As promised, I’ve completed the third section, as well as a short addendum to the second section, of my article on how experimenters at the Large Hadron Collider [LHC] can try to discover dark matter particles. The article is here; if you’ve already read what I wrote as of last Wednesday, you can pick up where you left off by clicking here.
Meanwhile, in the last week there were several dark-matter related stories that hit the press.
There has been a map made by the Dark Energy Survey of dark matter’s location across a swathe of the universe, based on the assumption that weak signals of gravitational lensing (bending of light by gravity) that cannot be explained by observed stars and dust is due to dark matter. This will be useful down the line as we test simulations of the universe such as the one I referred you to on Wednesday.
There’s been a claim that dark matter interacts with itself, which got a lot of billing in the BBC; however one should be extremely cautious with this one, and the BBC editor should have put the word “perhaps” in the headline! It’s certainly possible that dark matter interacts with itself much more strongly than it interacts with ordinary matter, and many scientists (including myself) have considered this possibility over the years. However, the claim reported by the BBC is considered somewhat dubious even by the authors of the study, because the little group of four galaxies they are studying is complicated and has to be modeled carefully. The effect they observed may well be due to ordinary astrophysical effects, and in any case it is less than 3 Standard Deviations away from zero, which makes it more a hint than evidence. We will need many more examples, or a far more compelling one, before anyone will get too excited about this.
Finally, the AMS experiment (whose early results I reported on here; you can find their September update here) has released some new results, but not yet in papers, so there’s limited information. The most important result is the one whose details will apparently take longest to come out: this is the discovery (see the figure below) that the ratio of anti-protons to protons in cosmic rays of energies above 100 GeV is not decreasing as was expected. (Note this is a real discovery by AMS alone — in contrast the excess positron-to-electron ratio at similar energies, which was discovered by PAMELA and confirmed by AMS.) The only problem is that they’ve made the discovery seem very exciting and dramatic by comparing their work to expectations from a model that is out of date and that no one seems to believe. This model (the brown swathe in the Figure below) tries to predict how high-energy anti-protons are produced (“secondary production”) from even higher energy protons in cosmic rays. Newer versions of this models are apparently significantly higher than the brown curve. Moreover, some scientists claim also that the uncertainty band (the width of the brown curve) on these types of models is wider than shown in the Figure. At best, the modeling needs a lot more study before we can say that this discovery is really in stark conflict with expectations. So stay tuned, but again, this is not yet something that in which one can have confidence. The experts will be busy.