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Positrons in space!!

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A fair amount of what I write about here is about accelerator physics, done at facilities like the Large Hadron Collider. But you can also do particle physics in space, which is filled with fast-moving particles from natural sources. "All" you need to do is build a particle detector, analogous to ATLAS or CMS, and put it in Earth orbit. That's exactly what the Alpha Magnetic Spectrometer (AMS) is. Since 2011, when it was installed on the International Space Station, AMS has been detecting cosmic electrons and positrons, looking for anomalous patterns, and today they presented their first data release.

Let's jump straight to the results:

AMS data with other experiments

This plot shows the number of positrons with a given energy as a fraction of the total number of electrons and positrons with that energy, \frac{N_\text{positrons}}{N_\text{electrons} + N_\text{positrons}}. The key interesting feature, which confirms a result from the previous experiments PAMELA and Fermi-LAT, is that the plot rises at energies higher than about \SI{10}{GeV}. That's not what you'd normally expect, because most physical processes produce fewer particles at higher energies. (Think about it: it's less likely that you'll accumulate a lot of energy in one particle.) So there must be some process, not completely understood, which is producing positrons.

As part of their data analysis, AMS has tested a model which describes the flux (total number) of positrons as the sum of two contributions:

  • A power law (the first term in the below equations), representing known, "typical" sources, and
  • A "source spectrum" with an exponential cutoff, representing something new

\begin{align}\Phi_{\ealp} &= C_{\ealp}E^{-\gamma_{\ealp}} + C_s E^{-\gamma_s} e^{-E/E_s} \\ \Phi_{\elp} &= C_{\elp}E^{-\gamma_{\elp}} + C_s E^{-\gamma_s} e^{-E/E_s}\end{align}

The model fits pretty well to the data so far:

Fit of model to AMS data

This means that the AMS data are consistent with the existence of a new massive particle, one that might make up the universe's dark matter. But a new particle is not the only explanation. You'll see a lot of news articles, blog posts, comments, etc. saying that AMS has detected evidence of dark matter, but that's just not true. For example, there are known astrophysical sources, such as pulsars, which could conceivably be making these high-energy positrons. The results found by AMS so far are not precise enough, and don't go up to high enough energies, to allow us to tell the difference with any confidence.

There are a couple of signs we'll be looking for that could help identify this unknown source of positrons:

  • The main one is that, if the positrons are coming from the decays of some as-yet-undiscovered particle, they can't be produced at energies higher than that particle's mass. Now yes, the new particle could be moving fast when it decays, and that would produce fast-moving positrons with high energies — but for a variety of reasons, we don't expect that to be the case. In the standard model of cosmology, the dark matter is "cold," which means that it's not moving very fast relative to other things in the universe.
  • The other main sign to look for is any anisotropy in the positrons' direction — that would mean that there are more positrons coming from some directions in the sky than others. If an anisotropy is detected, that indicates that these positrons are coming from specific, localized sources, and not from something that exists pretty much everywhere, as dark matter would. AMS has checked for this effect in the data they've collected so far, and they see a pretty isotropic distribution; positrons are coming from every direction in the sky with roughly the same frequency. That is consistent with the idea that they come from dark matter particle decays, but again, it is not actual evidence. Even if the positrons are coming from something like pulsars, they could be bent around to approach in different directions by the volatile magnetic fields of the Sun, Earth, and galaxy.

AMS will continue to collect data for a long time to come, so we can look forward to ever more precise data releases in the future, data which will hopefully put a rest to the mystery of the not-missing positrons. In the meantime, you may want to check out the PRL viewpoint — a not-too-technical explanation — of this research, or even read the original paper, which can be downloaded for free from the gray citation on the viewpoint page.

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