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Cosmological Dark Matter and the Isotropic Gamma-Ray Background: Measurements and Upper Limits
Stockholm University, Faculty of Science, Department of Physics. (Cosmology, Particle Astrophysics and String Theory)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis addresses the isotropic diffuse gamma-ray background, as measured by the Fermi gamma ray space telescope, and its implications for indirect detection of dark matter. We describe the measurement of the isotropic background, including also an alternative analysis method besides the one published by the Fermi-LAT collaboration. The measured isotropic diffuse background is compatible with a power law differential energy spectrum with a spectral index of  -2.41 ± 0.05 and -2.39 ± 0.08, for the two analysis methods respectively. This is a softer spectrum than previously reported by the EGRET experiment. This rules out any dominant contribution with a significantly different shape, e.g. from dark matter, in the energy range 20 MeV to 102.4 GeV. Instead we present upper limits on a signal originating from annihilating dark matter of extragalactic origin. The uncertainty in the dark matter signal is primarily dependent on the cosmological evolution of the dark matter distribution. We use recent N-body simulations of structure formation, as well as a semi-analytical calculation, to assess this uncertainty. We investigate three main annihilation channels and find that in some, but not in all, of our scenarios we can start to probe, and sometimes rule out, interesting parameter spaces of particle physics models beyond the standard model.We also investigate the possibility to use the angular anisotropies of the annihilation signal to separate it from a background originating from conventional sources, e.g. from active galactic nuclei. By carefully modelling the performance of the Fermi gamma-ray space telescope and galactic foregrounds we find that this method could be as sensitive as using information from the energy spectrum only.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2010. , 118 p.
Keyword [en]
dark matter theory, dark matter simulations, dark matter experiments, isotropic gamma-ray background, Fermi gamma-ray space telescope
National Category
Subatomic Physics
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-38900ISBN: 978-91-7447-082-6 (print)OAI: oai:DiVA.org:su-38900DiVA: diva2:317422
Public defence
2010-06-03, FD5, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2010-05-11 Created: 2010-05-03 Last updated: 2013-01-22Bibliographically approved
List of papers
1. Pre-launch estimates for GLAST sensitivity to Dark Matter annihilation signals
Open this publication in new window or tab >>Pre-launch estimates for GLAST sensitivity to Dark Matter annihilation signals
Show others...
2008 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, Vol. 0807, no 013Article in journal (Refereed) Published
Abstract [en]

We investigate the sensitivity of the Gamma-ray Large Area Space Telescope (GLAST) for indirectly detecting weakly interacting massive particles (WIMPs) through the γ-ray signal that their pair annihilation produces. WIMPs are among the favorite candidates for explaining the compelling evidence that about 80% of the mass in the Universe is non-baryonic dark matter (DM). They are serendipitously motivated by various extensions of the standard model of particle physics such as supersymmetry and universal extra dimensions (UED). With its unprecedented sensitivity and its very large energy range (20 MeV to more than 300 GeV) the main instrument on board the GLAST satellite, the Large Area Telescope (LAT), will open a new window of discovery. As our estimates show, the LAT will be able to detect an indirect DM signature for a large class of WIMP models given a cuspy profile for the DM distribution. Using the current state of the art Monte Carlo and event reconstruction software developed within the LAT collaboration, we present preliminary sensitivity studies for several possible sources inside and outside the Galaxy. We also discuss the potential of the LAT to detect UED via the electron/positron channel. Diffuse background modeling and other background issues that will be important in setting limits or seeing a signal are presented

Keyword
mörk materia, gamma-strålning, Fermi/GLAST
National Category
Subatomic Physics
Identifiers
urn:nbn:se:su:diva-15550 (URN)10.1088/1475-7516/2008/07/013 (DOI)000258022800013 ()
Note
Totalt 28 författare.Available from: 2008-12-05 Created: 2008-12-05 Last updated: 2010-05-31Bibliographically approved
2. Constraints on cosmological dark matter annihilation from the Fermi-LAT isotropic diffuse gamma-ray measurement
Open this publication in new window or tab >>Constraints on cosmological dark matter annihilation from the Fermi-LAT isotropic diffuse gamma-ray measurement
2010 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 4, 014- p.Article in journal (Refereed) Published
Abstract [en]

The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. We use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilat- ing into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly aected by the underlying distribution of dark matter, and by using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe’s transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersym- metry models) annihilating predominantly into quarks. For the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals.

Keyword
dark matter experiments, dark matter simulations, dark matter theory
National Category
Physical Sciences Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-38905 (URN)10.1088/1475-7516/2010/04/014 (DOI)000277684600019 ()
Note
Totalt 149 författare.Available from: 2010-05-03 Created: 2010-05-03 Last updated: 2017-12-12Bibliographically approved
3. Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived from First-Year Fermi Large Area Telescope Data
Open this publication in new window or tab >>Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived from First-Year Fermi Large Area Telescope Data
2010 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 104, no 101101Article in journal (Refereed) Published
Abstract [en]

We report on the first Fermi Large Area Telescope (LAT) measurements of the so-called ‘‘extra- galactic’’ diffuse gamma-ray emission (EGB). This component of the diffuse Υ-ray emission is generally considered to have an isotropic or nearly isotropic distribution on the sky with diverse contributions discussed in the literature. The derivation of the EGB is based on detailed modeling of the bright foreground diffuse Galactic gamma-ray emission, the detected LAT sources, and the solar gamma-ray emission. We find the spectrum of the EGB is consistent with a power law with a differential spectral index of 2.41 \pm 0.05 and intensity I(>100 MeV)=(1.03\pm 0.17) e-5 cm**2 s **-1 sr**-1 , where the error is systematics dominated. Our EGB spectrum is featureless, less intense, and softer than that derived from EGRET data.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-38906 (URN)10.1103/PhysRevLett.104.101101 (DOI)000275543500007 ()
Note
Totalt 183 författare.Available from: 2010-05-03 Created: 2010-05-03 Last updated: 2017-12-12Bibliographically approved
4. Anisotropies in the diffuse gamma-ray background from dark matter with Fermi LAT: a closer look
Open this publication in new window or tab >>Anisotropies in the diffuse gamma-ray background from dark matter with Fermi LAT: a closer look
2011 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 3, 2040-2054 p.Article in journal (Refereed) Published
Abstract [en]

We perform a detailed study of the sensitivity to the anisotropies related to dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) as measured by the Fermi Large Area Telescope (Fermi LAT). For the first time, we take into account the effects of the Galactic foregrounds and use a realistic representation of the Fermi LAT. We implement an analysis pipeline which simulates Fermi LAT data sets starting from model maps of the Galactic foregrounds, the Fermi-resolved point sources, the extragalactic diffuse emission and the signal from DM annihilation. The effects of the detector are taken into account by convolving the model maps with the Fermi LAT instrumental response. We then use the angular power spectrum to characterize the anisotropy properties of the simulated data and to study the sensitivity to DM. We consider DM anisotropies of extragalactic origin and of Galactic origin (which can be generated through annihilation in the MilkyWay substructures) as opposed to a background of anisotropies generated by sources of astrophysical origin, blazars for example. We find that with statistics from 5 yr of observation, Fermi is sensitive to a DM contribution at the level of 1-10 per cent of the measured IGRB depending on the DM mass m. and annihilation mode. In terms of the thermally averaged cross-section <sigma(A)v >, this corresponds to similar to 10(-25) cm(3) s(-1), i. e. slightly above the typical expectations for a thermal relic, for low values of the DM mass m(x) less than or similar to 100 GeV. The anisotropy method for DM searches has a sensitivity comparable to the usual methods based only on the energy spectrum and thus constitutes an independent and complementary piece of information in the DM puzzle.

Keyword
methods: statistical, dark matter, large-scale structure of Universe, gamma-rays: diffuse background
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-80714 (URN)10.1111/j.1365-2966.2011.18525.x (DOI)000292977600018 ()
Note

AuthorCount:4;

Available from: 2012-09-27 Created: 2012-09-27 Last updated: 2017-12-07Bibliographically approved

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