The cosmic ray signature of dark matter? November 24, 2008Posted by Sarah in science.
Tags: astronomy, cosmic rays, cosmology, dark matter, fermi, lhc, nasa, pamela
A very interesting paper was published in last week’s issue of Nature – I blogged about it before after reading the NASA press release. It’s wasn’t all that helpful without reading the actual paper but the cosmic ray – dark matetr link caught my attention.
Just today a conference paper (i.e. not peer-reviewed) appeared on astro-ph about some preliminary results from PAMELA – another cosmic ray detector that focuses on antiparticles (positrons and antiprotons). Recall that PAMELA was the source of some controversy earlier this year. Another paper on PAMELA data was posted on astro-ph back in October, it’s listed as being submitted to Nature so again, not reviewed yet. But perhaps another cosmic rays Nature paper soon, and there’s certainly a lot of buzz!
I had a read through these papers and some background stuff – it’s something I didn’t know much about and it’s very cool. Cosmic rays: inneresting akshually!
A quick recap first. The Nature paper in question was written by Chang of the Purple Mountain Observatory in Nanjing, China, and colleagues from the US, Germany and Russia. It describes findings from a balloon-borne experiment in Antarctica called ATIC, aimed at detecting high-energy cosmic radiation, in order to learn more about the distribution of energies at which these charged particles arrive at the Earth’s atmosphere, their composition, and how they get to be so energetic. The team essentially found a “bump” in number of electron detections around energies of 300-800 billion electron volts (GeV) that cannot be explained by theoretical models. Cosmic ray electrons are detected at a wide range of energies, and outside this bump range the ATIC data follow the models pretty well.
The model they compare the data to is produced with a code called GALPROP. This simulates the propagation of all species of cosmic rays (electrons, protons, alpha particles) from the source through the galaxy to our atmosphere; including processes that make the particles lose or gain energy, or interactions with interstellar material that produce secondary particles. You can read more about the science behind GALPROP here.
The PAMELA papers also report an excess in positrons above the model in the particle spectrum above 10 GeV.
A commonly accepted theory of the nature of dark matter proposes the existence of weakly interacting massive particles (WIMPs). So in that theory, the unseen matter would be made up of some kind of subatomic particle that is quite heavy, but doesn’t interact in the standard ways of “normal” baryonic matter – that’s why they’re “dark”. We would have to detect them indirectly, for example through the secondary particles produced when WIMPs annihilate each other. Various theories exist explaining the nature of these WIMPS, such as supersymmetry, and these theories predict the kinds of secondary particles signatures we may expect from the dark matter particles annihilating. In fact, the predictions seem to point to just the kinds of excesses these experiments are seeing.
Although it’s tempting to get very excited about the first dark matter detection, it’s important to remember that (A) these missions are all ongoing and new data is being gathered all the time, (B) models can be off the mark, and they too are constantly being tweaked and updated, and most importantly (C) there is an alternative explanation. If our current models are based on the known sources of cosmic rays in the Galaxy, maybe there are more sources we haven’t discovered yet? Indeed, the electron excess detected by ATIC may well be caused by a nearby pulsar or intermediate mass black hole. Getting more data with information on directionality, for example from Fermi, would tell us that. And that would be a very cool result too!
As the amounts of information from these missions are still quite small, scientists working on these data are limited to playing a numbers game with the various prediction theories, models, error bars, boost factors – but the fact that the early data from the experiments shows some consensus is very promising indeed.
Image credit: Sky & Telescope/Gregg Dinderman