Excess Antimatter in Space–A Possible Sign of Dark Matter


CERN has just published a press release about the newest results from the Alpha Magnetic Spectrometer (AMS). This is a particle detector that has been installed on the International Space Station (ISS), which should reduce the criticism a little bit that the ISS doesn’t do mush interesting science. The reason this detector has to be in space is because the particles of interest would have interacted with the matter in the atmosphere, either being destroyed along the way or simply producing a bunch of “shrapnel” that would make the science difficult.

What AMS detects are cosmic ray positrons. Positrons are a form of antimatter; in this case, they are the antimatter form of electrons, having the same mass but the opposite charge. When a positron and electron meet, they annihilate and form energy in the form of a light particle (photon). So, first off, the cool thing is that we are observing antimatter in space.

But there is more than just the actual observation of antimatter that is of interest here. We have known there has been this stuff out there for a while. What AMS and scientists have been looking at has been an excess of positrons out in space, especially at a particular level of energy. As mentioned, when positrons and electrons meet, they become photons, and those photons have to carry all the energy of their mass (from Einstein’s E=mc^2) as well as their kinetic energy. The faster the positron comes in, the more energetic the photons created.

AMS is reporting on 25 billion events, (25 billion particle detections), 400,000 of which are due to positrons. They looked at an energy range of 0.5 GeV and 350 GeV. For comparison, an electron has a mass-energy of 511 keV, so a minimum energy electron-positron annihilation would produce about 1 MeV or 0.001 GeV of energy. So this range is considering positrons which are moving at a pretty good click.

In those results, the AMS teem sees something going on at the 10 to 250 GeV range. They are seeing not only lots of positrons in this energy range, but there seems to be no dependence on time of observations or direction of the detector. This suggests a production of positrons that is pretty much all over the sky and with no time dependence.

This is of interest because this is what we expect to see on the widely-believed hypothesis about dark matter. For the last decade or so, the dominant hypothesis about dark matter has been that it is due to some sort of particle or particles; these particles have been called WIMPs (weakly interacting massive particles). The name was a contrast to the other hypothesis which has fallen by the way-side, MACHOs. In this case of the revenge of the (particle) nerds, the little particle explanation of dark matter seems the best explanation, and dark matter now makes up the vast majority of the mass of the universe (there dark energy is even more dominant as a constituent of the universe).

What is more exciting here is that these WIMPs have a possible explanation under super-symmetry. The standard idea is that dark matter particles will be their own antiparticle, so if they come together they annihilate, just like matter and antimatter. The resulting energy will then be enough to produce particle-antiparticle pairs that we can detect. The best models of galaxies predict that dark matter forms a halo about the whole galaxy, so these annihilations should be happening in all directions as seen by an observer inside the Milky Way (i.e. us). Their distribution should also be fairly even, so there should not be some times of greater or lesser annihilation, at least in the long-term. Also, the particles in the halo will be going about in orbit in this halo at high speeds, so there are predictions as to what sorts of energies the particles created in dark matter interactions should have. So far, all these predictions are consistent with the AMS results. In other words, AMS could be able to prove not on the WIMP hypothesis of dark matter, but some of the predictions of super-symmetry.

Now, this has not yet been proven, because other hypotheses about what is producing the positrons exist and have not yet been ruled out. The press release mentions how pulsars in the plane of the galaxy are another candidate. Since positrons are charged particles, the magnetic fields in the galaxy could send them in just about any direction, making them come to the Earth near randomly. This makes the observation that the positrons are coming in all directions equally less than decisive. However, it is indicated that over the next several months more results will come in and be analyzed, and at that point it may be possible to distinguish positrons from pulsars and from dark matter. How this is done is not specified, but I suspect it is because of the shape of the positron detection curve. The curve predicted by dark matter halo interactions will have a particular shape that fits the expected velocities of these particles; pulsars will likely have a different distribution of positron energies. Also, more detections from the sky will help differentiate between random-direction positrons from pulsars and uniform distributions from the dark matter halo.

I would also imagine these results will help particle physicists on Earth zero in on the energy regions to look for dark matter candidates. Super-symmetry in some ways has been producing DM candidates too well (since there are several versions of super-symmetry), but with theories better fitting the new data, and with experimentalists on the ground better able to know where to tune their instruments, we may make a lot of progress in advanced particle physics in the near future.

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