## Saturday, November 18, 2017 ... //

### Dark matter source of cosmic ray positrons more likely: a paper

The old intriguing PAMELA experiment and perhaps newer ones by Fermi-LAT and AMS-02 have discovered lots of positrons – the antiparticles of electrons – in the cosmic rays whose flux is generally higher than expected. Such positrons may originate from dark matter and would amount to an "almost direct detection" of the particles that make up dark matter.

That would be very exciting. However, there may also be more boring, astrophysical, sources of these positrons, and pulsars in particular (magnetized neutron stars or white dwarfs emitting a beam of electromagnetic radiation). Some pieces of matter may emit lots of junk and these processes may be completely compatible with the Standard Model – i.e. just slightly more extreme than when you cook a food in your kitchen.

OK, so are these positrons due to boring astrophysical sources or dark matter – or something more exotic? We don't know but The Physics World has summarized a fresh paper in Science,

Extended gamma-ray sources around pulsars constrain the origin of the positron flux at Earth
by Abeysekara and a hundred of other authors using the High-Altitude Water Cherenkov Observatory, HAWC (you usually have lots of authors when the first author's name starts with AB LOL). They claim to basically exclude the pulsar explanation – so they make the more exciting explanations more likely.

They look at two nearby pulsars and decide that they must be covered by a murky cloud, because of some features of the gamma-rays coming from them, and that's why the positrons can't really make it through this blanket. So almost no positrons should get to Earth from such ordinary sources, and more ambitious sources such as dark matter (the most appealing among them) should be preferred.

That sounds exciting but the claims don't necessarily have to be true, despite one hundred of their authors. Dan Hooper and three co-authors – and he isn't quite an idiot – claims to have analyzed the same data in similar ways and still believes their very opposite conclusion to be right. Those pulsars seem to be totally OK to explain the observed positron excess. Convective winds and shocks in the interstellar medium are supposed to help.

I have no idea who is right – it's too dirty a discipline of science for me, of course. Even such "seemingly mundane questions" seem to be murky enough so that a paper considered persuasive by a team of 100 authors may be considered wrong by another team of experts.