Here’s a brief overview of six hints of dark matter — signals (two in the sky, and four underground) that just might be from dark matter particles doing something interesting — that are being explored, as of April 2013. One or two of them might turn out to be true, but not all six can be, since they contradict each other. And this shouldn’t worry you, because this situation is perfectly normal with forefront science; research is difficult, and most hints of something exciting turn out to be mirages — statistical flukes, previously unknown quirks of a measurement technique, or even pure mistakes. With the Higgs particle, for instance, we had several false alarms (one as recent as July 2011) before the alarms finally rang true in July 2012. So we have to be patient and cautious, yet hopeful; discoveries are rare, but they do happen.
Dark Matter Overhead
First, there is a hint from the Fermi satellite (pointed out by Christoph Weniger) that photons [particles of light] of a particular energy (about 135 GeV, i.e. about 143 times the mass-energy [i.e., the E=mc2 energy] of a proton) are streaming out of the center of our galaxy. This could potentially be a signal of dark matter particles (which, moving slowly around our galaxy, would be especially numerous at the galactic center) in the process of colliding, annihilating, and turning into photons.
Click here for more detailed explanation of how this works. Briefly: Energy conservation assures that the energy in two annihilating dark matter particles (which is mostly in mass-energy, since dark matter particles move slowly inside a galaxy) is converted into the motion-energy of two photons — and thus the energy of each photon is equal to the mass of the dark matter particle times c².
Should we worry that this signal isn’t what it seems to be? A minor issue is that a standard “WIMP” (a Massive Particle that Interacts with matter via the Weak nuclear force — roughly speaking) couldn’t produce this signal without giving other signals that we would also see (such as lots of photons at lower energy.) But WIMPs are more popular than they deserve to be (see, for example, this paper), and other types of dark matter particles, ones that various particle theorists have imagined over the years, could potentially do the job.
A much more serious concern is that not only is the signal coming from the galactic center, it also appears to be coming from the limb of the earth and maybe from the sun. You wouldn’t expect that from dark matter annihilation. And the fact that this signal shows up in strange places where it isn’t expected might mean that the whole business actually reflects a subtle problem with Fermi’s photon detector. Nobody knows for sure at this point.
Here’s another example. The AMS experiment attached to the International Space Station recently claimed to have made a big “discovery” (though actually, as most press articles forgot to mention, what they really did was confirm something that the PAMELA experiment already discovered back in 2008 — indeed I don’t know why Dennis Overbye didn’t emphasize this more in his New York Times article on AMS, given that he himself had written about the PAMELA experiment’s discovery back in 2008.) What PAMELA discovered, and AMS confirmed and explored in much greater detail, is that in outer space, one finds an excess, compared to what one would have expected, of high-energy positrons (the anti-particles of electrons). The extra positrons are observed to have energies from 10 GeV on up to at least 350 GeV, where AMS’s data currently fades out.
Now maybe — maybe — those positrons are from the annihilation of dark matter particles! But if so, they can’t be the same type of dark matter particles that FERMI might be seeing from the galactic center. Any dark matter particles responsible for AMS’s signal would have to have mass above 350 GeV/c², in order to produce positrons of 350 GeV, while if there are dark matter particles making FERMI’s photons, they’d never give positrons with energies above 135 GeV. [Again this is just energy conservation; if your two annihilating dark matter particles have mass of 135 GeV/c² each, and are moving slowly enough that their motion-energy is small, electrons and positrons produced in the annihilation can't have energy above 135 GeV each.] So FERMI and AMS can’t both be seeing effects of dark matter; at least one of them is seeing something else.
As was suggested all the way back in 2008 (and as the AMS people were careful to admit), the positrons PAMELA saw then, and that AMS sees now, might be created by an astrophysical effect, such as a nearby pulsar (a rapidly spinning star with a powerful magnetic field, which can serve as a natural particle accelerator and become a source for extra electron-positron pairs.) And as everyone has known since 2008 (and as the AMS people were careful not to admit), a simple garden-variety neutralino from supersymmetry (or any other WIMP) could never give you such a big signal unless there was a previously unknown force able to enhance the annihilation rate. And even then, you wouldn’t get these positrons without getting other signals we also should have seen, unless you imagine this is from a very non-garden-variety type of dark matter — let’s call it a “jungle-variety” type of dark matter, not a bad name, because a whole host of particles are hidden away and hard to see. Jungle-variety theories are really cool, but the dark matter particles in such theories are not the simple supersymmetry WIMPs that the press articles about AMS all mentioned. (Talk about mixing your applets and your orangoutangs...)
Dark Matter Underfoot
Let’s move on! Anyone remember DAMA (now DAMA/LIBRA)? They’ve been claiming evidence of dark matter for over ten years! And they definitely have a real signal of something! Maybe it’s dark matter, and maybe it’s not.
You see, one clever way to look for dark matter is to let it find you. Just put a chunk or a vat of a carefully chosen and purified material in a mine deep underground. (Going deep underground drastically reduces effects from cosmic rays, high-energy particles from outer space.) Since dark matter must go right through ordinary matter, rarely leaving any trace, dark matter particles stream right through the rock and down into the mine and into the chunk or the vat. And if you’re very very patient, one of those dark matter particles might hit an atomic nucleus inside your material, and that little love tap may be just loud enough for you to detect it, if you’ve designed a very clever experiment. This is what DAMA, XENON, CoGeNT, CRESST, CDMS, and a host of other experiments are doing and have been doing for a long while now.
This is harder than it sounds. Radioactivity — the process by which an atomic nucleus changes from one type to another, spitting off a high energy particle or two along the way — can mimic the effects of a dark matter particle. [A process that mimics your ``signal'' --- the thing you're trying to detect --- is called a ``background''. For some conceptual discussion of signals and backgrounds, you can read the first part of this otherwise outdated article.] Backgrounds in this dark matter detection business are often much larger than the signals, and the experimenters need to understand all of the possible backgrounds very, very well if they’re going to detect something so small.
But now — getting back to DAMA specifically — here’s something you can do that’s diabolically clever. As the earth moves round the sun during the year, its velocity relative to the average velocity of the dark matter particles is changing. It’s similar to how, if you bike around a circular track on a windy day, sometimes the wind is brutally in your face, and sometimes it is comfortably at your back. So just as the strength of the wind changes as you circle the track, the speed of any dark matter “wind” changes strength during the year. And if the probability that a dark matter particle interacts with a nucleus depends on the relative velocity between the two (as is true in many examples of what dark matter might be), then the number of dark matter collisions that an experiment will measure will vary up and down, cycling once a year. So rather than just looking for signs of a few collisions, which might just be radioactivity that you’ve misinterpreted, perhaps you should look for variations of the number of collisions during the year! If you can convince yourself that radioactivity and other backgrounds themselves can’t cycle during the year, then any oscillation of this type is a smoking gun for dark matter.
Unfortunately, although this sounds great, the backgrounds can in fact cycle during the year after all, perhaps because minor temperature changes cause more or less radioactive gas to float around inside the mine, or something like that. So although DAMA/LIBRA’s data definitely shows an oscillating number of candidate dark-matter/nucleus collisions, it’s still not entirely clear this is due to dark matter. So far no one’s been able to verify their signal, but no one’s been able to show it’s a false alarm either.
DAMA/LIBRA is not alone. Over the past year or so, the CoGeNT experiment reported that they see an excess of possible collisions too, which, like DAMA/LIBRA, have a rate that oscillates up and down during the year.
That’s not all. Last year, the CRESST experiment also reported seeing a bunch of candidates for dark-matter particles hitting an atomic nucleus inside their detectors. There are several possible effects that could give candidates of this type, but — they say — when they add up all of those effects, they should see something like 42 candidates; but in fact they see 67, an excess of about 4 standard deviations, which is very strong evidence that something’s amiss.
Finally, the newest hint: the CDMS experiment has reported that they have seen three candidates for dark matter collisions in their chunks of silicon. [They have both silicon and germanium detectors; the new result is for data taken some years ago in the silicon chunks. Since a silicon nucleus is much lighter than a germanium nucleus, silicon is more responsive than germanium to a collision with a light-weight dark matter particle.] This is really very interesting! But, as they themselves are careful to say, hardly definitive yet. What’s almost certain is that it’s not from a background they expected. It’s not so obvious at first glance; the backgrounds they know about should only have generated about half an event on average, and the probability of getting these three events is about 5% — not very unlikely at all, when you consider how many different unlikely things can happen in an experiment. But when they account for the energies of these candidate collisions, the probability drops to 0.2%. Now that’s starting to get serious! But remember: all this means is that either (a) they’ve discovered dark matter, or (b) they’ve discovered a previously unknown background that gives them a false signal.
Now when we take these last four experiments all together, the news is both good and bad. The good news is that all four of these experiments — DAMA/LIBRA, CRESST, CoGeNT and CDMS — are consistent with a dark matter particle that is somewhere in the vicinity of 10 GeV/c², more or less.
The minor bad news is that the four measurements are inconsistent with each other; the interaction probabilities the experiments infer, for a given dark matter particle’s mass, aren’t all the same, and differ by as much as a factor of ten. This is illustrated in the figure below (taken from the CDMS paper), showing that the four allowed bands associated with the four experiments’ observations don’t generally overlap with one another. So this would suggest that at least two of the experiments must be false alarms.
The major bad news is that there’s another experiment which should (naively) be more sensitive to this type of dark matter particle than any of these four experiments. I refer to XENON100. For most of these signals, XENON100 should have seen lots of candidate events, typically tens or more. But they see only two. And so, nominally at least, all of these signals are ruled out by XENON100, and/or by a special analysis of the data from its predecessor, XENON10. Perhaps it can be argued that the CoGeNT and CDMS experiments’ signals are just barely ruled out, and so perhaps they’re still really worth taking seriously.
Yet one more sobering fact (NYU’s Neal Weiner emphasized this in his talk last week in Princeton) is that in all of these underground experiments, a failure to account for a small background will typically show up as a few extra low-energy collision candidates, which will then closely resemble what you’d expect for a low-mass dark matter particle. In other words, lightweight dark matter is what an oops! will look like.
As University of Chicago professor Juan Collar himself (leader of the CoGeNT experiment) said at a conference at CUNY’s graduate center in New York City about two years ago, the saga of dark matter searches is likely to be a long story of discovering one unexpected background after another — and it’s a story which could go on quite a while before dark matter is actually found, if ever, by one of these experiments. Indeed, this is reflected in the many false alarms that we’ve seen in recent years. [Interestingly, Collar stopped talking this way when CoGeNT started seeing a signal that could be interpreted as dark matter. But we remember what you said, Juan. We remember.]
Meanwhile, this type of puzzle is what theoretical physicists live for. A conundrum! A challenge! Invent a theory of dark matter that is such that CDMS and CoGeNT can easily observe its effects but XENON100 cannot! The experiments work differently — CDMS and CoGeNT are made from chunks of silicon and germanium respectively, while XENON100 uses — surprise! — a vat of xenon. There are already many papers on the matter. The most likely outcome is that XENON100 is right and CDMS and CoGeNT are seeing backgrounds of some type. But perhaps it will turn out otherwise.
To sum up: we have at least six hints of dark matter, mostly inconsistent with one another. The new CDMS hint is roughly consistent with CoGeNT; but if they’re both really seeing dark matter, why didn’t XENON100 see a big signal? Well… all of these experiments are working hard to improve their methods and their measurements, so if any of these hints are really signs of dark matter, we should start seeing more impressive evidence within a year or so.