The Alpha Magnetic Spectrometer [AMS] finally reported its first scientific results today. AMS, a rather large particle physics detector attached to the International Space Station, is designed to study the very high-energy particles found flying around in outer space. These “cosmic rays” (as they are called, for historical reasons) have been under continuous study since their discovery a century ago, but they are still rather mysterious, and we continue to learn new things about them. They are known to be of various different types — commonly found objects such as photons, electrons, neutrinos, protons, and atomic nuclei, and less common ones like positrons (antiparticles of electrons) and anti-protons. They are known to be produced by a variety of different processes. It is quite possible that some of these high-energy particles come from physical or astronomical processes, perhaps very exciting ones, that we have yet to discover. And AMS is one of a number of experiments designed to help us seek signs of these new phenomena.
The plan to build AMS was hatched in 1995, and the detector was finally launched, after various delays, in 2011, on a specially-ordered Space Shuttle mission. Today, Sam Ting, winner of the Nobel Prize for a co-discovery of the charm quark back in 1974, presented AMS’s first results — a first opportunity to justify all the time, effort and money that went into this project. And? The results look very nice, indicating the AMS experiment is working very well. Yet the conclusions from the results so far are not very dramatic, and, in my opinion, have been significantly over-sold in the press. Despite what you may read, we are no closer to finding dark matter than we were last week. Any claims to the contrary are due to scientists spinning their results (and to reporters who are being spun).
The logic behind AMS is this. If dark matter is indeed made from particles, those particles, as they drift through space, will occasionally encounter one another. And when two dark matter particles collide, if they are of an appropriate type (for instance, if dark matter particles are their own anti-particles, as is also true of photons, gluons, Z particles and Higgs particles), they may annihilate into other, more familiar particles, including (but not limited to) electrons and positrons. Assuming the dark matter particles are rather heavy, their large mass-energy [i.e. E = mc² energy] will get transformed in the annihilation process into large motion-energy of the lighter-weight, familiar particles. In other words, the familiar particles produced in the annihilation of dark matter particles will be high-energy cosmic rays, of the sort that AMS is designed to measure.
Thus annihilation — or, alternatively, if they can occasionally occur, decays — of dark matter particles can serve as a new source of high-energy particles, including perhaps positrons, and also perhaps anti-protons or even entire anti-nuclei.
To see a hint that this may be happening, the simplest trick is to count, in the cosmic rays, the number of electrons and positrons of a certain energy, and see what fraction of them are positrons. Naively, without a source like dark matter annihilation, one would expect this “positron fraction” to become gradually smaller at high energy. That is because electrons are abundant in the universe and are rather easily accelerated to high energy, whereas most positrons are expected to produced only as a by-product — when the high-energy electrons hit something out in space — which means the positrons would generally have lower energy than the electrons that were needed to produce them. But dark matter annihilation or decay would typically produce as many positrons as electrons, and with the same energies; and the typical energy would be comparable to or a bit lower than the mass-energy of the dark matter particles. So if dark matter annihilation or decay is occurring, then, at energies near to (but below) the mass-energy of dark matter particles, the positron fraction might begin to grow larger, instead of smaller, with increasing energy.
Thus, an increasing positron fraction is a signal of dark matter particles annihilating or decaying into known particles. But it isn’t a smoking gun. Why not? Many reasons. There’s the potential for false negatives. Dark matter may be made of particles that don’t annihilate or decay. Dark matter may be made of a small number of very heavy particles, in which case annihilation may occur but may be too rare to contribute many cosmic rays. Or annihilation may not produce that many electrons and positrons compared to other particles that we would already have observed. (In fact, the latter two issues typically are the case for many popular models of dark matter, such as those from simple variants of supersymmetry.) Meanwhile, false positives are possible too: there might be astronomical processes in nature that we are unaware of that can also create high-energy positrons; pulsars have been suggested as a source. So when interpreting the results from AMS and similar experiments, appropriate caution and clarity of thought [not universally observed in today's press articles] is always necessary.
The positron fraction was already examined by the PAMELA satellite, which, though smaller and less powerful, beat AMS into space by several years. Like AMS, PAMELA can measure the energies of particles that enter the satellite, and can distinguish electrons from positrons. Famously, PAMELA did indeed find a positron fraction that is growing, over energies of 10 – 100 GeV or so. (Their data are the blue squares in Figure 1.) The FERMI satellite has unexpectedly and cleverly managed to measure this too, up to 200 GeV. (Their data is the green triangles in Figure 1; note the large uncertainties given by the green band.) So we already knew AMS would find such an increase! We already were confident that there is an unknown source of positrons above 10 GeV. What we wanted to know from AMS was whether the effect continues at even higher energy, well above 100-200 GeV, and whether their more detailed observations would give us insight into whether this increase is due to a new astronomical effect or a new particle physics phenomenon.
Well? What did AMS say? Their data are the red dots in Figure 1. What do they mean?
First, AMS confirms what PAMELA and FERMI observe, that the positron fraction is increasing for some reason (though amusingly AMS phrases this “confirmation” as a “discovery”). The detail with which they make this measurement is very impressive! Look at all the red dots and how small the uncertainties are compared to previous measurements! Unfortunately, this detail does not reveal anything striking; there are no interesting features in the data, which instead is rather smooth. Perhaps the most interesting aspect is that the rate of increase of the positron fraction appears to be slowing down gradually as the energy increases. But this doesn’t have any obvious meaning, at least not yet.
Second, AMS is able to go a little higher in energy than PAMELA and FERMI. But not that much — only to 250-350 GeV — and not with enough data, at the moment, to really give us any insight as to what is going to happen to the positron fraction at higher energies. So we don’t really know much more about the high-energy behavior than we did before.
Third, AMS is working well and will be able, eventually, to give us more information. But it will take time. They both need to understand their experiment better and to collect more data. My guess? it will not be months, but years. Maybe 3. Maybe 5. Maybe 10. I don’t know. Let’s hope the space station doesn’t have any glitches or serious difficulties over that time.
One problem for the higher-energy measurements that are yet to come is that the systematic uncertainties on the measurements are becoming larger. This is due mainly to the difficulty of measuring the charge of the particles — which you absolutely have to measure if you are going to distinguish electrons from positrons. For instance, AMS reports that for positrons and electrons in the range of 206 – 260 GeV, the positron fraction is 15.3% with a statistical uncertainty of +- 1.6% and a systematic uncertainty (dominated by the charge measurement) of +-1.0% — which is about a 6% relative systematic uncertainty on the fraction itself. In the highest available range of 260-350 GeV shown today, the fraction is 15.5% with a statistical uncertainty of 2.0% and a systematic uncertainty of 1.5% — a 10% relative systematic uncertainty. At still higher energies the relative uncertainty will get worse: a particle’s electric charge is determined by measuring which direction the particle’s trajectory bends in a magnetic field, but the higher the energy, the straighter the trajectory becomes, so the challenge of determining the bending direction becomes greater. Can AMS meet this challenge? And can they convince us that they have met this challenge? That will probably be the question for AMS in the coming decade.
A final point that I don’t yet understand in detail: AMS finds that the positrons seem to come equally from all directions. That’s somewhat important, in that astronomical causes of the growing positron fraction might not be distributed equally across the sky, in contrast to dark matter. But I think AMS’s result is still too crude to tell us much at the moment.
To conclude, a word of caution: no matter what AMS finds, unless it is hugely spectacular, it will not be easy to settle the controversy over the source of the positrons. Current consensus among experts is that it is very unlikely we will see a smoking-gun of dark matter from any experiment like AMS. There’s nothing in today’s data, nor in the projection of that data into the future, that suggests we’re on the verge of a definitive discovery. We can hope for a surprise, though.
(You can read a similar point of view at Resonaances.)