What dark matter isn't

It’s 10 years since the recently restarted particle accelerator in CERN, the Large Hadron Collider, discovered the elusive Higgs Boson or “God Particle” as some also refer to it. So let’s talk particle physics now! Most of the time in science, researchers tell us about what they have found. But occasionally important papers are presented that tell us what hasn't been found; and this can help to rule out how something doesn’t work or what something doesn't look like and points us more in the right direction for where we should look. This week Ben McAllister, from the University of Western Australia and Swinburne University of Technology, has done just this in grappling with what is literally a massive problem in physics, as he explained to Chris Smith...

Ben - We have been trying to get to the bottom of one of the biggest mysteries in the universe, as I like to call it. For about a hundred years, we have known that all of the stuff we can see in the universe, that's the kind of matter that makes up you and me and the planet and the sun, is not even close to all the stuff that there is. We can't explain the way things move around. If we only consider the stuff we can see, particularly, it seems like the gravity that's provided by the stuff that we see, wouldn't be strong enough to pull stuff in space around as strongly as we see it pulled, if you like. So we infer that there must be a lot of invisible stuff out there about five times as much as there is of all the stuff we can see providing additional gravity. So it's this huge cosmic mystery. And we've known it's for a long time. And we just don't know exactly what it is.

Chris - If we are only inferring its existence, we can't see it. And we don't actually know how to detect it. How can we therefore work out what it is?

本——是的,这是一个很好的问题. So there's lots of different ways that people try and get to the bottom of dark matter. The most popular theories rely on introducing new particles. So new kinds of matter that are different to the kinds of things we already know about that can account for the dark matter. But generally speaking, the lines of attack for detecting dark matter are you can do something like what they do at the large Hadron Collider at CERN. Take particles we already know about and smash them together and hope that some interactions occur that cause them to generate dark matter. Or you can look in space with telescopes and hope that maybe some dark matter particles out there are interacting in such a way that they generate particles we can see like photons, particles of light, or other things like that, and we can catch them with telescopes. Or you can do what we are doing here in the study that we're talking about today, which is called direct detection, where you build a detector on earth and you try and catch an interaction of one of the dark matter particles that's passing through the earth with something in your detector.

Chris - You've therefore, presumably got in mind some kind of model for what shape or structure that dark matter entity might take. So you are saying, if it looks like this, it should do the following things in our detector, can we detect it?

Ben - Yeah, that's exactly right. So again, you need a hypothesis to start with of what the dark matter is. There are a few of them. In particular, the work we're talking about here is trying to detect, a very popular dark matter candidate, a thing called an axion, which is a new particle that was proposed in the seventies. Funny story it's actually named after a brand of dish soap, for the way it cleans up problems. And the nice thing about the axion is that it is supposed to have an interaction with photons, which are particles of light, which is great because it means we can take dark matter, this invisible thing that we can't see at all and convert it into a little flash of light, a thing that we're great at trying to detect. So that's what we do with the organ experiment, which is the work that is being published this week.

Chris - And what have you seen so far?

本,这是有趣的。当you do these kinds of experiments, you obviously hope to see something you hope to see that little flash of light in excess of the background that says, oh great, we found this dark matter signal, but more often than not, you don't. What you can then do as we've done here, and as other experiments have done in the past is you can place stringent limits on the existence of the dark matter. You can say, okay, our experiment is this sensitive in this region of parameter space of dark matter? And we didn't see anything, which means we can put a big block through it and say, okay, the dark matter is not here. So what you can do is you set it to be sensitive to a certain axion mass. You wait some amount of time to see if you get a signal and then you move to another axion mass and you do that for some range of masses. And if you don't see the axons, you say, okay, we can rule them out to this level of sensitivity within this mass range. And that's what we've done here. We've ruled out some new parameter space, the most sensitive search for axions to date in the mass ranging question.

Chris - How do you know your machine's just not broken?

Ben - Yeah. Right. Well, you can do all kinds of different calibrations and tests where you inject signals. That would mimic what you'd expect to see from an axion converting into photons and see if you pick those up. But the fundamental answer is really that it does depend on your model. You're saying, we're assuming for this thing, where the axion is the dark matter and it's interacting in this way inside the detector. And then we can generate simulated versions of that signal and see that we would see them and, say with some confidence, some degree of confidence that we have ruled out the axions in that range.

Chris - If they don't exist in that range and your experiments are correct. What effect does that have in terms of constraining what axion are?

Ben - Yeah, it's quite funny to be reporting a negative result, but it is really the way science works isn't it? I mean, we're doing a big search here. It's sort of like the wild west. There's all this territory that we have to go out and search. And what we're doing is essentially saying in concordance with the international community, well, we've looked here and it's not here, so we don't need to spend any more time looking here and we can start moving our detectors into other ranges and work with international collaborators in the field to gradually sweep through this huge range and hopefully eventually detect the dark matter.

Chris - And if it turns out your right and you find the elusive axion, what does that do to physics?

本——好吧,我的意思是,这就像一个巨大的对位digm shift because we would have answered one of the biggest mysteries in the universe, what the nature of the dark matter is. And at the same time detected this new particle, and it would be the start of a sort of new era of axion physics. You could imagine doing all kinds of interesting things. You can do astronomy using these axons. People have various proposals for different kinds of seemingly futuristic technologies that they might be able to be used for. But it is one of those questions where we almost don't wanna get too far ahead of ourselves in terms of possible applications of these things at the moment. I think it's more like we know there's this huge unexplored realm out there. There's all this dark matter and we just don't know what it is. And we will never learn anything new if we only ask questions we already know the answer to, like we never would've discovered radio waves, we never would've discovered modern electricity, if we'd only set out to do experiments where we said, okay, we know exactly what's gonna come out of this by the time we're finished doing it. We just need to start probing away at these mysterious things that seem at the time, like just unexplained physical phenomena. And that's exactly what we have here. And we know throughout history, when we probe these big unexplained physical phenomena, we make big discoveries and we change the way the world works.