposed DARWIN project, an international experiment that my group recently joined.

One of the proposed ways to see through the neutrino fog is something called directional detection. In fact, this is an area that is currently being worked on in Australia. At the moment, if we saw a signal in one of our dark matter detectors, we would simply see energy deposited and we wouldn’t know which direction the signal had come from. This is true for both dark matter and neutrinos signals. But if we were able to tell which direction the interacting particles were coming from, we’d be able to figure out whether or not we were seeing a genuine dark matter signal.

This works because we expect the direction of the dark matter particles to be aligned with a particular direction on the sky, in the direction of the constellation Cygnus. On the other hand, if we saw a signal coming from the direction of the Sun, we would know it was probably just a solar neutrino. So the development of directional technologies will ultimately be necessary to see below the neutrino floor, and to be sure that we are really detecting dark matter. I’m involved with an Australian group called CYGNUS-Oz working to develop these future directional detectors, which is an exciting project to be involved with right now.

As told to Graem Sims

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