Cosmic ray backgrounds for dark matter indirect detection

Abstract

Recently, dark matter indirect searches have gained a lot of attention, mostly due to the possibility of recent anomalies in cosmic rays and microwave sky maps being due to the annihilation or decay of dark matter. In this thesis, we argue however that these signals are plagued by irreducible astrophysical backgrounds and show how plausible conventional physics can mimic the alleged dark matter signals. In particular, we consider the possibility that the rise in the positron fraction observed by the PAMELA satellite is due to the production through (hadronic) cosmic ray spallation and subsequent acceleration of positrons, in the same sources as the primary cosmic rays. We present a new analytical estimate of the range of possible fluctuations in the high energy electron flux due to the discreteness of plausible cosmic ray sources. Fitting our result for the total electron-positron flux measured by the Fermi satellite allows us to fix the only free parameter of the model and make an independent prediction for the positron fraction. This model can be tested by considering similar effects expected for nuclear secondary-to-primary ratios such as B/C. A rise predicted above O(100) GeV/n would be an unique confirmation of our explanation for a rising positron fraction and rule out the dark matter explanation. Furthermore, we review the assumptions made in the extraction of the `WMAP haze' which has also been claimed to be due to electrons and positrons from dark matter annihilation in the Galactic centre region. We argue that the energy-dependence of their diffusion makes the extraction of the haze through template fitting unreliable. The systematic effects introduced by this can, under specific circumstances, reproduce the residual, suggesting that the `haze' may be just an artefact of the template subtraction.

161 pages, 50 figures, D.Phil. thesis, University of Oxford (2010)

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