Celine Boehm

MeV dark matter: Has it been detected?

We discuss the possibility that the recent detection of 511 keV gamma rays from the galactic bulge, as observed by INTEGRAL, is a consequence of low mass (1-100 MeV) particle dark matter annihilations. We discuss the type of halo profile favored by the observations as well as the size of the annihilation cross section needed to account for the signal. We find that such a scenario is consistent with the observed dark matter relic density and other constraints from astrophysics and particle physics.

Simplified models for dark matter searches at the LHC

This document outlines a set of simplified models for dark matter and its interactions with Standard Model particles. It is intended to summarize the main characteristics that these simplified models have when applied to dark matter searches at the LHC, and to provide a number of useful expressions for reference. The list of models includes both s-channel and t-channel scenarios. For s-channel, spin-0 and spin-1 mediation is discussed, and also realizations where the Higgs particle provides a portal between the dark and visible sectors. The guiding principles underpinning the proposed simplified models are spelled out, and some suggestions for implementation are presented.

Large Scale Structure in Bekenstein's theory of relativistic Modified Newtonian Dynamics

A relativistic theory of modified gravity has been recently proposed by Bekenstein. The tensor field in Einsteins theory of gravity is replaced by a scalar, a vector, and a tensor field which interact in such a way to give modified Newtonian dynamics (MOND) in the weak-field nonrelativistic limit. We study the evolution of the Universe in such a theory, identifying its key properties and comparing it with the standard cosmology obtained in Einstein gravity. The evolution of the scalar field is akin to that of tracker quintessence fields. We expand the theory to linear order to find the evolution of perturbations on large scales. The impact on galaxy distributions and the cosmic microwave background is calculated in detail. We show that it may be possible to reproduce observations of the cosmic microwave background and galaxy distributions with Bekensteins theory of MOND.

Interpretation of the Galactic Center excess of gamma rays with heavier dark matter particles

Previous attempts at explaining the gamma-ray excess near the Galactic Centre have focussed on dark matter annihilating directly into Standard Model particles. This results in a preferred dark matter mass of 30-40 GeV (if the annihilation is into b quarks) or 10 GeV (if it is into leptons). Here we show that the gamma-ray excess is also consistent with heavier dark matter particles; in models of secluded dark matter, dark matter with mass up to 76 GeV provides a good fit to the data. This occurs if the dark matter first annihilates to an on-shell particle that subsequently decays to Standard Model particles through a portal interaction. This is a generic process that works in models with annihilation, semi-annihilation or both. We explicitly demonstrate this in a model of hidden vector dark matter with an SU(2) gauge group in the hidden sector.

Can neutralinos in the MSSM and NMSSM scenarios still be light

Since the recent results of direct detection experiments at low mass, many authors have revisited the case of light (1–10) GeV weakly interacting massive particles. In particular, there have been a few attempts to explain the results from the DAMA/LIBRA, CDMS and/or CoGeNT experiments by invoking neutralinos lighter than 15 GeV. Here we show that in the minimal supersymmetric standard mode (MSSM), such light particles are completely ruled out by the Tevatron limits on the mass of the pseudoscalar Higgs. On the contrary, in the next-to-minimal supersymmetric standard model (NMSSM), we find that light neutralinos could still be viable candidates. In fact, in some cases, they may even have an elastic scattering cross section on nucleons in the range that is needed to explain either the DAMA/LIBRA, CoGeNT or CDMS recent results. Finally, we revisit the lowest limit on the neutralino mass in the MSSM and find that neutralinos should be heavier than ∼28  GeV to evade present experimental bounds.

Fitting the Fermi-LAT GeV excess: On the importance of including the propagation of electrons from dark matter

An excess of gamma rays at GeV energies has been pointed out in the Fermi-LAT data. This signal comes from a narrow region centred around the Galactic center and has been interpreted as possible evidence for light dark matter particles annihilating either into a mixture of leptons-antileptons and bb¯ or into bb¯ only. Focusing on the prompt gamma-ray emission, previous works found that the best fit to the data corresponds to annihilations proceeding predominantly into bb¯. However, here we show that omitting the photon emission originating from primary and secondary electrons produced in dark matter annihilations, and undergoing diffusion through the Galactic magnetic field, can actually lead to the wrong conclusion. Accounting for this emission, we find that not only are annihilations of ∼10  GeV particles into a purely leptonic final state allowed, but the democratic scenario actually provides a better fit to the spectrum of the excess than the pure bb¯ channel. We conclude our work with a discussion on constraints on these leptophilic scenarios based on the AMS data and the morphology of the excess.

Can annihilating dark matter be lighter than a few GeVs

The accurate measurement of galactic rotation curves,the CMB spectrum, the primordial abundances of lightelements, together with our understanding of structureformation provide convincing evidence in favor of theexistence of Dark Matter (DM) [1]. While the MA-CHOs searches [2] indicate that an astrophysical solu-tion is rather unlikely, most efforts are now concen-trated on searches for Weakly Interacting Massive Par-ticles (WIMPs) [3]. These particles would belong to theCold Dark Matter scenario (CDM); they would annihi-late and suffer from negligible damping effects at a cos-mological scale. Considering fermions only and assum-ing Fermi interactions, it was concluded [4] that the relicdensity argument constrains the DM mass (m

Interacting dark matter disguised as warm dark matter

We explore some of the consequences of dark-matter\char21{}photon interactions on structure formation, focusing on the evolution of cosmological perturbations and performing both an analytical and a numerical study. We compute the cosmic microwave background anisotropies and matter power spectrum in this class of models. We find, as the main result, that when dark matter and photons are coupled, dark matter perturbations can experience a new damping regime in addition to the usual collisional Silk damping effect. Such dark matter particles (having quite large photon interactions) behave like cold dark matter or warm dark matter as far as the cosmic microwave background anisotropies or matter power spectrum are concerned, respectively. These dark-matter\char21{}photon interactions leave specific imprints at sufficiently small scales on both of these two spectra, which may allow us to put new constraints on the acceptable photon\char21{}dark-matter interactions. Under the conservative assumption that the abundance of

Is it possible to explain neutrino masses with scalar dark matter

{10}^{12}{M}_{\ensuremath{\bigodot}}

Using the CMB angular power spectrum to study Dark Matter-photon interactions

galaxies is correctly given by the cold dark matter, and without any knowledge of the abundance of smaller objects, we obtain the limit on the ratio of the dark-matter\char21{}photon cross section to the dark matter mass