What are WIMPs?

Dark matter might be WIMPs - weakly-interacting massive particles. But what are they? How do we detect them?
14 May 2019

Interview with 

Katherine Freese, University of Michigan


A computer generated image of an atom


Historically, one popular candidate for dark matter is a particle which, ironically for something that’s actually supposed to be quite hefty, is called a “WIMP”; but don’t be misled: it actually stands for “weakly-interacting massive particle”. And these WIMPs may occasionally be knocking into the atoms in your body, which might help us detect them! Katie Haylor spoke to Katherine Freese from the University of Michigan to learn - what exactly IS a WIMP?

Katherine - There's a lot in that name. If we're right and these are the dark matter particles then there would be billions of them going through you every second. But they're not going to do anything to you because the interactions are really really weak. There are four forces of nature that we know about; there is of course gravity, electromagnetism, the strong force that holds your nuclei together, and the fourth one is the weak force. So we know that WIMPs feel gravity and we know that they don't have anything to do with electromagnetic force because they don't give off light, and the strong force when they bump into you they are not knocking you over so weakly interacting mass of particles means that they have the weak force only in addition to gravity and that they weigh about 1 to 10 thousand times as much as a proton.

Katie - Okay. So why do think that these are the dark matter particles then?

Katherine - One of the reasons that we think WIMPs are such good candidates is because they're automatically there in particle theories that have nothing to do with dark matter. Supersymmetry is an extension of the standard model of particle physics and if you postulate supersymmetry you are automatically get twice the number of particles. For every particle we have today, you have a partner which will be heavier than the particles we know about. And the lightest one of these supersymmetric candidates, that makes a perfect WIMP candidate. And we didn't put it in there to explain the dark matter problem, it automatically comes out of theories that have nothing to do with dark matter. Killing two birds with one stone is a good thing.

Katie - How do we go about trying to detect these particles in the first place if they're so hard to find?

Katherine - The way to look for WIMPs is to look for them scattering off of detectors. You have to put these detectors deep underground to get away from competing signals so you have to go about a mile underground. Some of these detectors are made of giant vats of xenon liquid. Some of the other detectors are made of crystals of very specialised material such as sodium iodide crystals. So deep underneath the mountains you have these detectors sitting there waiting for WIMPs to hit them, and then occasionally you'll get a signal, that's the idea.

Katie - How big are these things?

Katherine - The xenon detectors are a ton in size, you need a ton of material. Even with that, then the expected count rates you'd get one WIMP hitting one xenon nucleus a day. When WIMPs hit the detectors it eventually gives rise to a signal of light and there are specialised tubes that measure when the light hits them. Another thing to look for is when the WIMP hits the detectors they produce phonons; this is sort of like heat travelling through the detector and you look for that heat deposit.

Katie - Have there been any promising results?

Katherine - There is one experiment that has results. This is the DAMA/LIBRA experiment that is made of sodium iodide crystals and it’s sitting under the Apennine mountains outside of Rome. What the DAMA/LIBRA experiment sees is an effect which my collaborators and I predicted back in the 1980s. Because the Earth is going around the Sun, the count rate in the detector should go up and down with the time of year, and this is exactly the kind of signal that DAMA/LIBRA has pulled out of their data and they see it. They now have more than 10 years worth of data and there is absolutely no doubt that they're seeing something modulating, but the question of course is is it dark matter or something else.

Katie - Why would you expect the dark matter signal to vary at different times of year?

Katherine - The count rate on these detectors depends on our speed relative to the WIMPs. That speed is determined by two things: first of all, as the sun moves around the centre of the galaxy we are moving into what you might think of as a wind of WIMPs; it's like when you're driving, it looks like the raindrops are coming into your windshield because you're actually moving into them,so the same thing is true for us. However, on top of that the Earth moves around the sun which means that the speed with which we're moving into the wind of WIMPs changes with the time of year. So when you add those two together then we're moving the fastest into the WIMP wind in June and the slowest in December, so we expect a lot more counts in June than we would get in December.

Katie -  Is that what you get in the data then?

Katherine - Well, what they're measuring is the WIMP hits the sodium iodide crystals and then that causes a flash of light, and you can count how many light flashes you get. Now most of them are due to background, to junk that you have to remove but what you're hoping is every now and then one of them is really due to a WIMP. So they count the number of light signals they're getting at all different times of the year - you can see that it goes up and down with the time of year. It exactly matches our predictions which is pretty amazing.

Katie - Are there any potential issues with that because I can imagine there's quite a few things that vary with the different times of year, right? Do you know that it's the WIMPs?

Katherine - That's the problem, we don't know that it's there WIMPs. People, of course, at the beginning proposed many many alternative to WIMPs that could explain the seasonal variations. They thought ah well, it could be the temperature; it could be the atmosphere; it could be muons; it could be all kinds of different things, but it turns out that all of those alternative explanations are just plain wrong. So nobody has any alternative explanation to the WIMPs but the reason we are not all jumping up and down with joy is that when you get this kind of signal you have to repeat it in another experiment, but the vats of xenon are sitting in the same laboratory right next door and they don't see a signal. So what is going on? We have to have more sodium iodide crystals somewhere else on earth to check what is going on with DAMA.

Katie - And is that going on then?

Katherine - Yes it is. There are three experiments looking for it and these three experiments are the Cosine 100 experiment which is based in Korea. There's also another one called Sabre which is a joint Princeton-Australian collaboration, and a third one on in Eise in Spain. So within the next 3 to 5 years we will have an answer; or was DAMA actually seeing WIMPs or was it some noisy signal that we'll never be able to explain?

Katie: Katherine Freese from the University of Michigan.


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