As opposed to the usual case with ordinary recoils in e.g. liquid Xenon, we are relying on this quantum weirdness to allow us to look at dark matter which is lighter than a proton. The basic mechanism that we are looking at is one where dark matter comes in and scatters off a helium atom and then creates two quantum excitations (basically fancy quantum sound waves). Because of the weird quantum nature of the scattering, the excitations travel in opposite directions with nearly the same momentum. Just to complete the analogy and emphasize how weird this is, it's like if you break in pool and then only two billiard balls are affected (all the others stay *exactly* glued to where they are) and those two billiard balls travel exactly 180 degrees away from each other. In classical mechanics, this situation would be *extremely* unlikely.
With Kathryn Zurek
Dark matter. It's very real, it's very abundant, and it's very mysterious. It makes up most of the mass of the universe and we've known about it for almost 100 years, but we have no idea what it's made of. The main thing we know about it is that it's *not* made of obscured ordinary matter, like the kind that everything we know about is made of. There is definitely a new kind of stuff out there, and we have very few hints as to its properties. In this paper, we focused on proposing a brand new way of trying to learn more about dark matter.
One very well-motivated candidate for dark matter is called the WIMP, which stands for weakly-interacting massive particle. There were a series of cosmic coincidences that made us think the dark matter is made out of WIMPs, for instance the fact that WIMPs can account for the abundance of dark matter relative to ordinary matter and can also explain why the Higgs boson is lighter than we might have naively predicted. Because of these motivations, searches for dark matter in experiments have largely (and quite successfully) focused on searching for these WIMPs. However, we are now getting to the point where WIMPs are starting to look like they are almost ruled out by these experiments. In order for the dark matter to be made of WIMPs, you would need to work very hard to explain why we haven't seen it yet, and as your explanation gets increasingly complicated we have to start invoking Occam's razor. Sorry, WIMPs ;)
This is why in the past few years, lots of people have considered other kinds of well-motivated models. You're probably thinking "great, this means we can just use the experiments to look for these other models." Well, not really, no. See, the WIMP particles are relatively "heavy," typically around the mass of a proton or even heavier, while a lot of new models predict that the dark matter would be up to a million times lighter than that. The experiments that look for dark matter work similar to billiard balls scattering off each other: you have a bunch of target material in your experiment (for instance, liquid Xenon) and you wait for a dark matter particle to "break" off it like the white ball at the start of a game of pool. That works fine if the billiard balls are roughly the same mass, but what if the dark matter is a speck of dust instead of a billiard ball? Practically, if dark matter is too light, you won't see any effect in these experiments because the recoil effect will be too tiny to detect.
That's where we come in! We have proposed a novel method of detecting light dark matter using a pretty funky state of matter: superfluids. Superfluids are liquids with zero viscosity: picture dipping your hand in water and pulling it out and it's instantly dry, or picture a fluid no container can hold because it will just creep through the pores. Clearly, this is a very weird state of matter, and the weirdness comes about largely due to macroscopic quantum effects. As with all quantum magic, our classical intuition from interacting with objects around us is all wrong. So for the case of superfluids, when you play with billiard balls they don't recoil the way that you might expect.
In this paper, we computed the rate for this kind of quantum scattering to happen and found that we can experimentally probe and constrain dark matter up to one million times lighter than the proton. This relies on having very good energy resolution, which may be attainable in just a few short years. Using this same energy resolution and normal scattering, we can also cover dark matter up to one thousand times lighter than the proton. As shown in the above plot, the two-excitation process (labelled "2X") and the nuclear recoils ("NR") are very complementary in terms of the dark matter masses they can probe. And the dark matter cross section that they can plausibly reach rivals the sensitivity of existing experiments looking at higher-mass dark matter. Some physical dark matter models are shown in the plot as lines, which represent the cross sections that those models would have at various dark matter masses; the coupling constants and force-carrier masses are shown, and the sensitivity of this kind of experiment would be able to greatly constrain dark matter even with very weak couplings to ordinary matter.
I'm very excited about the prospects of this going forward, as we've been talking to some experimentalists who think that this might actually get built in the coming years. If this happens, this could be the first way to directly probe light dark matter all the way down to the warm dark matter limit (a limit which says that if dark matter was *really* light, it would have a measurable effect on the gravitational influences that form galaxies.) It's also likely that this kind of experiment will be very "clean" in terms of separating the dark matter signal from contaminants or noise. The reason for that goes back to the weird quantum process that causes the excitations to be "back to back" and moving in opposite directions. Random noise wouldn't have this property, so requiring that the excitations are back to back is a way of ensuring that we are detecting dark matter and not some artifact of the detectors or of the target material.
Right now the field is exploding with lots of new ideas for detecting dark matter, and this is one piece of that rich tapestry. Things should be quite exciting in the field of direct detection in the next few years, definitely something to watch! As for me, I had fun learning some condensed matter physics and now I have some more tools in my back pocket as I think about the universe and all its mysteries. :)