Life After FERMI

I mean, after FERMI's first electron data that shed new light on the currently hottest topic in astroparticle physics - the origin of the cosmic-ray positron excess measured by PAMELA. The PAMELA anomaly trumpeted last summer, combined with the data from ATIC (who claims a spectacular excess of electrons at few hundred GeV), prompted zillions of publications that speculate of its dark matter origin. A few weeks ago FERMI revealed their first measurement of the electron+positron cosmic ray spectrum up to 1 TeV. Here is my summary of what that implies for the models of dark matter.

Let's begin with a handful of facts:

• Thanks to FERMI and HESS we now have a pretty accurate picture of the cosmic-ray electron+positron spectrum. Up to 1 TeV, the spectrum is well approximated by power law, $\sim E^{-3}$, while above TeV it becomes softer (larger power).
• ATIC screwed: there is no clear feature in the electron spectrum. In principle, one should describe the present situation as "two experiments giving inconsistent results", given that ATIC's brand new data continue displaying a distinct bump with even smaller errorbars. But, somehow, the public opinion prefers a fancy high-tech satellite over a dirty leaking balloon somewhere in the cold Antarctica. More seriously, FERMI simply beats ATIC with a hundred times more statistics.
• The electron spectrum near TeV is above the background predicted by conventional cosmic-ray propagation models (which prefer a larger power, $\sim E^{-3.3}$) in which the electrons are produced by scattering of cosmic ray protons. But that alone is hardly an anomaly, and the spectrum can be easily fitted by cranking up the propagation model.
• However, combining FERMI with the PAMELA positron data strongly indicates the presence of a new primary component of electrons/positrons (as opposed to the secondary production by protons) .
• The primary component could be injected by nearby pulsars, or by another so far unidentified astrophysical process, or by dark matter annihilating or decaying in our galaxy. We have to wait for the jury to reach the verdict.
• But, light (less than 1 TeV) dark matter particle as a source of the PAMELA and FERMI excess is now strongly disfavored. The reason is that a light particle would lead to sharp spectral features at energies comparable to its mass, whereas FERMI sees none of that. If dark matter is the cause of the excess, it has to be relatively heavy, 1 TeV or more.

In fact, the new FERMI data did not really lead to a slaughter of the dark matter models or its authors. New papers keep appearing in which the excess is fitted with axions, neutralinos, winos, KK particles, F-theory or three little pigs. But Occam is waving his razor menacingly, and we are reaching the point where boring astrophysics becomes the simplest explanation of all available data. The dark matter models, although still viable, have to be intelligently designed to yield observable signals in the positron and electron channels, but none in antiprotons, gamma rays or neutrinos. Incidentally, the electron channel is the one where the astrophysical background is most difficult to control...

The cool thing is that by the end of this summer we may further disfavor dark matter or return it to grace. The new crucial piece of information will be FERMI's measurement of the diffuse gamma ray spectrum that will extend its previous measurement to larger energies. If the electrons and positrons observed by PAMELA and FERMI originate from dark matter it means that they are produced all over the galaxy. Once produced, the electrons lose their energy by scattering on starlight and on the CMB or by synchrotron radiation in the galactic magnetic fields, which leads to a diffuse flux of photons at energies of a few hundred GeV. Given the number of electrons needed to explain PAMELA and FERMI, the diffuse signal should be detectable by 1-year FERMI data. The important thing is that boring astrophysics cannot easily fake that signal. On the other hand, the absence of features in diffuse gamma would be a huge setback for the dark matter interpretation. The experimental data are expected on August 12, so just a little patience please...

For more details and plots see the slides of Alessandro Strumia's talk at Planck'09; check also for the connection between dark matter and dialectic materialism.