APS: Fermi Results

I am on my way back from the 2009 American Physical Society (APS)’s April meeting (in May). It was an exciting, well run conference and I thoroughly enjoyed myself.

The big news of the conference was the high energy electron+positron cosmic ray spectrum that the Fermi telescope released during the first talk of the meeting. As I mentioned in my previous post, cosmic rays are particles which are constantly bombarding the earth’s atmosphere. They consist of a mixture of protons, electrons, positrons (positively charged electrons), photons and a mix of heavier elements. They originate in a variety of sources ranging from astrophysical accelerators such as pulsars, supernovae remnants and active galactic nuclei which can accelerate particles to high energies to the sun. It is also believed that the cosmic ray flux could contain the decay products of dark matter annihilation. Dark matter comprises 25% of the known energy density in the universe, 5% goes to the normal matter that you, I and the stars are made of, and 70% percent goes to something so mysterious that we call it dark energy. Sounds spooky, right? There are many theories of what the dark matter could be made of but all we know for sure is that it interacts extremely rarely, giving it the property of ‘darkness’.

Newly released Fermi results (red) of the electron and positron flux compared with the conventional diffuse model of cosmic ray flux (blue). For more information Physics has a technical article about it.

Eluding detection, dark matter has been a persistent thorn in the side of particle physicists and astrophysicists alike since Fritz Zwicky proposed it in 1933 after observing the rotational velocity of stars in galaxies. Stars orbit the center of their galaxy at a speed that is dependent upon the mass inside their orbits. We can measure the speed of the stars using the doppler effect and we can determine how much mass we think is there by measuring the amount of light we see. However, once you compare the mass in the galaxy and the velocity of the stars you find that you need many more times the mass that you see to generate the speed of the stars. Dark matter was proposed to fill this gap and has been verified in diverse and numerous observations.

There are a number of experiments investigating the flux of cosmic rays to try to find evidence of dark matter annihilation. In the past 6 months or so there have been several exciting yet unconfirmed measurements published. The ATIC experiment found an excess of electrons+positrons at high energy and, in a different energy range, the Pamela experiment found an excess of positrons relative to the number of positrons+electrons. Fermi, a state of the art satellite telescope designed to look at gamma rays and other cosmic rays, was supposed to clarify the situation. They released the results of electron+positron flux on Saturday morning and rather than clearing up the picture, it is now fuzzier. They do not see the same excess that ATIC sees, yet their data does not agree with the standard simulations of galactic cosmic rays. Looking at their data you could see an excess if you wanted to but if you don’t you can convince yourself its not there as is often the case with ambiguous results.

My concern with all of these experiments is that the signal is very small compared to the flux of protons, their main background, and their background rejection efficiency decreases in the region where the excess grows. A mis-measurement of the efficiencies, resolutions or the errors could make a big change in the results. I’m not yet convinced that these results are evidence for dark matter, and I think there are many who would agree with me.

It's important to note that there is a simple astrophysical solution which could explain the data. People have proposed that there is one or more nearby pulsars that are creating the excess that we haven’t found yet. However pulsars are mundane to particle physicists so that explanation was mostly ignored, especially by the theorists. It was amusing to see how quickly they showed plots comparing their favorite models to the new data.

A composite image of the pulsar containing Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. The size of the X-ray image is smaller because the higher energy X-ray emitting electrons radiate away their energy more quickly than the lower energy optically emitting electrons as they move.

The experiments are in the process of further investigating their data. Fermi should release their high energy gamma ray data soon and Pamela will release their positron+electron flux measurement which can be directly compared to the Fermi data. If either of these signals confirm what we’ve seen already the case for dark matter will become more convincing. Until that time, however, I like the suggestion of a commenter in one of the sessions: “Members of each experiment should be locked in a room without weapons...and not let out until they figure this mess out”.