Dark matter indirect detection and Bremsstrahlung Processes

It is now well established that some form of Dark Matter (DM) makes a sizeable contribution to the total matter-energy abundance of the Universe, yet DM still evades detection and its particle properties remain unknown. Indirect detection provides an important probe of some of these fundamental properties. DM self-annihilation throughout the Universe is expected to lead to an observable signal of standard model particles at Earth, and any observed flux of standard model particles from a particular region acts as an upper limit on the annihilation signal from that region. In Chapter 1, we give an introduction to our current knowledge of DM. We begin with the historic and recent evidence for the existence of DM based on its gravitational effects, before describing our current knowledge of DM formation history and abundance. We then describe and compare a number of competing DM density profiles for our galaxy, highlighting the large uncertainties towards the Galactic center. There are currently a large number of DM candidates, sometimes called the `Candidate Zoo'. We briefly introduce several of the most popular candidates, describing their history and motivation. We then move on to describe current searches for DM, focusing on indirect detection, which aims to detect DM via an observable flux of its SM annihilation products. We detail the major constraints on the DM self-annihilation cross section, and examine some potential signals from DM annihilation. We also describe constraints on DM from direct detection and collider searches. Finally, we introduce bremsstrahlung processes in the context of DM annihilation, where a particle such as a gamma-ray is radiated from one of the DM annihilation products at the Feynman diagram level. In Chapter 2, we use gamma-ray data from observations of the Milky Way, Andromeda (M31), and the cosmic background to calculate robust upper limits on the dark matter self-annihilation cross section to monoenergetic gamma rays, $\langle v\sigma \rangle_{\gamma \gamma}$, over a wide range of dark matter masses. We do this in a model-independent and conservative way, such that our results are valid across a broad spectrum of DM models and astrophysical assumptions. In fact, over most of this range, our results are unchanged if one considers just the branching ratio to gamma rays with energies within a factor of a few of the endpoint at the dark matter mass. If the final-state branching ratio to gamma rays, $Br(\gamma \gamma)$, were known, then $\langle v\sigma \rangle_{\gamma \gamma} / Br(\gamma \gamma)$ would define an upper limit on the {\it total} cross section. %we conservatively assume $Br(\gamma \gamma) \gtrsim 10^{-4}$. In Chapter 3, we take advantage of the fact that annihilation to charged leptons will inevitably be accompanied by gamma rays due to radiative corrections to place similar limits on the the annihilation cross section to an electron-positron pair, $\langle v\sigma \rangle_{e^+e^-}$. Photon bremsstrahlung from the final state particles occurs at the Feynman diagram level, yet the gamma-ray spectrum per annihilation is approximately model independent, such that our analysis applies to a broad class of DM models. We compare the expected annihilation signal with the observed gamma-ray flux from the Galactic Center, and place conservative upper limits on the annihilation rate to an electron-positron pair. We also constrain annihilation to muon and tau lepton pairs. We again make conservative choices in the uncertain dark matter density profiles, and note that our constraints would only be strengthened if the density were more tightly constrained. The spectrum per annihilation produces hard gamma rays near the kinematic cutoff, and we find that the constraints on $\langle v\sigma \rangle_{e^+e^-}$ are weaker than those on $\langle v\sigma \rangle_{\gamma \gamma}$ only by a factor of $\sim 10^{-2}$, as expected since the $2\rightarrow 3$ process is suppressed relative to the $2\rightarrow 2$ process. Annihilation to leptons will also be accompanied by massive gauge bosons due to electroweak radiative corrections. In Chapter 4 we examine a case where DM annihilates exclusively to neutrinos at the $2\rightarrow 2$ level, and gamma rays, leptons and hadrons will inevitably be produced due to electroweak bremsstrahlung. We explicitly calculate the ratio of the rate for the three electroweak bremsstrahlung modes $\chi\chi\rightarrow \nu\bar\nu Z,\, e^+ \nu W^-\, e^- \bar \nu W^+$ to the rate for the $2\rightarrow 2$ process $\chi\chi\rightarrow \nu\bar\nu$. Electroweak bremsstrahlung plays a larger role in the special case where the annihilation rate to leptonic modes suffers helicity suppression. While it has long been known that photon bremsstrahlung can lift the helicity suppression, we show in Chapter 5 that electroweak bremsstrahlung is also capable of lifting this suppression, such that the branching ratio to the 3-body electroweak bremsstrahlung final states can greatly exceed the branching ratio to an electron-positron or neutrino pair. We explicitly calculate the electroweak bremsstrahlung cross section in a typical leptophilic model. In Chapter 6 we examine observational signatures of dark matter annihilation in the Milky Way arising from these electroweak bremsstrahlung contributions to the annihilation cross section. Here we calculate the spectra of stable annihilation products produced via $\gamma/W$/$Z$-bremsstrahlung. After modifying the fluxes to account for the propagation through the Galaxy, we set upper bounds on the annihilation cross section via a comparison with observational data. We show that stringent cosmic ray antiproton limits preclude a sizable dark matter contribution to observed cosmic ray positron fluxes in the class of models for which the bremsstrahlung processes dominate.