The Hunt For Dark Matter In The Universe

June 8, 2011


A DARK MATTER DETECTOR  about 700 meters below the ground in a Minnesota mine has recorded a seasonal modulation in staggeringly faint electrical pulses ““ the possible result of dark matter particles called WIMPs that envelope the Milky Way galaxy and collide with atoms in the detector’s germanium crystal.







THE KAVLI FOUNDATION  (TKF): Is your tentative result, which suggests a seasonal variation in the number of particle collisions with the nuclei in your detector, evidence for WIMPs ““ that is, evidence for dark matter?




COLLAR: When a dark matter particle, a WIMP, strikes the nucleus of an atom in a germanium crystal, the nucleus recoils, striking other nearby atoms and knocking off their electrons. Our detector reads this ionization as an electrical signal. What’s special about our detector is that it’s optimized to look for light-mass dark matter particles, arguably the best for that particular job.

The reason for this is that most other detectors are designed to detect particles about 100 times the mass of a proton. But if WIMPS are much lighter than that, say just a few times the mass of a proton, then you need a detector with very low electronic noise. You have to be able to distinguish the very tiny energy depositions that are triggered by these collisions, because that’s what you expect from lighter particles; they don’t pack as much of a punch as something heavier.

That’s what CoGeNT detectors are designed for, and that’s where they perform very well, dramatically so. They are sensitive to very low energies, and that’s where we got stuck. We’re observing something down there that we don’t quite understand yet. We cannot yet tell you what it is, but of course we can list many possibilities. The low energy events that we end up with ““ we don’t see anything anomalous in them. They look like they’re produced by some form of radiation.



We were to some extent lucky. The underground laboratory is pretty large, and our side was untouched by either fire or fire-extinguishing fluid. That’s what made much of the mess down there; so much foam had to be injected that it actually pushed the doors open to the laboratory and some experiments were completely covered by it. Our experiment was fine; but the detector went through a thermal cycle. Normally, we keep these devices at ninety degrees Kelvin, which is pretty chilly, and when you warm them up like this the quality of the vacuum inside the detector makes a critical difference. If it’s a good vacuum, you should be fine when you turn it on again; if it’s a bad vacuum, the detector could be useless for the delicate work we are trying to do. This is why we have analyzed the data collected up until the fire. We have 15 months of observations and we don’t know if we’ll have another day of good data with this device.

If this detector is not dead and the signal rate starts to decrease again toward next December and then picks up again afterward ““ the chances of seeing something like that start to become very small, very fast. The opposite side of the coin is, of course, if it doesn’t do what I just described. In that case, the modulation we have observed so far may have been just a fluke.

TKF: One possibility is that you’re seeing a systematic error, that is, false signals that originate from the detector itself. But if CoGeNT, DAMA and CRESST ““ all different experiments with different kinds of detectors and different methods ““ are seeing the same seasonal modulation in collisions, isn’t it unlikely that the signal is some kind of systematic effect?

COLLAR: Indeed. Yes, you’re right. The targets, or the material that the detectors are made of, are different. Also, each of these experiments collects and reads events differently.




COLLAR: They’ve been exquisitely careful about trying to eliminate other possibilities. Over the last year or so, they have implemented different precautions, and have made different tests of their detector ““ all to exclude the possibility that what they’re observing is a number of possible sources of background radiation. They’ve been discarding those one after another, and they’re now left with about 20 events that they just don’t know what to make of. They look like nuclear recoils, particularly affecting the oxygen in the crystal, and they cannot be explained away. They see evidence for 20 events that might be compatible with the same particle that might be producing a modulation in DAMA, or might be producing a modulation in our detector.




COLLAR: Well, exactly. It could be. It’s one of three things: It’s either dark matter, or it is something perfectly boring, systematic and instrumental ““ it wouldn’t be the first time in physics that we fool ourselves into thinking that something mundane is relevant. This sort of situation typically happens when agreement with other experiments is noticed, and soon after you are obsessed with observations that normally you wouldn’t pay any attention to.





TKF: You’ve been involved in other experiments to detect dark matter particles ““ namely COUPP, the Chicagoland Observatory for Underground Particle Physics at Fermi National Laboratory. This experiment uses a quartz bell jar or bubble chamber as a detector for WIMPs. But this type of detector was conceived by physicist Donald Glaser in 1952. Why turn to an old technology in the 21st century race to detect dark matter?



Let me explain something that is unique about the bubble chamber. In DAMA and CoGeNT, at the end of the day we are dealing with electrical pulses. In DAMA you have a flash of light that is invisible to the eye, and there’s a photomultiplier that converts it and amplifies it into a small current. All these things are happening very, very close to the electronic noise in these devices. In CoGeNT, it’s the same story. We pull the ionization signal out of the crystal and we amplify it. This is done close to the noise of the detector.

But a bubble is a bubble is a bubble. If you started to see an annual modulation in the rate of something that at the end of the day is macroscopic and can be detected via standard photography, there’s no concern that there’s some electronic noise or anything similar conspiring to give you signals you may mistake for a WIMP.

The whole beauty of the bubble chamber is that it all starts with the microscopic process of radiation interacting with matter. And you don’t have to move a finger to amplify it. It’s all coming from the fact that the liquid is out of equilibrium. It’s an unstable system, and when an episode of microscopic boiling happens because a particle interacted with the fluid, everything that ensues, the formation of visible bubbles, is a spontaneous transition from the microscopic world to macroscopic observables. You don’t have to amplify small signals through noisy chains of electronics or anything like it.




Under some conditions, you can create a microscopic bubble. Those conditions are met for nuclear recoils because the local heating is very high, precisely because the nucleus doesn’t go very far. The recoil track is very short. But for other types of radiation, the energy is deposited over much longer distances. It’s the same amount of energy, but it is spread out, and the particle doesn’t heat up the liquid enough locally to produce bubbles. We fine-tune the temperature and pressure of the bubble chamber to be able to detect only bubbles produced by nuclear recoils like those expected from WIMPs, and not the tracks produced by uninteresting known particles.





COLLAR: This is the same detector concept, but with a lot of improvements. It’s essentially the same thing, but hopefully even better in performance.


COLLAR: The bubble chamber, in principle, should allow us to test this hypothesis ““ that there should be a modulation in the rate of collisions between WIMPs and the nuclei in our detectors. We can make our bubble chamber sensitive to light WIMPs, and we have done that before. We have operated the chamber in conditions where we should be able to see the bubbles produced by these particles ““ if they exist.

The problem is that there are internal sources of neutrons in our bubble chamber. Some materials inside the bubble chamber are generating ““ at a very small rate ““ a neutron every so often. And we have evidence for that, because we see events containing multiple bubbles that can only be neutrons.

We identified where these neutrons are coming from, and we’re going to replace those parts of the detector this summer ““ these sources of internal neutrons in the bubble chamber. After that, we’ll attempt to look for light WIMPs with our bubble chambers.


COLLAR: In this particular case, they seem to be dominated by our ceramic piezos ““ they are electronic sensors that we use to detect the acoustic emission that accompanies the production of the bubbles. When bubbles form, it’s a rather dramatic process. You have a liquid that is out of equilibrium. It’s super-heated, so it’s a liquid that you’ve tricked into remaining a liquid when it should be a gas at the pressure and temperature you’ve set.



TKF: So as long as you are eliminating all the known sources of noise, such as the ceramic material that you’ve identified in your bubble chamber, then hopefully what you’re left with is an authentic signal of WIMPs, of dark matter?

COLLAR: Well”¦

TKF: That would be a leap of logic.

COLLAR: That would be a leap of faith. You would be left wondering, in the case of the bubble chamber, if you have rejected all the possibilities that your signals might be something else ““ for instance, some other background radiation.

Now, a few months ago, when the signals we have now identified to be these neutrons produced inside the chamber started to show, we had been pretty convinced that we hadn’t missed anything. We thought, “These must be WIMPS”. And then we saw a triple bubble, which a WIMP cannot produce. WIMPS cannot interact more than once. You’re lucky enough to get them to interact at all.


There is a lesson to be learned there. It teaches you that, not only for bubble chambers but any other WIMP detector, after you clean things up you’re always going to be left wondering what else it is that you missed. Particle physics and in particular dark matter searches can be very tricky.


COLLAR: That’s actually a technology that nobody has yet ““ a technology that can see the direction of the recoil. There are small prototypes that researchers have been building, and it is an extremely promising technique.


The day someone observes the modulation in the direction of the recoils ““ that would be very hard to mimic, and it would be the next step on the road to a discovery. It would be very hard to explain such data as anything else other than dark matter, because there’s essentially nothing else in nature that could imitate that.

The problem is that the only way that we know how to spot the direction of a recoil is by stretching the short tracks they make in a solid. They only span on the order of 500 angstroms, so there’s no way we could image that in a radiation detector. The solution, then, is to rarify the detector medium, to move to gaseous detectors and to operate them at very low pressure. Then, those tracks ““ those little tracks that the recoils might produce with a preferred direction if they’re coming from WIMPS ““ they can be stretched to a few millimeters. We have the technologies to image that ““ barely, but we do.



If all these results are replicated with different experiments around the world, which use different materials and different methods, would you feel confident that we have detected WIMPs ““ that is, dark matter?

COLLAR: I’m going to quote my colleague here in Chicago, (astrophysicist) Rocky Kolb. Rocky says, “ËœIt’s going to take a village to discover dark matter,’ and I agree with that. It’s going to take more than the direct detection community observing these recoils. Certainly the directional signature would be fantastic, but I think we’re really far away from getting to that point, if we ever get to that point.



On top of anything that the detection community might find, there’s a theoretical framework that explains why these particles exist, why they have the mass they have, the probability of interaction that we observe in experiments, etc. This theoretical framework also generates new predictions that you can go out and test.




Now, maybe the CRESST guys are paying more attention to those 20 events than they should, because they’re biased by DAMA’s pre-existing results. Same exact thing applies to us and our modulation within CoGeNT. So on and so forth.



Nobody knows what happened there. We do not have a good explanation as to why these objects were observed in the first place. The evidence just went away. So that’s why you should not rely exclusively on direct detection experiments for dark matter, in my opinion.


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