What are axions?

This week we’re delving into dark matter and so far we’ve heard about particles called WIMPs that might be what dark matter is made up of. But there are other possibilities too. One is a particle called an axion, and now scientists are developing experiments to try to detect them. Gray Rybka is from the Axion Dark Matter Experiment (ADMX) which is in Seattle, Washington, and claims to be the most sensitive axion dark matter detector on the planet, and he spoke to Chris Smith.

Chris - So Gray, first of all, what's actually wrong with a WIMP?

Gray - What's wrong with WIMPs? Well, I'd rather talk about what's right with axions. It's a particle that was predicted by nuclear physics to solve a very subtle problem, and yet when we look at how many of these axions should be produced in the early universe you get almost exactly the right amount of dark matter. I find that's too good a coincidence, you've got to look for it.

Chris - If I could see an axion what would it look like? How would I recognise one if it came and slapped me round the face?

Gray - An axion is very light and very weakly interacting with anything. If you imagine something like neutrino, it's a thousand times lighter, much less strongly interacting. It's behaviour is much more like a radio wave than even a particle.

Chris - Given that dark matter's all about gravity, if these things are incredibly light does that mean there must be lots and lots of them to subserve enough of an effect then?

Gray - That is absolutely correct. There is a huge, huge number of them.

Chris - And if they're a bit like a sort of radio wave, how are you trying to detect them because we heard from Katherine how she's trying to find WIMPs using their incidental occasional collisions between the WIMP and things like sodium iodide, how do you try and find an axion then?

Gray - Well, the axion should interact very weakly with electromagnetism and that means that if you've got a big strong magnet you can convert axion dark matter into microwaves, and after that you just have to detect the microwaves.

Chris - But I thought the whole point of this was that we thought dark matter doesn't really interact with anything?

Gray - Well, it interacts very, very weakly. You need an extremely strong magnet and to see the signal, which is in the yoctowatts, “yocto” being the bottom of the SI units scale, you need both an extremely sensitive detector and you need very very low noise levels. The thing has to be cooled to a 100 millikelvin just to keep the blackbody radiation from making it impossible to see the signal.

Chris - Talk us through actually how your detector works then, so where is it built how does it work, and what does it do when it registers a hit? How will you know when you found in axion or two coming through?

Gray - It's at the University of Washington and the outside is an 8 Tesla superconducting magnet with maybe a 50 cm bore, about a metre deep of active area. Inside that we have a big tin can, well it's a copper can, and we can tune the frequency that that can resonates at, we can tune what microwave photons it picks up, and as we tune that sweeping across frequencies, if we hit the frequency that corresponds to the axion mass it will start ringing up. And so we'll see extra power coming just out of the dark matter, and we pick that up with what's essentially a very very fancy AM radio.

克里斯,这是否意味着,当他们在this intense magnetic field being resonated at the right frequency, it's actually the particles or the waves themselves the axions that are moving backwards and forwards in generating that signal that you are detecting?

Gray - Yeah, that's absolutely right. You can view it as the axions are turning into microwave photons or you can view it as just an interaction between the axion waves and radio waves.

Chris - Actually how close are you to being able to detect what you think is a real axion now because, obviously, there are sensitivities with all these experiments? And I mentioned at the beginning that you are laying claim to having the most sensitive way of doing this, so how sensitive is sensitive?

Gray - Well, this is the exciting part. After decades of work we finally have an experiment that is sensitive to the theoretically predicted axion interactions, which means now we're operating just any day as we slowly tune this radio like experiment we could make a discovery.

Chris - Will there be a repetition of this experiment because are other people also building similar detectors because obviously it's important to do replication, isn't it?

Gray - There are a number of people who are working on detectors, mostly they're working on ways of making them more sensitive. There's kind of an interesting thing about axions which is that once you know the exact frequency to look at it’s much easier to build an experiment. So once we've made a discovery, I think that very quickly, all over the world there will be people who will be able to build experiments, repeat the signal, see that yes we see this all over the world and start doing some axion astronomy with it.

Chris - And how does this tally with what Colin was saying about his modelling? If you actually feed in axions into models like Colin's do they actually fit the bill?

Gray - Yes. That's kind of the interesting thing is there are people working very hard to say ‘Is there a difference between axions and WIMPs and what we would see astrophysically?’ So far the answer is no but there's always hoping that more subtle models will be able to fish out the difference.

Chris - And is there any grounds for considering that they might actually be more than one particle, because you're in the axion camp, we heard from Katherine in the WIMP camp, is it possible that more than one thing is happening at once?

Gray - It's certainly possible. Nature can be very very rich. Just Occam’s Razor that we hope that it's just one thing. If it is many things that's great, it'll keep us physicists in business for quite a while.