Why AMS hasn’t (yet!) demonstrated the existence of dark-matter

The recent publicity around AMS’s first observational results sparked quite a bit of speculation on what those observations mean for dark-matter. AMS is a particle detector attached to the International Space Station, and it observes a wide variety of high-energy particles that, for historical reasons, are generally called cosmic rays. The experimentalists running AMS hope to discover new physics by studying these cosmic rays.

The first announced results from AMS came out a few days ago, and despite what you might read in the press, dark-matter has not been detected. What has been detected is the largest sample of antimatter cosmic rays to date, in the form of positrons (positrons are the anti-particle of the electron). Since a particle/anti-particle collision results in the total annihilation of both particles (releasing pure energy), it would be easy to spot significant concentrations of antimatter in the universe, or at least the boundary between significant antimatter concentrations and ordinary matter. Just look for a blaze of gamma ray photons! We don’t see this phenomenon in general, so antimatter is rare in the observable universe. But AMS has observed about 400,000 positrons (out of roughtly 25 billion cosmic ray events) since it began taking observations in 2011, which is by far the most antimatter seen in Nature to date. Since we see little to no antimatter concentrations in the universe, the antiparticles detected by AMS must have been created somewhat near the Earth (the longer they travel, the higher the probability they will annihilate in a collision with matter). It turns out that we expect these positrons to have been created within our own galaxy–that’s ‘near’ by astronomical standards–by various physical processes. The earliest observations of positrons from satellites indicated an excess of positrons over what was predicted from known physical processes in our galaxy. AMS has confirmed that excess positron flux. That means either new physical processes nearby, or something is wrong with the prediction that positrons do not survive the trip across the intergalactic medium. Both explanations indicate new physics.

One explanation involving new physical processes involves dark-matter. Dark-matter consists of particles that do not interact with the electromagnetic field (hence the ‘dark’ part) but which do interact with the gravitational field (the ‘matter’ part). Dark-matter might also explain some other puzzling observations in astronomy, but if dark-matter is the correct explanation, there is a huge amount of it. In fact, the dark-matter explanation requires that dark-matter be the dominate form of particles with mass in the Universe. Most dark-matter theories have collisions between dark-matter particles that result in non-dark-matter fragments, including antimatter. It is the possibility that dark-matter collisions are producing the positron excess that has everyone so excited. But the best we can say at the moment is that AMS’s results are consistent with various theories of dark-matter that have been put forth. And that’s about it. The principal investigator, Dr. Sam Ting, has been very careful to avoid overstating the implications of these first observations. There are other possibilities, besides dark-matter, which might explain the positron excess. The ‘shape’ of the positron excess when plotted against positron energy can be used to rule out some of the various competing theories; in particular, a sharp drop off near the top of the energy range measured by AMS is consistent with some (but not all!) dark-matter theories, and incompatible with the most likely non-dark-matter explanations. Only more observations, enough to be statistically meaningful, will enable scientists to distinguish with confidence between the various explanations. So there is a lot more work to be done.

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