Lab Notes: Have we found dark matter?
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When you think about it, you can't see the wind. You only see the effects of it, moving trees or sheets on a clothesline. And that's how astronomers have viewed dark matter, this mysterious stuff that's apparently 85% of the universe, for over 80 years, only seeing its effects. That is Until recently. Maybe. This is Lab Notes from ABC Radio National. I'm Jonathan Webb and today I'm talking to Dr. Teresa Fruth, an astroparticle physicist from the University of Sydney, who was involved in the recent discovery that might be changing everything in this field. Hi Teresa.
Hi, Jonathan.
So just to make sure that we're all on the same page, if we can't see dark matter, why do scientists know it's there?
We see its gravitational interactions, so we see that there's Mars. It's almost like if you had an invisible friend who sits on the sofa next to you and you can only see the indent. We can see that it makes our galaxies rotate faster, it has really important consequences in how the universe formed, like to form the structure we see today. Dark matter was kind of the little initial clusters everything else could fall into. Oh right. So it's really important to explain where we are today and how the universe works.
And what do we think it is?
There are many different things dark matter could be. It could be a particle which is quite heavy. If you think about our atomic nucleus, the protons, which are one of those particles inside. If it's a particle that's about that size or a bit heavier than that, that's uh what we typically call WIMPS, weakly interacting massive particles. But WIMP is of course something you can easily remember. So I work um in A field which is called direct detection where we're really trying to see the direct interaction of dark matter with ordinary matter. So what I'm looking at is things that are heavier than the proton. And we do that with liquid xenon detectors. So out in the US, in South Dakota, we have seven tons of liquid xenon deep underground.
And so we try to make this really quiet space. And now what do we do? is basically sit and wait and see what we see.
And I'm very curious about what it is that you're actually trying to see.
We have these models which fit in well with the standard model of particle physics. And these theory predict that every now and then a dark matter particle might scatter off an atomic nucleus. So that's you can imagine it like a billiard ball collision. That's what we are actually looking for. And it would happen very rarely, and that's the only reason why we need to go underground and make it a big mass, because it would happen so few. times that in like a thousand days of running we would maybe see a handful of events.
And what did you see in this underground tank and when did it happen?
The event itself was from um I think sixteenth of May twenty twenty three.
Okay.
Um so a while back and you might be like, Why haven't you told us until now? So that's how how our detectors work. We leave it run for a while and then we take a big chunk of data. So we took like two hundred and twenty days of data and look at it at once.
So something happened in June twenty twenty three. At what point did you or other members of the team realise that it might have been something interesting? What was the first you saw of it?
So this event happened in a energy region of our detector where we don't always look. It's not our ideal energy region. Like it's a bit higher up. So that makes it even more interesting.
Does that mean it's heavier or lighter than you were expecting?
Heavier.
Right.
It's not necessarily a surprising dark matter type.
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