ATIC ATAC

I'm slowly recovering from the shock of starting a new life in a new time zone in a new haircut. Time to kick off with no-longer-from-CERN blogging. As a warm-up, I have an overdue rant. Some inspiration came from Tommaso's post who shares a few warm remarks about theorists teaching experimentalists. Here I elaborate on the opposite case.

The Nature magazine recently published a paper from the ATIC collaboration. ATIC is a balloon-borne experiment that studies high energy electrons and positrons (they cannot distinguish the two) coming from the cosmos. Many of these electrons and positrons are created by known astrophysical processes, mainly by cosmic rays scattering on interstellar matter (the secondary production). Astrophysicists can roughly estimate the flux due to the secondary production, although these estimates are subject to many uncertainties and should be taken with a whole container of salt. Anyway, assuming that the background estimates are correct, ATIC observes an excess of electrons at positrons at energies between 100 and 800 GeV. This fits well together with the positron excess between 10 and 100 GeV reported recently by the PAMELA satellite. Things are even more interesting. Rather than a mere excess, ATIC sees a distinct feature in the spectrum: a bump between 300 and 800 GeV. Astronomers are excited because this could be a signature of an interesting astrophysical object (a young nearby pulsar?, a microquasar?). Particle physicists are even more excited because the PAMELA and ATIC observations could be the first clear signals of annihilating or decaying dark matter particles with TeV scale masses.

The results from ATIC may turn out an important piece in the puzzle of what is the nature of dark matter. However, the collaboration must consider their results so uninteresting that they have to provide us with a cavalier theoretical interpretation. The signal is interpreted in the context of the so-called LKP - the lightest Kaluza-Klein particle in the Universal Extra Dimensions (UED) scenario. The names of Kaluza and Klein appear in the abstract and $e^N$ times in the main text. In case you missed it, they stress in conclusion that "if the Kaluza–Klein annihilation explanation proves to be correct, this will necessitate a fuller investigation of such multi-dimensional spaces, with potentially important implications for our understanding of the Universe." All in all, ATIC claims to have found in their data some hints toward the presence of extra dimensions of spacetime. What is their reason for such an extraordinary claim? It looks like they applied the modern tertium non datur: whatever is not the MSSM must be the UED.

That's philosophy. Physics, on the other hand, does not support ATIC's interpretation. From the theoretical point of view one can complain that the UED is an artificial and poorly motivated construction, that it does not address any problems of the SM (except for dark matter) while creating new problems of its own, and so on. But the main point here is that the LKP interpretation is not consistent with all available experimental data. Firstly, the LKP annihilation cross section is not large enough to explain the ATIC and PAMELA signals (if the dark matter abundance is of thermal origin). ATIC shrugs this off with a bla bla, where the former bla stands for a 100 boost factor from dark matter clumpiness (which does not come out of numerical simulations) and the latter for "other kids do it too". There is yet another serious problem that has to do with the fact that PAMELA observes no excess in cosmic anti-protons. Even though the LKP couples more strongly to leptons than to quarks (because the former have larger hypercharges to which the LKP couples), the decay rate into hadrons in the UED is still far too large. This issue is not addressed at all in ATIC's paper because it was submitted before PAMELA's data came out.

To summarize, it's such a pity to mar a beautiful experiment with a crappy theory.