PRL published a couple new papers from AMS today, including this one, which shows an updated plot of the positron fraction (e+/(e+ + e-)) as a function of energy. I don’t think these have been posted to the arXiv yet, which is unusual for our field. Unfortunately, PRL must be accessed either through a network with access or with an account, which requires money, so if you’re not at a university and aren’t an APS member you might have trouble looking at the paper. In conjunction with the papers being released, there was a seminar at CERN earlier today. I didn’t find out about the seminar until it was too late for me to try to call in remotely.
AMS is a large multipurpose detector based at the International Space Station that studies cosmic rays, which are high energy particles flying around in space. The search for dark matter is one of the main purposes of the experiment, but it can study many things about cosmic rays, such as their composition, energy spectra, directions, etc.
This paper shows the positron fraction up to 500 GeV, which is a bit higher than in previous measurements. This measurement is useful for looking for dark matter annihilation to electron-positron pairs. The expected signal is for the positron fraction to increase at some energy and then peak and drop fairly sharply at an energy of approximately the dark matter mass. Previous measurements from AMS, Fermi, PAMELA and ATIC have all seen an increase in the ratio and have even seen the spectrum seem to level off. This new measurement shows that the leveling off continues, and the spectrum may even be starting to fall in the highest energy bin. However, the measurement is limited by both statistics and systematics at the highest energies, so the apparent decrease in the highest bin is not going to be statistically significant at this point in time. That may change with more events in the future and hopefully a better understanding of the detector and of astrophysical models for the non-dark matter background.