Let me start with this: the AMS-02 detector on the International Space Station has not detected dark matter. It hasn’t found “indications” of dark matter, or even “hints.” It certainly is not providing the “best evidence yet” of dark matter’s existence.
What AMS has done is measure, to very high accuracy, the amount of antimatter the galaxy is bombarding us with. It might be coming from dark matter… but really there’s no compelling evidence that it is. And it’s certainly too soon to break out the champagne.
So what is AMS really telling us? The AMS experiment detects cosmic rays — protons, electrons, and the antimatter counterparts of each, antiprotons and positrons. Before the experiment ran, we had predictions of how the matter/antimatter fraction should vary with the energy of the particles.
AMS tells us our predictions were wrong. The antiprotons look about right, but there’s a huge excess of high-energy positrons over what astrophysical models predict, and a bump in the electron flux at high energies. All of these results were actually seen by earlier experiments PAMELA and Fermi, but AMS confirms them to higher precision and higher energies. There’s more antimatter than we thought; now we have to figure out why.
Here’s where dark matter comes in. One thing several models for the dark matter particle have in common is the idea that it might be its own antiparticle and thus be able to annihilate with other particles of its kind. (It’s a quirk of particle physics that a particle can be its own antiparticle, which always strikes me as a bit weird and kind of lonely, but there you go.) This annihilation produces particle-antiparticle pairs, which then interact with other particles through a particle cascade, and at the end you can get electron-positron pairs. Voila! There’s your positron excess! Problem solved!
Only, maybe not. Dark matter annihilation isn’t the only thing that makes positrons. And to make an excess of positrons without an accompanying excess of antiprotons, your dark matter model has to be… rather specific.
Basically all we can say is that there’s some kind of high-energy process out there that we haven’t accounted for. But if it’s not dark matter, what can it be?
The most popular non-dark-matter interpretation of the positron excess is that it comes from pulsars, which can use their extreme magnetic fields to accelerate particles and create electron-positron pairs. The fact that pulsars do this is solidly in the realm of known physics, and theoretical models can easily fit the signals seen in the cosmic ray experiments.
So why consider dark matter at all? Why not just give up and say, “well, it’s probably pulsars, we can all go home”?
While it’s tempting, accepting the pulsar hypothesis would be premature. Pulsar models are highly uncertain, and there could even be some other astrophysical process contributing to the problem. The data can’t yet tell the difference.
So what if it really is dark matter? What kind of dark matter would that be?
There are two major conditions for the dark matter model to fit the data.
1) It has to reproduce the positron flux. This requires the annihilation cross-section — which determines how direct a collision needs to be for annihilation — to be much larger locally than in the early universe. There are models that can get both annihilation rates right, but they need a boost: more and denser clumps of nearby dark matter, or a so-called Sommerfeld enhancement to increase the annihilation rate, or some kind of resonance.
2) While providing the positron excess, it has to ignore antiprotons. Most theories would produce excesses in each, so we have to make the model discriminate. One way is to suppose the particle only annihilates into electron-like particles. Another is to use a short-lived intermediary particle that is itself lighter than a proton. The dark matter particles annihilate into the intermediary, and since you can’t decay into something heavier than yourself, the intermediary can only decay into light things like electrons and positrons.
There are models that meet these criteria, and it’s basically a matter of taste whether you prefer them to more “traditional” models such as the supersymmetric neutralino, since the data can’t really tell the difference. On the one hand, you might have to invent a new force in the dark sector to explain the positron excess. On the other hand, sticking with the bog-standard neutralino might not be so great either — supersymmetry is under heavy pressure from the fact that the LHC so far hasn’t seen it. Maybe it’s a good time to think outside the box. Really, we just need more data, using different kinds of physics (gamma rays, direct detection, colliders) to give us some experimental handle on the theoretical possibilities.
As pointed out by the Resonaances Blog, the AMS press release mentioned dark matter nine times. They said the signal was consistent with dark matter, but “not yet sufficiently conclusive to rule out other explanations.” Unsurprisingly, several media outlets ran with this, announcing that hints of dark matter have been found, or that this is the first evidence for dark matter that we’ve seen. Neither of these statements is true. While intriguing, the AMS signal is not a “hint” of dark matter — it’s an anomaly that needs to be explained. And it’s certainly not the best evidence for dark matter we have. Check out the Bullet Cluster, or any number of extremely compelling astrophysical results that, however indirectly, make it abundantly clear that dark matter is real, and abundant.
Another misconception making the rounds (and encouraged by the press release) is that we are really close to confirming the signal is actually dark matter. The story goes that all we need is a sharp cut-off in the positron flux at energies just above what we’ve measured, and that will be a “smoking gun”; pulsar emission would only have a gradual drop. This is wrong on two counts. One, it’s not necessarily true that the dark matter cut-off would be sharp. Models that use an intermediary in the annihilation, such as described above, can have a much softer drop at high energies. And two, some models of pulsar emission also have a very sharp drop. The shape of a cut-off at higher energies will be interesting and useful, but will by no means definitively answer the question.
Another thing being held up as a sure sign of dark matter is the fact that the signal appears isotropic, coming equally from all parts of the sky. Contrary to what some media outlets are saying, dark matter isn’t distributed evenly throughout the universe — it traces regular matter, and in our galaxy is most dense in the center. But charged particles lose so much energy interacting with matter and radiation in their paths there that none of the positrons from the galactic center would make it to us. Even worse, charged particles are strongly deflected by the galaxy’s complex magnetic field, which makes it impossible to know with much accuracy which direction they came from initially. So while it’s true that the local dark matter density is close to constant, we’d expect the signal to be basically isotropic anyway. And the magnetic field means positrons from pulsars — even just one or two pulsars very nearby — would also appear close to isotropic. The directional data isn’t good enough to tell the difference yet, and might not be for the duration of the experiment.
We don’t yet know what the AMS signal comes from, but we know it’s something interesting. I wouldn’t say it’s necessarily new physics or astrophysics, but it’s intriguing nonetheless. Personally, I think it’s probably pulsars (partially because that’s a more conservative view), but I’d be extremely happy to find out it was dark matter all along. We just need more data. And in the meantime, theorists like me are going to have a lot of fun with the mystery.
Image: The International Space Station, space shuttle Endeavour remains docked with the station and the Alpha Magnetic Spectrometer-2 when it was newly installed. Credit: NASA